linux-kernel - Re: [PATCH 2/2] mmc: sdhci-acpi: AMDI0040: Allow changing HS200/HS400 driver strength

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Message-ID: <CAPDyKFrVtz29w=gONGO8_sGDQU-ZSXH2NyMuwRL7-27ae=jt_A@mail.gmail.com>
Date:   Mon, 5 Oct 2020 14:50:58 +0200
From:   Ulf Hansson <ulf.hansson@...aro.org>
To:     Raul E Rangel <rrangel@...omium.org>
Cc:     "linux-mmc@...r.kernel.org" <linux-mmc@...r.kernel.org>,
        Adrian Hunter <adrian.hunter@...el.com>, Shirish.S@....com,
        Akshu Agrawal <Akshu.Agrawal@....com>,
        "Shah, Nehal-bakulchandra" <Nehal-bakulchandra.Shah@....com>,
        "Wang, Chris" <chris.wang@....com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] mmc: sdhci-acpi: AMDI0040: Allow changing HS200/HS400
 driver strength

On Mon, 28 Sep 2020 at 23:59, Raul E Rangel <rrangel@...omium.org> wrote:
>
> This change will allow platform designers better control over signal
> integrity by allowing them to tune the HS200 and HS400 driver strengths.
>
> The driver strength was previously hard coded to A to solve boot
> problems with certain platforms. This driver strength does not
> universally apply to all platforms so we need a knob to adjust it.
>
> All older platforms currently have the SDR104 preset hard coded to A in
> the firmware. This means that switching from the hard coded value in
> the kernel to reading the SDR104 preset is a no-op for these platforms.
> Newer platforms will have properly set presets. So this change will
> support both new and old platforms.
>
> Signed-off-by: Raul E Rangel <rrangel@...omium.org>

Applied for next, thanks!

Kind regards
Uffe


> ---
>
>  drivers/mmc/host/sdhci-acpi.c | 39 ++++++++++++++++++++++++++++++++---
>  1 file changed, 36 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/mmc/host/sdhci-acpi.c b/drivers/mmc/host/sdhci-acpi.c
> index d335a34ad05b3..5c9a041af5b4b 100644
> --- a/drivers/mmc/host/sdhci-acpi.c
> +++ b/drivers/mmc/host/sdhci-acpi.c
> @@ -545,10 +545,43 @@ struct amd_sdhci_host {
>
>  static int amd_select_drive_strength(struct mmc_card *card,
>                                      unsigned int max_dtr, int host_drv,
> -                                    int card_drv, int *drv_type)
> +                                    int card_drv, int *host_driver_strength)
>  {
> -       *drv_type = MMC_SET_DRIVER_TYPE_A;
> -       return MMC_SET_DRIVER_TYPE_A;
> +       struct sdhci_host *host = mmc_priv(card->host);
> +       u16 preset, preset_driver_strength;
> +
> +       /*
> +        * This method is only called by mmc_select_hs200 so we only need to
> +        * read from the HS200 (SDR104) preset register.
> +        *
> +        * Firmware that has "invalid/default" presets return a driver strength
> +        * of A. This matches the previously hard coded value.
> +        */
> +       preset = sdhci_readw(host, SDHCI_PRESET_FOR_SDR104);
> +       preset_driver_strength =
> +               (preset & SDHCI_PRESET_DRV_MASK) >> SDHCI_PRESET_DRV_SHIFT;
> +
> +       /*
> +        * We want the controller driver strength to match the card's driver
> +        * strength so they have similar rise/fall times.
> +        *
> +        * The controller driver strength set by this method is sticky for all
> +        * timings after this method is called. This unfortunately means that
> +        * while HS400 tuning is in progress we end up with mismatched driver
> +        * strengths between the controller and the card. HS400 tuning requires
> +        * switching from HS400->DDR52->HS->HS200->HS400. So the driver mismatch
> +        * happens while in DDR52 and HS modes. This has not been observed to
> +        * cause problems. Enabling presets would fix this issue.
> +        */
> +       *host_driver_strength = preset_driver_strength;
> +
> +       /*
> +        * The resulting card driver strength is only set when switching the
> +        * card's timing to HS200 or HS400. The card will use the default driver
> +        * strength (B) for any other mode.
> +        */
> +       return preset_driver_strength;
> +
>  }
>
>  static void sdhci_acpi_amd_hs400_dll(struct sdhci_host *host, bool enable)
> --
> 2.28.0.709.gb0816b6eb0-goog
>