SD and MMC Block Device Attributes
  ==================================
  
  These attributes are defined for the block devices associated with the
  SD or MMC device.
  
  The following attributes are read/write.
  
  	force_ro		Enforce read-only access even if write protect switch is off.

  SD and MMC Device Attributes
  ============================
  
  All attributes are read-only.
  
  	cid			Card Identifaction Register
  	csd			Card Specific Data Register
  	scr			SD Card Configuration Register (SD only)
  	date			Manufacturing Date (from CID Register)
  	fwrev			Firmware/Product Revision (from CID Register) (SD and MMCv1 only)
  	hwrev			Hardware/Product Revision (from CID Register) (SD and MMCv1 only)
  	manfid			Manufacturer ID (from CID Register)
  	name			Product Name (from CID Register)
  	oemid			OEM/Application ID (from CID Register)
  	serial			Product Serial Number (from CID Register)
  	erase_size		Erase group size
  	preferred_erase_size	Preferred erase size
  
  Note on Erase Size and Preferred Erase Size:
  
  	"erase_size" is the  minimum size, in bytes, of an erase
  	operation.  For MMC, "erase_size" is the erase group size
  	reported by the card.  Note that "erase_size" does not apply
  	to trim or secure trim operations where the minimum size is
  	always one 512 byte sector.  For SD, "erase_size" is 512
  	if the card is block-addressed, 0 otherwise.
  
  	SD/MMC cards can erase an arbitrarily large area up to and
  	including the whole card.  When erasing a large area it may
  	be desirable to do it in smaller chunks for three reasons:
  		1. A single erase command will make all other I/O on
  		the card wait.  This is not a problem if the whole card
  		is being erased, but erasing one partition will make
  		I/O for another partition on the same card wait for the
  		duration of the erase - which could be a several
  		minutes.
  		2. To be able to inform the user of erase progress.
  		3. The erase timeout becomes too large to be very
  		useful.  Because the erase timeout contains a margin
  		which is multiplied by the size of the erase area,
  		the value can end up being several minutes for large
  		areas.
  
  	"erase_size" is not the most efficient unit to erase
  	(especially for SD where it is just one sector),
  	hence "preferred_erase_size" provides a good chunk
  	size for erasing large areas.
  
  	For MMC, "preferred_erase_size" is the high-capacity
  	erase size if a card specifies one, otherwise it is
  	based on the capacity of the card.
  
  	For SD, "preferred_erase_size" is the allocation unit
  	size specified by the card.
  
  	"preferred_erase_size" is in bytes.

  
  SD/MMC/SDIO Clock Gating Attribute
  ==================================
  
  Read and write access is provided to following attribute.
  This attribute appears only if CONFIG_MMC_CLKGATE is enabled.

  	clkgate_delay	Tune the clock gating delay with desired value in milliseconds.

  
  echo <desired delay> > /sys/class/mmc_host/mmcX/clkgate_delay