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Documentation/DMA-API-HOWTO.txt
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Dynamic DMA mapping Guide ========================= |
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David S. Miller <davem@redhat.com> Richard Henderson <rth@cygnus.com> Jakub Jelinek <jakub@redhat.com> |
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This is a guide to device driver writers on how to use the DMA API with example pseudo-code. For a concise description of the API, see |
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DMA-API.txt. |
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CPU and DMA addresses There are several kinds of addresses involved in the DMA API, and it's important to understand the differences. The kernel normally uses virtual addresses. Any address returned by kmalloc(), vmalloc(), and similar interfaces is a virtual address and can be stored in a "void *". The virtual memory system (TLB, page tables, etc.) translates virtual addresses to CPU physical addresses, which are stored as "phys_addr_t" or "resource_size_t". The kernel manages device resources like registers as physical addresses. These are the addresses in /proc/iomem. The physical address is not directly useful to a driver; it must use ioremap() to map the space and produce a virtual address. I/O devices use a third kind of address: a "bus address" or "DMA address". If a device has registers at an MMIO address, or if it performs DMA to read or write system memory, the addresses used by the device are bus addresses. In some systems, bus addresses are identical to CPU physical addresses, but in general they are not. IOMMUs and host bridges can produce arbitrary mappings between physical and bus addresses. Here's a picture and some examples: CPU CPU Bus Virtual Physical Address Address Address Space Space Space +-------+ +------+ +------+ | | |MMIO | Offset | | | | Virtual |Space | applied | | C +-------+ --------> B +------+ ----------> +------+ A | | mapping | | by host | | +-----+ | | | | bridge | | +--------+ | | | | +------+ | | | | | CPU | | | | RAM | | | | Device | | | | | | | | | | | +-----+ +-------+ +------+ +------+ +--------+ | | Virtual |Buffer| Mapping | | X +-------+ --------> Y +------+ <---------- +------+ Z | | mapping | RAM | by IOMMU | | | | | | | | +-------+ +------+ During the enumeration process, the kernel learns about I/O devices and their MMIO space and the host bridges that connect them to the system. For example, if a PCI device has a BAR, the kernel reads the bus address (A) from the BAR and converts it to a CPU physical address (B). The address B is stored in a struct resource and usually exposed via /proc/iomem. When a driver claims a device, it typically uses ioremap() to map physical address B at a virtual address (C). It can then use, e.g., ioread32(C), to access the device registers at bus address A. If the device supports DMA, the driver sets up a buffer using kmalloc() or a similar interface, which returns a virtual address (X). The virtual memory system maps X to a physical address (Y) in system RAM. The driver can use virtual address X to access the buffer, but the device itself cannot because DMA doesn't go through the CPU virtual memory system. In some simple systems, the device can do DMA directly to physical address Y. But in many others, there is IOMMU hardware that translates bus addresses to physical addresses, e.g., it translates Z to Y. This is part of the reason for the DMA API: the driver can give a virtual address X to an interface like dma_map_single(), which sets up any required IOMMU mapping and returns the bus address Z. The driver then tells the device to do DMA to Z, and the IOMMU maps it to the buffer at address Y in system RAM. |
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So that Linux can use the dynamic DMA mapping, it needs some help from the drivers, namely it has to take into account that DMA addresses should be mapped only for the time they are actually used and unmapped after the DMA transfer. The following API will work of course even on platforms where no such |
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hardware exists. Note that the DMA API works with any bus independent of the underlying |
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microprocessor architecture. You should use the DMA API rather than the bus-specific DMA API, i.e., use the dma_map_*() interfaces rather than the pci_map_*() interfaces. |
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First of all, you should make sure |
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#include <linux/dma-mapping.h> |
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is in your driver, which provides the definition of dma_addr_t. This type can hold any valid DMA or bus address for the platform and should be used everywhere you hold a DMA address returned from the DMA mapping functions. |
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What memory is DMA'able? The first piece of information you must know is what kernel memory can be used with the DMA mapping facilities. There has been an unwritten set of rules regarding this, and this text is an attempt to finally write them down. If you acquired your memory via the page allocator (i.e. __get_free_page*()) or the generic memory allocators (i.e. kmalloc() or kmem_cache_alloc()) then you may DMA to/from that memory using the addresses returned from those routines. This means specifically that you may _not_ use the memory/addresses returned from vmalloc() for DMA. It is possible to DMA to the _underlying_ memory mapped into a vmalloc() area, but this requires walking page tables to get the physical addresses, and then translating each of those pages back to a kernel address using something like __va(). [ EDIT: Update this when we integrate Gerd Knorr's generic code which does this. ] |
