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

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Date:   Wed, 5 May 2021 12:09:47 +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] mmc: enable UHS voltage switch for SDSC if supported

On Thu, 22 Apr 2021 at 15:18, Christian Löhle <CLoehle@...erstone.com> wrote:
>
> Ignore the reported capacity if the card otherwise reports UHS support.
>
> Currently SDSC cards reporting UHS support except for the CCS do not run
> through 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.

Maybe not, but I think the code just tries to conform to the SD spec.

> Especially for pSLCs in practice such cards benefit greatly by this patch,
> as they can be new and UHS supporting, but must not lie about their CCS.

Can you provide some concrete examples of SDSC cards that support UHS-I?

> The behaviour of linux-mmc for SDSC is deviating from the standard anyway
> in such a case, as the card is treated as UHS card not supporting the
> voltage switch in general.
> Such a card will come up as
> mmc0: new ultra high speed SDR25 SD card at address 0001.
> Thus the subsystem will support CMD23 and others to the card.
> But if we deviate from the standard anyway, then we might as well
> not throttle SDSC to 25MB/s.

Not sure I understand this correctly? Can you elaborate a bit more, so
I understand how we deviate from the SD spec here?

As far as I understand the spec, SDSC cards shouldn't support UHS-I.
If we decide to deviate from the spec to support this anyway, at least
we need to know about a couple of cards that actually supports this.

Kind regards
Uffe

>
> Signed-off-by: Christian Loehle <cloehle@...erstone.com>
> ---
>  drivers/mmc/core/sd.c | 7 +++----
>  1 file changed, 3 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/mmc/core/sd.c b/drivers/mmc/core/sd.c
> index 6fa51a6ed058..281ca2da8e0b 100644
> --- a/drivers/mmc/core/sd.c
> +++ b/drivers/mmc/core/sd.c
> @@ -841,11 +841,10 @@ 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
> -        * Switch procedure. SPI mode doesn't support CMD11.
> +        * In case S18A in the response is set, start Signal Voltage Switch
> +        * procedure. SPI mode doesn't support CMD11.
>          */
> -       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--;
> --
> 2.31.1
>
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