linux-kernel - Re: [PATCH] mmc: core: limit probe clock frequency to configured f

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Message-ID: <CAPDyKFpZWnkK7UmCZ8M4UnM05wR3MQsPrpEjOJuwkKcN2gePSg@mail.gmail.com>
Date:   Thu, 16 Jan 2020 15:07:22 +0100
From:   Ulf Hansson <ulf.hansson@...aro.org>
To:     Michał Mirosław <mirq-linux@...e.qmqm.pl>
Cc:     "linux-mmc@...r.kernel.org" <linux-mmc@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mmc: core: limit probe clock frequency to configured f_max

On Thu, 2 Jan 2020 at 11:54, Michał Mirosław <mirq-linux@...e.qmqm.pl> wrote:
>
> Currently MMC core disregards host->f_max during card initialization
> phase. Obey upper boundary for the clock frequency and skip faster
> speeds when they are above the limit.

Is this a hypothetical problem or a real problem?

>
> Signed-off-by: Michał Mirosław <mirq-linux@...e.qmqm.pl>
> ---
>  drivers/mmc/core/core.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/mmc/core/core.c b/drivers/mmc/core/core.c
> index abf8f5eb0a1c..aa54d359dab7 100644
> --- a/drivers/mmc/core/core.c
> +++ b/drivers/mmc/core/core.c
> @@ -2330,7 +2330,13 @@ void mmc_rescan(struct work_struct *work)
>         }
>
>         for (i = 0; i < ARRAY_SIZE(freqs); i++) {
> -               if (!mmc_rescan_try_freq(host, max(freqs[i], host->f_min)))
> +               unsigned int freq = freqs[i];
> +               if (freq > host->f_max) {
> +                       if (i + 1 < ARRAY_SIZE(freqs))
> +                               continue;
> +                       freq = host->f_max;

This looks wrong to me. For example, what if f_max = 250KHz and f_min = 50 KHz.

Then we should try with 250KHz, then 200KHz and then 100KHz. This
isn't what the above code does, I think.

Instead it will try with 200KHz and then 100KHz, thus skip 250KHz.

Maybe we should figure out what index of freqs[] to start the loop for
(before actually starting the loop), depending on the value of f_max -
rather than always start at 0.

> +               }
> +               if (!mmc_rescan_try_freq(host, max(freq, host->f_min)))
>                         break;
>                 if (freqs[i] <= host->f_min)
>                         break;
> @@ -2344,7 +2350,7 @@ void mmc_rescan(struct work_struct *work)
>
>  void mmc_start_host(struct mmc_host *host)
>  {
> -       host->f_init = max(freqs[0], host->f_min);
> +       host->f_init = max(min(freqs[0], host->f_max), host->f_min);
>         host->rescan_disable = 0;
>         host->ios.power_mode = MMC_POWER_UNDEFINED;
>
> --
> 2.20.1
>

Kind regards
Uffe