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This rule also means that you may use neither kernel image addresses (items in data/text/bss segments), nor module image addresses, nor stack addresses for DMA. These could all be mapped somewhere entirely different than the rest of physical memory. Even if those classes of memory could physically work with DMA, you'd need to ensure the I/O buffers were cacheline-aligned. Without that, you'd see cacheline sharing problems (data corruption) on CPUs with DMA-incoherent caches. (The CPU could write to one word, DMA would write to a different one in the same cache line, and one of them could be overwritten.) |
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Also, this means that you cannot take the return of a kmap() call and DMA to/from that. This is similar to vmalloc(). What about block I/O and networking buffers? The block I/O and networking subsystems make sure that the buffers they use are valid for you to DMA from/to. DMA addressing limitations Does your device have any DMA addressing limitations? For example, is |
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your device only capable of driving the low order 24-bits of address? If so, you need to inform the kernel of this fact. |
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By default, the kernel assumes that your device can address the full |
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32-bits. For a 64-bit capable device, this needs to be increased. And for a device with limitations, as discussed in the previous paragraph, it needs to be decreased. Special note about PCI: PCI-X specification requires PCI-X devices to support 64-bit addressing (DAC) for all transactions. And at least one platform (SGI SN2) requires 64-bit consistent allocations to operate correctly when the IO bus is in PCI-X mode. For correct operation, you must interrogate the kernel in your device probe routine to see if the DMA controller on the machine can properly support the DMA addressing limitation your device has. It is good style to do this even if your device holds the default setting, |
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because this shows that you did think about these issues wrt. your device. |
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The query is performed via a call to dma_set_mask_and_coherent(): |
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int dma_set_mask_and_coherent(struct device *dev, u64 mask); |
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|
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which will query the mask for both streaming and coherent APIs together. If you have some special requirements, then the following two separate queries can be used instead: |
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The query for streaming mappings is performed via a call to dma_set_mask(): int dma_set_mask(struct device *dev, u64 mask); The query for consistent allocations is performed via a call to dma_set_coherent_mask(): int dma_set_coherent_mask(struct device *dev, u64 mask); |
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|
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Here, dev is a pointer to the device struct of your device, and mask is a bit mask describing which bits of an address your device supports. It returns zero if your card can perform DMA properly on the machine given the address mask you provided. In general, the |
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device struct of your device is embedded in the bus-specific device struct of your device. For example, &pdev->dev is a pointer to the device struct of a PCI device (pdev is a pointer to the PCI device |
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struct of your device). |
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If it returns non-zero, your device cannot perform DMA properly on |
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this platform, and attempting to do so will result in undefined behavior. You must either use a different mask, or not use DMA. This means that in the failure case, you have three options: 1) Use another DMA mask, if possible (see below). 2) Use some non-DMA mode for data transfer, if possible. 3) Ignore this device and do not initialize it. It is recommended that your driver print a kernel KERN_WARNING message when you end up performing either #2 or #3. In this manner, if a user of your driver reports that performance is bad or that the device is not even detected, you can ask them for the kernel messages to find out exactly why. |
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The standard 32-bit addressing device would do something like this: |
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if (dma_set_mask_and_coherent(dev, DMA_BIT_MASK(32))) { |
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dev_warn(dev, "mydev: No suitable DMA available "); |
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goto ignore_this_device; } |
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Another common scenario is a 64-bit capable device. The approach here is to try for 64-bit addressing, but back down to a 32-bit mask that should not fail. The kernel may fail the 64-bit mask not because the platform is not capable of 64-bit addressing. Rather, it may fail in this case simply because 32-bit addressing is done more efficiently than 64-bit addressing. For example, Sparc64 PCI SAC addressing is more efficient than DAC addressing. |
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Here is how you would handle a 64-bit capable device which can drive all 64-bits when accessing streaming DMA: int using_dac; |
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if (!dma_set_mask(dev, DMA_BIT_MASK(64))) { |
