ANDROID: mmc: Add CONFIG_MMC_SIMULATE_MAX_SPEED

When CONFIG_MMC_SIMULATE_MAX_SPEED is enabled, Expose max_read_speed,
max_write_speed and cache_size default module parameters and sysfs
controls to simulate a slow eMMC device. Default values are 0 (off),
0 (off) and 4 MB respectively.

Signed-off-by: Mark Salyzyn <salyzyn@google.com>
Bug: 26976972
Change-Id: I342bfbd8b85f9b790e3f0e1e4e51a900ae07e05d

2 files changed, 44 insertions, 0 deletions

diff --git a/Documentation/block/00-INDEX b/Documentation/block/00-INDEX
index e840b47613f7..bc5148757edb 100644
--- a/Documentation/block/00-INDEX
+++ b/Documentation/block/00-INDEX
@@ -26,3 +26,9 @@ switching-sched.txt
 	- Switching I/O schedulers at runtime
 writeback_cache_control.txt
 	- Control of volatile write back caches
+mmc-max-speed.txt
+	- eMMC layer speed simulation, related to /sys/block/mmcblk*/
+          attributes:
+            max_read_speed
+            max_write_speed
+            cache_size
diff --git a/Documentation/block/mmc-max-speed.txt b/Documentation/block/mmc-max-speed.txt
new file mode 100644
index 000000000000..3f052b9fb999
--- /dev/null
+++ b/Documentation/block/mmc-max-speed.txt
@@ -0,0 +1,38 @@
+eMMC Block layer simulation speed controls in /sys/block/mmcblk*/
+===============================================
+
+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.