linux-kernel - Re: [PATCH v2] mmc: enable UHS voltage switch for SDSC if supported

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Message-ID: <CAPDyKFq_eM0EvLk_M2nGhJMv4nouZLKnhowtyHbH-MM8U5Awpw@mail.gmail.com>
Date:   Fri, 4 Jun 2021 14:10:36 +0200
From:   Ulf Hansson <ulf.hansson@...aro.org>
To:     Christian Löhle <CLoehle@...erstone.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mmc@...r.kernel.org" <linux-mmc@...r.kernel.org>,
        "pali@...nel.org" <pali@...nel.org>,
        "huyue2@...ong.com" <huyue2@...ong.com>,
        "tiantao6@...ilicon.com" <tiantao6@...ilicon.com>
Subject: Re: [PATCH v2] mmc: enable UHS voltage switch for SDSC if supported

On Wed, 12 May 2021 at 18:03, Christian Löhle <CLoehle@...erstone.com> wrote:
>
> Ignore the reported capacity if the card otherwise reports UHS support.
>
> Let SDSC cards reporting UHS support (except for the CCS) attempt the
> voltage switch.
> Up until now such cards would be initialized as UHS,
> supporting UHS features SDSC cards are otherwise barred from,
> but skip the voltage switch.
> While strictly speaking a SDSC card cannot support UHS in compliance
> with the standard, there is no good reason to throttle them that way.
> Especially for pSLCs in practice such cards benefit greatly,
> as they can be new and UHS supporting, but must not lie about their CCS.
>
> Signed-off-by: Christian Loehle <cloehle@...erstone.com>

Apologize for the delay. I have tested a bunch of my old (and new)
SD/uSD cards, this seems to work fine.

I have applied this for next, but I think this actually should be
considered as bug fix. I let it cook in linux-next for a while, then I
will probably add a stable tag for it.

Thanks and kind regards
Uffe

> ---
>  drivers/mmc/core/sd.c | 21 +++++++++++++++++----
>  1 file changed, 17 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/mmc/core/sd.c b/drivers/mmc/core/sd.c
> index 2c48d6504101..62d02f1dc924 100644
> --- a/drivers/mmc/core/sd.c
> +++ b/drivers/mmc/core/sd.c
> @@ -847,11 +847,17 @@ int mmc_sd_get_cid(struct mmc_host *host, u32 ocr, u32 *cid, u32 *rocr)
>                 return err;
>
>         /*
> -        * In case CCS and S18A in the response is set, start Signal Voltage
> +        * In case S18A in the response is set, start Signal Voltage
>          * Switch procedure. SPI mode doesn't support CMD11.
> +        * Strictly speaking, S18A is not valid if CCS is not set (= not SDSC),
> +        * so one would have to OCR for 0x41000000.
> +        * We choose to ignore this as SDSC cards that report UHS voltage
> +        * support should not be throttled artificially by the standard this
> +        * way.
> +        * SDSC cards that 'accidentally' reporting UHS support by setting the
> +        * reserved bits don't seem to be an issue in practice.
>          */
> -       if (!mmc_host_is_spi(host) && rocr &&
> -          ((*rocr & 0x41000000) == 0x41000000)) {
> +       if (!mmc_host_is_spi(host) && rocr && (*rocr & 0x01000000)) {
>                 err = mmc_set_uhs_voltage(host, pocr);
>                 if (err == -EAGAIN) {
>                         retries--;
> @@ -1109,7 +1115,14 @@ static int mmc_sd_init_card(struct mmc_host *host, u32 ocr,
>                 }
>         }
>
> -       /* Initialization sequence for UHS-I cards */
> +       /* Initialization sequence for UHS-I cards
> +        * Strictly speaking, S18A in the OCR is only valid if CCS is set, too.
> +        * So SDSC cards should be excluded. We choose to deviate from the
> +        * standard here to allow SDSC cards to utilize UHS if they report
> +        * supporting it.
> +        * Fortunately, SDSC cards reporting S18A and the related bus speed
> +        * modes on accident, by setting reserved bits, don't seem to exist.
> +        */
>         if (rocr & SD_ROCR_S18A && mmc_host_uhs(host)) {
>                 err = mmc_sd_init_uhs_card(card);
>                 if (err)
> --
> 2.31.1
>
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