Re: [PATCH 1/2] mmc: core: Validate UHS/DDR/HS200 timing selection for 1-bit bus width

[Ulf Hansson]

On Fri, 6 Mar 2026 at 13:48, Shawn Lin <shawn.lin@xxxxxxxxx> wrote:
>
> From: Luke Wang <ziniu.wang_1@xxxxxxx>
>
> UHS/DDR/HS200 modes require at least 4-bit bus support. Host controllers
> that lack relevant capability registers rely on paring properties provided
> by firmware, which may incorrectly set these modes. Now that mmc_validate_host_caps()
> has been introduced to validate such configuration violations, let's also
> add checks for UHS/DDR/HS200 modes.
>
> This fixes an issue where, if the HS200/HS400 property is set while only a
> 1-bit bus width is used, mmc_select_hs200() returns 0 without actually performing
> the mode switch. Consequently, mmc_select_timing() proceeds without falling back to
> mmc_select_hs(), leaving the eMMC device operating in legacy mode (26 MHz) instead
> of switching to High Speed mode (52 MHz).
>
> Signed-off-by: Luke Wang <ziniu.wang_1@xxxxxxx>
> [Shawn: reword the commit msg and drop HS400 change]
> Signed-off-by: Shawn Lin <shawn.lin@xxxxxxxxx>
> ---
>
>  drivers/mmc/core/host.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
>
> diff --git a/drivers/mmc/core/host.c b/drivers/mmc/core/host.c
> index 88c95db..d1d4870 100644
> --- a/drivers/mmc/core/host.c
> +++ b/drivers/mmc/core/host.c
> @@ -624,6 +624,15 @@ static int mmc_validate_host_caps(struct mmc_host *host)
>                 return -EINVAL;
>         }
>
> +       /* UHS/DDR/HS200 modes require at least 4-bit bus */
> +       if (!(caps & (MMC_CAP_4_BIT_DATA | MMC_CAP_8_BIT_DATA)) &&
> +           ((caps & (MMC_CAP_UHS | MMC_CAP_DDR)) || (caps2 & MMC_CAP2_HS200))) {
> +               dev_warn(dev, "drop UHS/DDR/HS200 support since 1-bit bus only\n");
> +               host->caps = caps = caps & ~(MMC_CAP_UHS | MMC_CAP_DDR);
> +               host->caps2 = caps2 = caps2 & ~MMC_CAP2_HS200;

Hmm.

How about just updating the temporary variables, caps and caps2, here?

Then at the end of the function when we have validated all "caps", we
can update the variables in the host, like:
host->caps = caps;
host->caps2 = caps2;

> +       }
> +
> +       /* HS400 and HS400ES modes require 8-bit bus */
>         if (caps2 & (MMC_CAP2_HS400_ES | MMC_CAP2_HS400) &&
>             !(caps & MMC_CAP_8_BIT_DATA) && !(caps2 & MMC_CAP2_NO_MMC)) {
>                 dev_warn(dev, "drop HS400 support since no 8-bit bus\n");
> --
> 2.7.4
>

Kind regards
Uffe