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using_dac = 1; |
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} else if (!dma_set_mask(dev, DMA_BIT_MASK(32))) { |
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using_dac = 0; } else { |
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dev_warn(dev, "mydev: No suitable DMA available "); |
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goto ignore_this_device; } If a card is capable of using 64-bit consistent allocations as well, the case would look like this: int using_dac, consistent_using_dac; |
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if (!dma_set_mask_and_coherent(dev, DMA_BIT_MASK(64))) { |
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using_dac = 1; consistent_using_dac = 1; |
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} else if (!dma_set_mask_and_coherent(dev, DMA_BIT_MASK(32))) { |
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using_dac = 0; consistent_using_dac = 0; |
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} else { |
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dev_warn(dev, "mydev: No suitable DMA available "); |
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goto ignore_this_device; } |
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The coherent mask will always be able to set the same or a smaller mask as the streaming mask. However for the rare case that a device driver only uses consistent allocations, one would have to check the return value from dma_set_coherent_mask(). |
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Finally, if your device can only drive the low 24-bits of |
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address you might do something like: |
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if (dma_set_mask(dev, DMA_BIT_MASK(24))) { |
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dev_warn(dev, "mydev: 24-bit DMA addressing not available "); |
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goto ignore_this_device; } |
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When dma_set_mask() or dma_set_mask_and_coherent() is successful, and returns zero, the kernel saves away this mask you have provided. The kernel will use this information later when you make DMA mappings. |
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There is a case which we are aware of at this time, which is worth mentioning in this documentation. If your device supports multiple functions (for example a sound card provides playback and record functions) and the various different functions have _different_ DMA addressing limitations, you may wish to probe each mask and only provide the functionality which the machine can handle. It |
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is important that the last call to dma_set_mask() be for the |
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most specific mask. Here is pseudo-code showing how this might be done: |
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#define PLAYBACK_ADDRESS_BITS DMA_BIT_MASK(32) |
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#define RECORD_ADDRESS_BITS DMA_BIT_MASK(24) |
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struct my_sound_card *card; |
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struct device *dev; |
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... |
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if (!dma_set_mask(dev, PLAYBACK_ADDRESS_BITS)) { |
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card->playback_enabled = 1; } else { card->playback_enabled = 0; |
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dev_warn(dev, "%s: Playback disabled due to DMA limitations ", |
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card->name); } |
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if (!dma_set_mask(dev, RECORD_ADDRESS_BITS)) { |
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card->record_enabled = 1; } else { card->record_enabled = 0; |
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dev_warn(dev, "%s: Record disabled due to DMA limitations ", |
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card->name); } A sound card was used as an example here because this genre of PCI devices seems to be littered with ISA chips given a PCI front end, and thus retaining the 16MB DMA addressing limitations of ISA. Types of DMA mappings There are two types of DMA mappings: - Consistent DMA mappings which are usually mapped at driver initialization, unmapped at the end and for which the hardware should guarantee that the device and the CPU can access the data in parallel and will see updates made by each other without any explicit software flushing. Think of "consistent" as "synchronous" or "coherent". The current default is to return consistent memory in the low 32 |
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bits of the bus space. However, for future compatibility you should set the consistent mask even if this default is fine for your |
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driver. Good examples of what to use consistent mappings for are: - Network card DMA ring descriptors. - SCSI adapter mailbox command data structures. - Device firmware microcode executed out of main memory. The invariant these examples all require is that any CPU store to memory is immediately visible to the device, and vice versa. Consistent mappings guarantee this. IMPORTANT: Consistent DMA memory does not preclude the usage of proper memory barriers. The CPU may reorder stores to consistent memory just as it may normal memory. Example: if it is important for the device to see the first word of a descriptor updated before the second, you must do something like: desc->word0 = address; wmb(); desc->word1 = DESC_VALID; in order to get correct behavior on all platforms. |
