linux-wandboard

ENGR00219601-02: mmc: sdhci: revise pre_req & post_req to improve performance

Test Env:
1. MX6DL SabreSD board.
2. On board eMMC (Sandisk: SDIN5C2-8G) running at 8-bit DDR @ 52MHz.
3. Test commands:
  3.1 Writing command:
  # dd if=/dev/zero of=/dev/mmcblk0 bs=1M count=100 conv=fsync
  3.2 Reading command:
  # echo 1 > /proc/sys/vm/drop_caches
  # echo 1 > /proc/sys/vm/drop_caches
  # sleep 1
  # dd if=/dev/mmcblk0 of=/dev/null bs=1M count=100

Performance result with this patch:
-------------------------------------------------------
| CPU freq | SDMA (512KB) | SDMA (64KB) |    ADMA     |
|----------+--------------+-------------+-------------|
|   1Ghz   |  ~11MB/s (w) | ~5MB/s (w)  | ~11MB/s (w) |
|          |  ~25MB/s (r) | ~25MB/s (r) | ~23MB/s (r) |
|----------+--------------+-------------+-------------|
|  200Mhz  |  ~8MB/s (w)  | ~5MB/s (w)  | ~9MB/s (w)  |
|          |  ~16MB/s (r) | ~20MB/s (r) | ~13MB/s (r) |
-------------------------------------------------------

Performance result without this patch:
-------------------------------------------------------
| CPU freq | SDMA (512KB) | SDMA (64KB) |    ADMA     |
|----------+--------------+-------------+-------------|
|   1Ghz   |  ~10MB/s (w) | ~5MB/s (w)  | ~10MB/s (w) |
|          |  ~22MB/s (r) | ~23MB/s (r) | ~22MB/s (r) |
|----------+--------------+-------------+-------------|
|  200Mhz  |  ~8MB/s (w)  | ~4MB/s (w)  | ~8MB/s (w)  |
|          |  ~13MB/s (r) | ~16MB/s (r) | ~11MB/s (r) |
-------------------------------------------------------

Signed-off-by: Ryan QIAN <b32804@freescale.com>