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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.
num_wr_reqs_to_start_packing This attribute is used to determine
the trigger for activating the write packing, in case the write
packing control feature is enabled.
When the MMC manages to reach a point where num_wr_reqs_to_start_packing
write requests could be packed, it enables the write packing feature.
This allows us to start the write packing only when it is beneficial
and has minimum affect on the read latency.
The number of potential packed requests that will trigger the packing
can be configured via sysfs by writing the required value to:
The default value of num_wr_reqs_to_start_packing was determined by
running parallel lmdd write and lmdd read operations and calculating
the max number of packed writes requests.
bkops_check_threshold This attribute is used to determine whether
the status bit that indicates the need for BKOPS should be checked.
The value should be given in percentages of the card size.
This value is used to calculate the minimum number of sectors that
needs to be changed in the device (written or discarded) in order to
require the status-bit of BKOPS to be checked.
The value can modified via sysfs by writing the required value to:
SD and MMC Device Attributes
All attributes are read-only.
cid Card Identification 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)
prv Product Revision (from CID Register) (SD and MMCv4 only)
serial Product Serial Number (from CID Register)
erase_size Erase group size
preferred_erase_size Preferred erase size
raw_rpmb_size_mult RPMB partition size
rel_sectors Reliable write sector count
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
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
"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.
Note on raw_rpmb_size_mult:
"raw_rpmb_size_mult" is a mutliple of 128kB block.
RPMB size in byte is calculated by using the following equation:
RPMB partition size = 128kB x raw_rpmb_size_mult
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
SD/MMC/SDIO Clock Scaling Attributes
Read and write accesses are provided to following attributes.
polling_interval Measured in milliseconds, this attribute
defines how often we need to check the card
usage and make decisions on frequency scaling.
up_threshold This attribute defines what should be the
average card usage between the polling
interval for the mmc core to make a decision
on whether it should increase the frequency.
For example when it is set to '35' it means
that between the checking intervals the card
needs to be on average more than 35% in use to
scale up the frequency. The value should be
between 0 - 100 so that it can be compared
against load percentage.
down_threshold Similar to up_threshold, but on lowering the
frequency. For example, when it is set to '2'
it means that between the checking intervals
the card needs to be on average less than 2%
in use to scale down the clocks to minimum
frequency. The value should be between 0 - 100
so that it can be compared against load
enable Enable clock scaling for hosts (and cards)
that support ultrahigh speed modes
(SDR104, DDR50, HS200).
echo <desired value> > /sys/class/mmc_host/mmcX/clk_scaling/polling_interval
echo <desired value> > /sys/class/mmc_host/mmcX/clk_scaling/up_threshold
echo <desired value> > /sys/class/mmc_host/mmcX/clk_scaling/down_threshold
echo <desired value> > /sys/class/mmc_host/mmcX/clk_scaling/enable