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Also, on some platforms your driver may need to flush CPU write buffers in much the same way as it needs to flush write buffers found in PCI bridges (such as by reading a register's value after writing it). |
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- Streaming DMA mappings which are usually mapped for one DMA transfer, unmapped right after it (unless you use dma_sync_* below) and for which hardware can optimize for sequential accesses. |
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This of "streaming" as "asynchronous" or "outside the coherency domain". Good examples of what to use streaming mappings for are: - Networking buffers transmitted/received by a device. - Filesystem buffers written/read by a SCSI device. The interfaces for using this type of mapping were designed in such a way that an implementation can make whatever performance optimizations the hardware allows. To this end, when using such mappings you must be explicit about what you want to happen. |
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Neither type of DMA mapping has alignment restrictions that come from the underlying bus, although some devices may have such restrictions. |
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Also, systems with caches that aren't DMA-coherent will work better when the underlying buffers don't share cache lines with other data. |
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Using Consistent DMA mappings. To allocate and map large (PAGE_SIZE or so) consistent DMA regions, you should do: dma_addr_t dma_handle; |
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cpu_addr = dma_alloc_coherent(dev, size, &dma_handle, gfp); |
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|
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where device is a struct device *. This may be called in interrupt context with the GFP_ATOMIC flag. |
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Size is the length of the region you want to allocate, in bytes. This routine will allocate RAM for that region, so it acts similarly to |
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__get_free_pages() (but takes size instead of a page order). If your |
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driver needs regions sized smaller than a page, you may prefer using |
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the dma_pool interface, described below. The consistent DMA mapping interfaces, for non-NULL dev, will by default return a DMA address which is 32-bit addressable. Even if the device indicates (via DMA mask) that it may address the upper 32-bits, consistent allocation will only return > 32-bit addresses for DMA if the consistent DMA mask has been explicitly changed via dma_set_coherent_mask(). This is true of the dma_pool interface as well. |
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dma_alloc_coherent() returns two values: the virtual address which you |
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can use to access it from the CPU and dma_handle which you pass to the card. |
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The CPU virtual address and the DMA bus address are both |
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guaranteed to be aligned to the smallest PAGE_SIZE order which is greater than or equal to the requested size. This invariant exists (for example) to guarantee that if you allocate a chunk which is smaller than or equal to 64 kilobytes, the extent of the buffer you receive will not cross a 64K boundary. To unmap and free such a DMA region, you call: |
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dma_free_coherent(dev, size, cpu_addr, dma_handle); |
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|
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where dev, size are the same as in the above call and cpu_addr and |
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dma_handle are the values dma_alloc_coherent() returned to you. |
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This function may not be called in interrupt context. If your driver needs lots of smaller memory regions, you can write |
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custom code to subdivide pages returned by dma_alloc_coherent(), |
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or you can use the dma_pool API to do that. A dma_pool is like |
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a kmem_cache, but it uses dma_alloc_coherent(), not __get_free_pages(). |
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Also, it understands common hardware constraints for alignment, like queue heads needing to be aligned on N byte boundaries. |
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Create a dma_pool like this: |
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struct dma_pool *pool; |
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pool = dma_pool_create(name, dev, size, align, boundary); |
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The "name" is for diagnostics (like a kmem_cache name); dev and size |
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are as above. The device's hardware alignment requirement for this type of data is "align" (which is expressed in bytes, and must be a power of two). If your device has no boundary crossing restrictions, |
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pass 0 for boundary; passing 4096 says memory allocated from this pool |
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must not cross 4KByte boundaries (but at that time it may be better to |
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use dma_alloc_coherent() directly instead). |
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Allocate memory from a DMA pool like this: |
