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

[Adrian Hunter <adrian.hunter@intel.com>]

On 29/09/20 12:59 am, Raul E Rangel 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@chromium.org>

Acked-by: Adrian Hunter <adrian.hunter@intel.com>

> ---
> 
>  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)
>