dma-buf: Expose buffer size to userspace (v2)

Each dma-buf has an associated size and it's reasonable for userspace to want to know what it is. Since userspace already has an fd, expose the size using the size = lseek(fd, SEEK_END, 0); lseek(fd, SEEK_CUR, 0); idiom. v2: Added Daniel's sugeested documentation, with minor fixups Signed-off-by: Christopher James Halse Rogers <christopher.halse.rogers@canonical.com> Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch> Tested-by: Daniel Vetter <daniel.vetter@ffwll.ch> Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>

dma-buf: Expose buffer size to userspace (v2)
Each dma-buf has an associated size and it's reasonable for userspace to want to know what it is. Since userspace already has an fd, expose the size using the size = lseek(fd, SEEK_END, 0); lseek(fd, SEEK_CUR, 0); idiom. v2: Added Daniel's sugeested documentation, with minor fixups Signed-off-by: Christopher James Halse Rogers <christopher.halse.rogers@canonical.com> Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch> Tested-by: Daniel Vetter <daniel.vetter@ffwll.ch> Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>
Christopher James Halse Rogers · Sumit Semwal
1 parent 9022e24e89
Showing 2 changed files with 41 additions and 0 deletions Side-by-side Diff
Documentation/dma-buf-sharing.txt
drivers/base/dma-buf.c
@@ -407,6 +407,18 @@
    interesting ways depending upong the exporter (if userspace starts depending
    upon this implicit synchronization).
  
+Other Interfaces Exposed to Userspace on the dma-buf FD
+------------------------------------------------------
+
+- Since kernel 3.12 the dma-buf FD supports the llseek system call, but only
+  with offset=0 and whence=SEEK_END|SEEK_SET. SEEK_SET is supported to allow
+  the usual size discover pattern size = SEEK_END(0); SEEK_SET(0). Every other
+  llseek operation will report -EINVAL.
+
+  If llseek on dma-buf FDs isn't support the kernel will report -ESPIPE for all
+  cases. Userspace can use this to detect support for discovering the dma-buf
+  size using llseek.
+
 Miscellaneous notes
 -------------------
  
@@ -77,9 +77,36 @@
 	return dmabuf->ops->mmap(dmabuf, vma);
 }
  
+static loff_t dma_buf_llseek(struct file *file, loff_t offset, int whence)
+{
+	struct dma_buf *dmabuf;
+	loff_t base;
+
+	if (!is_dma_buf_file(file))
+		return -EBADF;
+
+	dmabuf = file->private_data;
+
+	/* only support discovering the end of the buffer,
+	   but also allow SEEK_SET to maintain the idiomatic
+	   SEEK_END(0), SEEK_CUR(0) pattern */
+	if (whence == SEEK_END)
+		base = dmabuf->size;
+	else if (whence == SEEK_SET)
+		base = 0;
+	else
+		return -EINVAL;
+
+	if (offset != 0)
+		return -EINVAL;
+
+	return base + offset;
+}
+
 static const struct file_operations dma_buf_fops = {
 	.release	= dma_buf_release,
 	.mmap		= dma_buf_mmap_internal,
+	.llseek		= dma_buf_llseek,
 };
  
 /*
@@ -137,6 +164,8 @@
 		kfree(dmabuf);
 		return ERR_CAST(file);
 	}
+
+	file->f_mode |= FMODE_LSEEK;
 	dmabuf->file = file;
  
 	mutex_init(&dmabuf->lock);
...	...	@@ -407,6 +407,18 @@
407	407	interesting ways depending upong the exporter (if userspace starts depending
408	408	upon this implicit synchronization).
409	409
	410	+Other Interfaces Exposed to Userspace on the dma-buf FD
	411	+------------------------------------------------------
	412	+
	413	+- Since kernel 3.12 the dma-buf FD supports the llseek system call, but only
	414	+ with offset=0 and whence=SEEK_END\|SEEK_SET. SEEK_SET is supported to allow
	415	+ the usual size discover pattern size = SEEK_END(0); SEEK_SET(0). Every other
	416	+ llseek operation will report -EINVAL.
	417	+
	418	+ If llseek on dma-buf FDs isn't support the kernel will report -ESPIPE for all
	419	+ cases. Userspace can use this to detect support for discovering the dma-buf
	420	+ size using llseek.
	421	+
410	422	Miscellaneous notes
411	423	-------------------
412	424
...	...	@@ -77,9 +77,36 @@
77	77	return dmabuf->ops->mmap(dmabuf, vma);
78	78	}
79	79
	80	+static loff_t dma_buf_llseek(struct file *file, loff_t offset, int whence)
	81	+{
	82	+ struct dma_buf *dmabuf;
	83	+ loff_t base;
	84	+
	85	+ if (!is_dma_buf_file(file))
	86	+ return -EBADF;
	87	+
	88	+ dmabuf = file->private_data;
	89	+
	90	+ /* only support discovering the end of the buffer,
	91	+ but also allow SEEK_SET to maintain the idiomatic
	92	+ SEEK_END(0), SEEK_CUR(0) pattern */
	93	+ if (whence == SEEK_END)
	94	+ base = dmabuf->size;
	95	+ else if (whence == SEEK_SET)
	96	+ base = 0;
	97	+ else
	98	+ return -EINVAL;
	99	+
	100	+ if (offset != 0)
	101	+ return -EINVAL;
	102	+
	103	+ return base + offset;
	104	+}
	105	+
80	106	static const struct file_operations dma_buf_fops = {
81	107	.release = dma_buf_release,
82	108	.mmap = dma_buf_mmap_internal,
	109	+ .llseek = dma_buf_llseek,
83	110	};
84	111
85	112	/*
...	...	@@ -137,6 +164,8 @@
137	164	kfree(dmabuf);
138	165	return ERR_CAST(file);
139	166	}
	167	+
	168	+ file->f_mode \|= FMODE_LSEEK;
140	169	dmabuf->file = file;
141	170
142	171	mutex_init(&dmabuf->lock);