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cpu_addr = dma_pool_alloc(pool, flags, &dma_handle); |
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flags are GFP_KERNEL if blocking is permitted (not in_interrupt nor holding SMP locks), GFP_ATOMIC otherwise. Like dma_alloc_coherent(), |
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this returns two values, cpu_addr and dma_handle. |
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Free memory that was allocated from a dma_pool like this: |
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dma_pool_free(pool, cpu_addr, dma_handle); |
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|
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where pool is what you passed to dma_pool_alloc(), and cpu_addr and dma_handle are the values dma_pool_alloc() returned. This function |
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may be called in interrupt context. |
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Destroy a dma_pool by calling: |
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dma_pool_destroy(pool); |
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Make sure you've called dma_pool_free() for all memory allocated |
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from a pool before you destroy the pool. This function may not be called in interrupt context. DMA Direction The interfaces described in subsequent portions of this document take a DMA direction argument, which is an integer and takes on one of the following values: |
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DMA_BIDIRECTIONAL DMA_TO_DEVICE DMA_FROM_DEVICE DMA_NONE |
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You should provide the exact DMA direction if you know it. |
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DMA_TO_DEVICE means "from main memory to the device" DMA_FROM_DEVICE means "from the device to main memory" |
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It is the direction in which the data moves during the DMA transfer. You are _strongly_ encouraged to specify this as precisely as you possibly can. If you absolutely cannot know the direction of the DMA transfer, |
216bf58f4 Documentation: co... |
470 |
specify DMA_BIDIRECTIONAL. It means that the DMA can go in |
1da177e4c Linux-2.6.12-rc2 |
471 472 473 |
either direction. The platform guarantees that you may legally specify this, and that it will work, but this may be at the cost of performance for example. |
216bf58f4 Documentation: co... |
474 |
The value DMA_NONE is to be used for debugging. One can |
1da177e4c Linux-2.6.12-rc2 |
475 476 477 478 479 480 481 482 483 |
hold this in a data structure before you come to know the precise direction, and this will help catch cases where your direction tracking logic has failed to set things up properly. Another advantage of specifying this value precisely (outside of potential platform-specific optimizations of such) is for debugging. Some platforms actually have a write permission boolean which DMA mappings can be marked with, much like page protections in the user program address space. Such platforms can and do report errors in the |
216bf58f4 Documentation: co... |
484 |
kernel logs when the DMA controller hardware detects violation of the |
1da177e4c Linux-2.6.12-rc2 |
485 486 487 488 |
permission setting. Only streaming mappings specify a direction, consistent mappings implicitly have a direction attribute setting of |
216bf58f4 Documentation: co... |
489 |
DMA_BIDIRECTIONAL. |
1da177e4c Linux-2.6.12-rc2 |
490 |
|
be7db055d [PATCH] remove ol... |
491 492 493 |
The SCSI subsystem tells you the direction to use in the 'sc_data_direction' member of the SCSI command your driver is working on. |
1da177e4c Linux-2.6.12-rc2 |
494 495 |
For Networking drivers, it's a rather simple affair. For transmit |
216bf58f4 Documentation: co... |
496 |
packets, map/unmap them with the DMA_TO_DEVICE direction |
1da177e4c Linux-2.6.12-rc2 |
497 |
specifier. For receive packets, just the opposite, map/unmap them |
216bf58f4 Documentation: co... |
498 |
with the DMA_FROM_DEVICE direction specifier. |
1da177e4c Linux-2.6.12-rc2 |
499 500 501 502 503 504 505 506 507 |
Using Streaming DMA mappings The streaming DMA mapping routines can be called from interrupt context. There are two versions of each map/unmap, one which will map/unmap a single memory region, and one which will map/unmap a scatterlist. To map a single region, you do: |
216bf58f4 Documentation: co... |
508 |
struct device *dev = &my_dev->dev; |
1da177e4c Linux-2.6.12-rc2 |
509 510 511 |
dma_addr_t dma_handle; void *addr = buffer->ptr; size_t size = buffer->len; |
216bf58f4 Documentation: co... |
512 |
dma_handle = dma_map_single(dev, addr, size, direction); |
8d7f62e6a Documentation DMA... |
513 514 515 516 517 518 519 520 |
if (dma_mapping_error(dma_handle)) { /* * reduce current DMA mapping usage, * delay and try again later or * reset driver. */ goto map_error_handling; } |
1da177e4c Linux-2.6.12-rc2 |
521 522 |
and to unmap it: |
216bf58f4 Documentation: co... |
523 |
dma_unmap_single(dev, dma_handle, size, direction); |
1da177e4c Linux-2.6.12-rc2 |
524 |
|
8d7f62e6a Documentation DMA... |
525 |
You should call dma_mapping_error() as dma_map_single() could fail and return |
77f2ea2f8 DMA-API: Clarify ... |
526 |
error. Not all DMA implementations support the dma_mapping_error() interface. |
8d7f62e6a Documentation DMA... |
527 528 |
However, it is a good practice to call dma_mapping_error() interface, which will invoke the generic mapping error check interface. Doing so will ensure |
77f2ea2f8 DMA-API: Clarify ... |
529 |
that the mapping code will work correctly on all DMA implementations without |
8d7f62e6a Documentation DMA... |
530 531 |
