Re: [PATCH] mmc: core: fix host controller programming for fixed driver type

[Shawn Lin]

在 2026/04/18 星期六 5:02, Kamal Dasu 写道:
> When using the fixed-emmc-driver-type device tree property, the MMC core
> correctly selects the driver strength for the card but fails to program
> the host controller accordingly. This causes a mismatch where the card
> uses the specified driver type while the host controller defaults to
> Type B (since ios->drv_type remains zero).
>
> Split the driver type programming logic to handle both fixed and dynamic
> driver type selection paths. For fixed driver types, program the host
> controller with the selected drive_strength value. For dynamic selection,
> use the existing drv_type as before.
>
> This ensures both the eMMC device and host controller use matching driver
> strengths, preventing potential signal integrity issues.

I agree the host and card drive strengths should match. However, it
highlights a deeper issue: the drv_type(A/B/C/D) is only implemented by
sdhci-based hosts. Many other controllers configure their drive strength
in their GPIO or IOMUX/Pinmux modules, which currently have no
association with ios->drv_type. Moreover, the strength levels defined by
the type A/B/C/D standard may not have a direct mapping to the drive 
strength settings of these modules.

Therefore, the fixed-emmc-driver-type property may have introduced a
"signal configuration gap" from the start: it can specify a type for the 
eMMC device side, but the corresponding drive strength configuration on
the host side may not take effect, or may not even exist. This is not
just a missing driver implementation, but a lack of coordination at the
framework level.

Nevertheless, within the existing framework, this patch is a reasonable
and positive improvement to me. With one suggestion below, feel free to add:

Reviewed-by: Shawn Lin <shawn.lin@xxxxxxxxxxxxxx>


> Fixes: 6186d06c519e ("mmc: parse new binding for eMMC fixed driver type")
> Signed-off-by: Kamal Dasu <kamal.dasu@xxxxxxxxxxxx>
> ---
>   drivers/mmc/core/mmc.c | 6 +++++-
>   1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/mmc/core/mmc.c b/drivers/mmc/core/mmc.c
> index 8846550a8892..3507a8d8610a 100644
> --- a/drivers/mmc/core/mmc.c
> +++ b/drivers/mmc/core/mmc.c
> @@ -1371,8 +1371,12 @@ static void mmc_select_driver_type(struct mmc_card *card)
>   
>   	card->drive_strength = drive_strength;
>   
> -	if (drv_type)
> +	if (fixed_drv_type >= 0) {

How about check in a single line: if (fixed_drv_type >= 0 && drive_strength)

> +		if (drive_strength)
> +			mmc_set_driver_type(card->host, drive_strength);
> +	} else if (drv_type) {
>   		mmc_set_driver_type(card->host, drv_type);
> +	}
>   }
>   
>   static int mmc_select_hs400es(struct mmc_card *card)