Documentation/block/mmc-max-speed.txt

eMMC Block layer simulation speed controls in /sys/block/mmcblk*/
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Turned on with CONFIG_MMC_SIMULATE_MAX_SPEED which enables MMC device speed
limiting. Used to test and simulate the behavior of the system when
confronted with a slow MMC.

Enables max_read_speed, max_write_speed and cache_size attributes and module
default parameters to control the write or read maximum KB/second speed
behaviors.

NB: There is room for improving the algorithm for aspects tied directly to
eMMC specific behavior. For instance, wear leveling and stalls from an
exhausted erase pool. We would expect that if there was a need to provide
similar speed simulation controls to other types of block devices, aspects of
their behavior are modelled separately (e.g. head seek times, heat assist,
shingling and rotational latency).

/sys/block/mmcblk0/max_read_speed:

Number of KB/second reads allowed to the block device. Used to test and
simulate the behavior of the system when confronted with a slow reading MMC.
Set to 0 or "off" to place no speed limit.

/sys/block/mmcblk0/max_write_speed:

Number of KB/second writes allowed to the block device. Used to test and
simulate the behavior of the system when confronted with a slow writing MMC.
Set to 0 or "off" to place no speed limit.

/sys/block/mmcblk0/cache_size:

Number of MB of high speed memory or high speed SLC cache expected on the
eMMC device being simulated. Used to help simulate the write-back behavior
more accurately. The assumption is the cache has no delay, but draws down
in the background to the MLC/TLC primary store at the max_write_speed rate.
Any write speed delays will show up when the cache is full, or when an I/O
request to flush is issued.