any dependency on the specifics of the underlying implementation. Using the returned address without checking for errors could result in failures ranging |
be62bc410 Documentation/DMA... |
532 |
from panics to silent data corruption. A couple of examples of incorrect ways |
77f2ea2f8 DMA-API: Clarify ... |
533 |
to check for errors that make assumptions about the underlying DMA |
be62bc410 Documentation/DMA... |
534 535 |
implementation are as follows and these are applicable to dma_map_page() as well. |
8d7f62e6a Documentation DMA... |
536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 |
Incorrect example 1: dma_addr_t dma_handle; dma_handle = dma_map_single(dev, addr, size, direction); if ((dma_handle & 0xffff != 0) || (dma_handle >= 0x1000000)) { goto map_error; } Incorrect example 2: dma_addr_t dma_handle; dma_handle = dma_map_single(dev, addr, size, direction); if (dma_handle == DMA_ERROR_CODE) { goto map_error; } |
77f2ea2f8 DMA-API: Clarify ... |
552 |
You should call dma_unmap_single() when the DMA activity is finished, e.g., |
1da177e4c Linux-2.6.12-rc2 |
553 |
from the interrupt which told you that the DMA transfer is done. |
f311a724a DMA-API: Capitali... |
554 |
Using CPU pointers like this for single mappings has a disadvantage: |
1da177e4c Linux-2.6.12-rc2 |
555 |
you cannot reference HIGHMEM memory in this way. Thus, there is a |
77f2ea2f8 DMA-API: Clarify ... |
556 |
map/unmap interface pair akin to dma_{map,unmap}_single(). These |
f311a724a DMA-API: Capitali... |
557 |
interfaces deal with page/offset pairs instead of CPU pointers. |
1da177e4c Linux-2.6.12-rc2 |
558 |
Specifically: |
216bf58f4 Documentation: co... |
559 |
struct device *dev = &my_dev->dev; |
1da177e4c Linux-2.6.12-rc2 |
560 561 562 563 |
dma_addr_t dma_handle; struct page *page = buffer->page; unsigned long offset = buffer->offset; size_t size = buffer->len; |
216bf58f4 Documentation: co... |
564 |
dma_handle = dma_map_page(dev, page, offset, size, direction); |
8d7f62e6a Documentation DMA... |
565 566 567 568 569 570 571 572 |
if (dma_mapping_error(dma_handle)) { /* * reduce current DMA mapping usage, * delay and try again later or * reset driver. */ goto map_error_handling; } |
1da177e4c Linux-2.6.12-rc2 |
573 574 |
... |
216bf58f4 Documentation: co... |
575 |
dma_unmap_page(dev, dma_handle, size, direction); |
1da177e4c Linux-2.6.12-rc2 |
576 577 |
Here, "offset" means byte offset within the given page. |
8d7f62e6a Documentation DMA... |
578 579 |
You should call dma_mapping_error() as dma_map_page() could fail and return error as outlined under the dma_map_single() discussion. |
77f2ea2f8 DMA-API: Clarify ... |
580 |
You should call dma_unmap_page() when the DMA activity is finished, e.g., |
8d7f62e6a Documentation DMA... |
581 |
from the interrupt which told you that the DMA transfer is done. |
1da177e4c Linux-2.6.12-rc2 |
582 |
With scatterlists, you map a region gathered from several regions by: |
216bf58f4 Documentation: co... |
583 |
int i, count = dma_map_sg(dev, sglist, nents, direction); |
1da177e4c Linux-2.6.12-rc2 |
584 |
struct scatterlist *sg; |
4c2f6d4c2 use sg helper fun... |
585 |
for_each_sg(sglist, sg, count, i) { |
1da177e4c Linux-2.6.12-rc2 |
586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604 |
hw_address[i] = sg_dma_address(sg); hw_len[i] = sg_dma_len(sg); } where nents is the number of entries in the sglist. The implementation is free to merge several consecutive sglist entries into one (e.g. if DMA mapping is done with PAGE_SIZE granularity, any consecutive sglist entries can be merged into one provided the first one ends and the second one starts on a page boundary - in fact this is a huge advantage for cards which either cannot do scatter-gather or have very limited number of scatter-gather entries) and returns the actual number of sg entries it mapped them to. On failure 0 is returned. Then you should loop count times (note: this can be less than nents times) and use sg_dma_address() and sg_dma_len() macros where you previously accessed sg->address and sg->length as shown above. To unmap a scatterlist, just call: |
216bf58f4 Documentation: co... |
605 |
dma_unmap_sg(dev, sglist, nents, direction); |
1da177e4c Linux-2.6.12-rc2 |
606 607 |
Again, make sure DMA activity has already finished. |
216bf58f4 Documentation: co... |
608 609 |
PLEASE NOTE: The 'nents' argument to the dma_unmap_sg call must be the _same_ one you passed into the dma_map_sg call, |
1da177e4c Linux-2.6.12-rc2 |
610 |
it should _NOT_ be the 'count' value _returned_ from the |
216bf58f4 Documentation: co... |
611 |
dma_map_sg call. |
1da177e4c Linux-2.6.12-rc2 |
612 |
|
77f2ea2f8 DMA-API: Clarify ... |
613 614 615 |
Every dma_map_{single,sg}() call should have its dma_unmap_{single,sg}() counterpart, because the bus address space is a shared resource and you could render the machine unusable by consuming all bus addresses. |
1da177e4c Linux-2.6.12-rc2 |
616 617 618 |
If you need to use the same streaming DMA region multiple times and touch the data in between the DMA transfers, the buffer needs to be synced |
f311a724a DMA-API: Capitali... |
619 |
properly in order for the CPU and device to see the most up-to-date and |
1da177e4c Linux-2.6.12-rc2 |
620 |
correct copy of the DMA buffer. |
77f2ea2f8 DMA-API: Clarify ... |
621 |
So, firstly, just map it with dma_map_{single,sg}(), and after each DMA |
1da177e4c Linux-2.6.12-rc2 |
622 |
transfer call either: |
216bf58f4 Documentation: co... |
623 |
dma_sync_single_for_cpu(dev, dma_handle, size, direction); |
1da177e4c Linux-2.6.12-rc2 |
624 625 |
or: |
216bf58f4 Documentation: co... |
626 |
dma_sync_sg_for_cpu(dev, sglist, nents, direction); |
1da177e4c Linux-2.6.12-rc2 |
627 628 629 630 |
as appropriate. Then, if you wish to let the device get at the DMA area again, |
f311a724a DMA-API: Capitali... |
631 |
finish accessing the data with the CPU, and then before actually |
1da177e4c Linux-2.6.12-rc2 |
