[PATCH v4 2/2] mmc: mediatek: extend number of tuning steps

[AngeloGioacchino Del Regno]

Il 07/12/23 07:35, Axe Yang ha scritto:
> 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 at mediatek.com>

Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno at collabora.com>