mmc: msm_sdcc: add support for AUTO_CMD19 control
SDR104 (supported by SD3.0 spec compliant SD cards) bus speed mode
requires DLL (Delay Locked Loop for sampling clock generation) HW
block to be tuned if clock rate is >100MHz. To help the sampling clock
tuning SD3.0 specification has added CMD19.
During card initialization, driver uses these tuning commands to search
for optimal sampling point and then programs the DLL HW with this optimal
sampling point.
In addition, our SDCC controller's DLL HW has a mechansim of data
tracking (CDR) which can be enabled during read transaction so the host
will not lose the sampling point as a result of voltage and, especially,
temperature variations. Tuning pattern commands (CMD19) can be sent
automatically by HW (by using AUTO_CMD19 bit) before any RX transaction
for helping the CDR to track after the correct sampling point.
Although sending CMD19 automatically before every read commands is
overhead and reduces read performance by ~1 MB/s. SD spec does
not mandate to send these commands before every read operations.
This change keeps AUTO_CMD19 disabled by default but provides sysfs
attribute to enable or disable AUTO_CMD19 as per need.
Change-Id: I9ef99a8dde9b9733ea99639cdab2b6714ac50d76
Signed-off-by: Subhash Jadavani <subhashj@codeaurora.org>
2 files changed