632 |
giving the buffer to the hardware call either: |
216bf58f4 Documentation: co... |
633 |
dma_sync_single_for_device(dev, dma_handle, size, direction); |
1da177e4c Linux-2.6.12-rc2 |
634 635 |
or: |
216bf58f4 Documentation: co... |
636 |
dma_sync_sg_for_device(dev, sglist, nents, direction); |
1da177e4c Linux-2.6.12-rc2 |
637 638 639 640 |
as appropriate. After the last DMA transfer call one of the DMA unmap routines |
77f2ea2f8 DMA-API: Clarify ... |
641 642 643 |
dma_unmap_{single,sg}(). If you don't touch the data from the first dma_map_*() call till dma_unmap_*(), then you don't have to call the dma_sync_*() routines at all. |
1da177e4c Linux-2.6.12-rc2 |
644 645 |
Here is pseudo code which shows a situation in which you would need |
216bf58f4 Documentation: co... |
646 |
to use the dma_sync_*() interfaces. |
1da177e4c Linux-2.6.12-rc2 |
647 648 649 650 |
my_card_setup_receive_buffer(struct my_card *cp, char *buffer, int len) { dma_addr_t mapping; |
216bf58f4 Documentation: co... |
651 |
mapping = dma_map_single(cp->dev, buffer, len, DMA_FROM_DEVICE); |
8d7f62e6a Documentation DMA... |
652 653 654 655 656 657 658 659 |
if (dma_mapping_error(dma_handle)) { /* * reduce current DMA mapping usage, * delay and try again later or * reset driver. */ goto map_error_handling; } |
1da177e4c Linux-2.6.12-rc2 |
660 661 662 663 664 665 666 667 668 669 670 671 672 673 674 675 676 677 678 679 680 681 682 |
cp->rx_buf = buffer; cp->rx_len = len; cp->rx_dma = mapping; give_rx_buf_to_card(cp); } ... my_card_interrupt_handler(int irq, void *devid, struct pt_regs *regs) { struct my_card *cp = devid; ... if (read_card_status(cp) == RX_BUF_TRANSFERRED) { struct my_card_header *hp; /* Examine the header to see if we wish * to accept the data. But synchronize * the DMA transfer with the CPU first * so that we see updated contents. */ |
216bf58f4 Documentation: co... |
683 684 685 |
dma_sync_single_for_cpu(&cp->dev, cp->rx_dma, cp->rx_len, DMA_FROM_DEVICE); |
1da177e4c Linux-2.6.12-rc2 |
686 687 688 689 |
/* Now it is safe to examine the buffer. */ hp = (struct my_card_header *) cp->rx_buf; if (header_is_ok(hp)) { |
216bf58f4 Documentation: co... |
690 691 |
dma_unmap_single(&cp->dev, cp->rx_dma, cp->rx_len, DMA_FROM_DEVICE); |
1da177e4c Linux-2.6.12-rc2 |
692 693 694 |
pass_to_upper_layers(cp->rx_buf); make_and_setup_new_rx_buf(cp); } else { |
3f0fb4e85 Documentation/DMA... |
695 696 697 698 699 700 |
/* CPU should not write to * DMA_FROM_DEVICE-mapped area, * so dma_sync_single_for_device() is * not needed here. It would be required * for DMA_BIDIRECTIONAL mapping if * the memory was modified. |
1da177e4c Linux-2.6.12-rc2 |
701 |
*/ |
1da177e4c Linux-2.6.12-rc2 |
702 703 704 705 |
give_rx_buf_to_card(cp); } } } |
77f2ea2f8 DMA-API: Clarify ... |
706 707 708 |
Drivers converted fully to this interface should not use virt_to_bus() any longer, nor should they use bus_to_virt(). Some drivers have to be changed a little bit, because there is no longer an equivalent to bus_to_virt() in the |
1da177e4c Linux-2.6.12-rc2 |
709 |
dynamic DMA mapping scheme - you have to always store the DMA addresses |
77f2ea2f8 DMA-API: Clarify ... |
710 711 |
returned by the dma_alloc_coherent(), dma_pool_alloc(), and dma_map_single() calls (dma_map_sg() stores them in the scatterlist itself if the platform |
1da177e4c Linux-2.6.12-rc2 |
712 713 |
supports dynamic DMA mapping in hardware) in your driver structures and/or in the card registers. |
216bf58f4 Documentation: co... |
714 715 |
All drivers should be using these interfaces with no exceptions. It is planned to completely remove virt_to_bus() and bus_to_virt() as |
1da177e4c Linux-2.6.12-rc2 |
716 717 |
they are entirely deprecated. Some ports already do not provide these as it is impossible to correctly support them. |
4ae9ca825 Documentation: mo... |
718 719 720 721 |
Handling Errors DMA address space is limited on some architectures and an allocation failure can be determined by: |
77f2ea2f8 DMA-API: Clarify ... |
722 |
- checking if dma_alloc_coherent() returns NULL or dma_map_sg returns 0 |
4ae9ca825 Documentation: mo... |
723 |
|
77f2ea2f8 DMA-API: Clarify ... |
724 |
- checking the dma_addr_t returned from dma_map_single() and dma_map_page() |
4ae9ca825 Documentation: mo... |
725 726 727 728 729 730 731 732 733 734 735 |
by using dma_mapping_error(): dma_addr_t dma_handle; dma_handle = dma_map_single(dev, addr, size, direction); if (dma_mapping_error(dev, dma_handle)) { /* * reduce current DMA mapping usage, * delay and try again later or * reset driver. */ |
8d7f62e6a Documentation DMA... |
736 737 738 739 740 741 742 743 744 745 746 747 748 749 750 751 752 753 754 755 756 757 758 759 760 761 762 763 764 765 766 767 768 769 770 |
goto map_error_handling; } - unmap pages that are already mapped, when mapping error occurs in the middle of a multiple page mapping attempt. These example are applicable to dma_map_page() as well. Example 1: dma_addr_t dma_handle1; dma_addr_t dma_handle2; dma_handle1 = dma_map_single(dev, addr, size, direction); if (dma_mapping_error(dev, dma_handle1)) { /* * reduce current DMA mapping usage, * delay and try again later or * reset driver. */ goto map_error_handling1; } dma_handle2 = dma_map_single(dev, addr, size, direction); if (dma_mapping_error(dev, dma_handle2)) { /* * reduce current DMA mapping usage, * delay and try again later or * reset driver. */ goto map_error_handling2; } ... map_error_handling2: dma_unmap_single(dma_handle1); map_error_handling1: |
11cd3db01 Documentation/DMA... |
771 |
Example 2: (if buffers are allocated in a loop, unmap all mapped buffers when |
8d7f62e6a Documentation DMA... |
772 773 774 775 776 777 778 779 780 781 782 783 784 785 786 787 788 789 790 791 792 793 794 795 796 797 798 799 800 801 802 803 |
