Previously, during the MSDC calibration process, a full clock cycle
actually not be covered, which in some cases didn't yield the best
results and could cause CRC errors. This problem is particularly
evident when MSDC is used as an SDIO host. In fact, MSDC support
tuning up to a maximum of 64 steps, but by default, the step number
is 32. By increase the tuning step, we are more likely to cover more
parts of a clock cycle, and get better calibration result.

To illustrate, when tuning 32 steps, if the obtained window has a hole
near the middle, like this: 0xffc07ff (hex), then the selected delay
will be the 6 (counting from right to left).

(32 <- 1)
1111 1111 1100 0000 0000 0111 11(1)1 1111

However, if we tune 64 steps, the window obtained may look like this:
0xfffffffffffc07ff. The final selected delay will be 44, which is
safer as it is further away from the hole:

(64 <- 1)
1111 ... (1)111 1111 1111 1111 1111 1100 0000 0000 0111 1111 1111

In this case, delay 6 selected through 32 steps tuning is obviously
not optimal, and this delay is closer to the hole, using it would
easily cause CRC problems.

As per mesaurements taken on mediatek SoC platform, the tuning phase
will take:
	eMMC	- 32 steps: ~3ms
		- 64 steps: ~6ms
	SDIO	- 32 steps: ~4ms
		- 64 steos: ~7ms
Tuning more steps won't prolong boot times by any meaningful amount
of time, so for SD/SDIO the default tuning steps will be adjust to
64. But for eMMC, it is still preferred to use 32 steps tuning as
otherwise there would be performance lose when accessing the RPMB
partition(requiring retuning each time).

You can configure property "mediatek,tuning-step" in MSDC dts node
to adjust the step number.
Signed-off-by: Axe Yang <axe.yang@mediatek.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/20231207063535.29546-3-axe.yang@mediatek.comSigned-off-by: Ulf Hansson <ulf.hansson@linaro.org>