mapping error is detected in the middle) dma_addr_t dma_addr; dma_addr_t array[DMA_BUFFERS]; int save_index = 0; for (i = 0; i < DMA_BUFFERS; i++) { ... dma_addr = dma_map_single(dev, addr, size, direction); if (dma_mapping_error(dev, dma_addr)) { /* * reduce current DMA mapping usage, * delay and try again later or * reset driver. */ goto map_error_handling; } array[i].dma_addr = dma_addr; save_index++; } ... map_error_handling: for (i = 0; i < save_index; i++) { ... dma_unmap_single(array[i].dma_addr); |
4ae9ca825 Documentation: mo... |
804 |
} |
77f2ea2f8 DMA-API: Clarify ... |
805 |
Networking drivers must call dev_kfree_skb() to free the socket buffer |
4ae9ca825 Documentation: mo... |
806 807 808 809 810 811 812 |
and return NETDEV_TX_OK if the DMA mapping fails on the transmit hook (ndo_start_xmit). This means that the socket buffer is just dropped in the failure case. SCSI drivers must return SCSI_MLQUEUE_HOST_BUSY if the DMA mapping fails in the queuecommand hook. This means that the SCSI subsystem passes the command to the driver again later. |
1da177e4c Linux-2.6.12-rc2 |
813 |
Optimizing Unmap State Space Consumption |
216bf58f4 Documentation: co... |
814 |
On many platforms, dma_unmap_{single,page}() is simply a nop. |
1da177e4c Linux-2.6.12-rc2 |
815 816 817 818 819 820 821 |
Therefore, keeping track of the mapping address and length is a waste of space. Instead of filling your drivers up with ifdefs and the like to "work around" this (which would defeat the whole purpose of a portable API) the following facilities are provided. Actually, instead of describing the macros one by one, we'll transform some example code. |
216bf58f4 Documentation: co... |
822 |
1) Use DEFINE_DMA_UNMAP_{ADDR,LEN} in state saving structures. |
1da177e4c Linux-2.6.12-rc2 |
823 824 825 826 827 828 829 830 831 832 833 834 |
Example, before: struct ring_state { struct sk_buff *skb; dma_addr_t mapping; __u32 len; }; after: struct ring_state { struct sk_buff *skb; |
216bf58f4 Documentation: co... |
835 836 |
DEFINE_DMA_UNMAP_ADDR(mapping); DEFINE_DMA_UNMAP_LEN(len); |
1da177e4c Linux-2.6.12-rc2 |
837 |
}; |
77f2ea2f8 DMA-API: Clarify ... |
838 |
2) Use dma_unmap_{addr,len}_set() to set these values. |
1da177e4c Linux-2.6.12-rc2 |
839 840 841 842 843 844 |
Example, before: ringp->mapping = FOO; ringp->len = BAR; after: |
216bf58f4 Documentation: co... |
845 846 |
dma_unmap_addr_set(ringp, mapping, FOO); dma_unmap_len_set(ringp, len, BAR); |
1da177e4c Linux-2.6.12-rc2 |
847 |
|
77f2ea2f8 DMA-API: Clarify ... |
848 |
3) Use dma_unmap_{addr,len}() to access these values. |
1da177e4c Linux-2.6.12-rc2 |
849 |
Example, before: |
216bf58f4 Documentation: co... |
850 851 |
dma_unmap_single(dev, ringp->mapping, ringp->len, DMA_FROM_DEVICE); |
1da177e4c Linux-2.6.12-rc2 |
852 853 |
after: |
216bf58f4 Documentation: co... |
854 855 856 857 |
dma_unmap_single(dev, dma_unmap_addr(ringp, mapping), dma_unmap_len(ringp, len), DMA_FROM_DEVICE); |
1da177e4c Linux-2.6.12-rc2 |
858 859 860 861 862 863 864 865 866 867 868 869 |
It really should be self-explanatory. We treat the ADDR and LEN separately, because it is possible for an implementation to only need the address in order to perform the unmap operation. Platform Issues If you are just writing drivers for Linux and do not maintain an architecture port for the kernel, you can safely skip down to "Closing". 1) Struct scatterlist requirements. |
b02de8716 Documentation: up... |
870 871 872 873 |
Don't invent the architecture specific struct scatterlist; just use <asm-generic/scatterlist.h>. You need to enable CONFIG_NEED_SG_DMA_LENGTH if the architecture supports IOMMUs (including software IOMMU). |
1da177e4c Linux-2.6.12-rc2 |
874 |
|
ce00f7feb Documentation: DM... |
875 |
2) ARCH_DMA_MINALIGN |
2fd74e25d Documentation/DMA... |
876 877 878 879 880 |
Architectures must ensure that kmalloc'ed buffer is DMA-safe. Drivers and subsystems depend on it. If an architecture isn't fully DMA-coherent (i.e. hardware doesn't ensure that data in the CPU cache is identical to data in main memory), |
ce00f7feb Documentation: DM... |
881 |
ARCH_DMA_MINALIGN must be set so that the memory allocator |
2fd74e25d Documentation/DMA... |
882 883 |
makes sure that kmalloc'ed buffer doesn't share a cache line with the others. See arch/arm/include/asm/cache.h as an example. |
ce00f7feb Documentation: DM... |
884 |
Note that ARCH_DMA_MINALIGN is about DMA memory alignment |
2fd74e25d Documentation/DMA... |
885 886 887 |
constraints. You don't need to worry about the architecture data alignment constraints (e.g. the alignment constraints about 64-bit objects). |
1da177e4c Linux-2.6.12-rc2 |
888 |
|
c31e74c4c Documentation: DM... |
889 890 891 892 893 894 895 896 897 |
3) Supporting multiple types of IOMMUs If your architecture needs to support multiple types of IOMMUs, you can use include/linux/asm-generic/dma-mapping-common.h. It's a library to support the DMA API with multiple types of IOMMUs. Lots of architectures (x86, powerpc, sh, alpha, ia64, microblaze and sparc) use it. Choose one to see how it can be used. If you need to support multiple types of IOMMUs in a single system, the example of x86 or powerpc helps. |
1da177e4c Linux-2.6.12-rc2 |
898 |
Closing |
a33f32244 Documentation/: i... |
899 |
This document, and the API itself, would not be in its current |
1da177e4c Linux-2.6.12-rc2 |
900 901 902 903 904 905 906 907 908 909 910 |
form without the feedback and suggestions from numerous individuals. We would like to specifically mention, in no particular order, the following people: Russell King <rmk@arm.linux.org.uk> Leo Dagum <dagum@barrel.engr.sgi.com> Ralf Baechle <ralf@oss.sgi.com> Grant Grundler <grundler@cup.hp.com> Jay Estabrook <Jay.Estabrook@compaq.com> Thomas Sailer <sailer@ife.ee.ethz.ch> Andrea Arcangeli <andrea@suse.de> |
26bbb29a2 Update Jens Axboe... |
911 |
Jens Axboe <jens.axboe@oracle.com> |
1da177e4c Linux-2.6.12-rc2 |
912 |
David Mosberger-Tang <davidm@hpl.hp.com> |