Re: [PATCH v3 3/13] mmc: sunxi: Always set signal delay to 0 for A64

[Ulf Hansson <ulf.hansson@linaro.org>]

On 16 January 2017 at 17:56, Maxime Ripard
<maxime.ripard@free-electrons.com> wrote:
> Experience have shown that the using the  autocalibration could severely
> degrade the performances of the MMC bus.
>
> Allwinner is using in its BSP a delay set to 0 for all the modes but HS400.
> Remove the calibration code for now, and add comments to document our
> findings.

So doesn't this break some platforms using HS400? Or are you saying
those are already broken?

Kind regards
Uffe

>
> Reviewed-by: Andre Przywara <andre.przywara@arm.com>
> Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
> ---
>  drivers/mmc/host/sunxi-mmc.c | 50 ++++++++++++-------------------------
>  1 file changed, 17 insertions(+), 33 deletions(-)
>
> diff --git a/drivers/mmc/host/sunxi-mmc.c b/drivers/mmc/host/sunxi-mmc.c
> index 019f95e8e7c5..b9c8a62bc212 100644
> --- a/drivers/mmc/host/sunxi-mmc.c
> +++ b/drivers/mmc/host/sunxi-mmc.c
> @@ -683,41 +683,19 @@ static int sunxi_mmc_oclk_onoff(struct sunxi_mmc_host *host, u32 oclk_en)
>
>  static int sunxi_mmc_calibrate(struct sunxi_mmc_host *host, int reg_off)
>  {
> -       u32 reg = readl(host->reg_base + reg_off);
> -       u32 delay;
> -       unsigned long timeout;
> -
>         if (!host->cfg->can_calibrate)
>                 return 0;
>
> -       reg &= ~(SDXC_CAL_DL_MASK << SDXC_CAL_DL_SW_SHIFT);
> -       reg &= ~SDXC_CAL_DL_SW_EN;
> -
> -       writel(reg | SDXC_CAL_START, host->reg_base + reg_off);
> -
> -       dev_dbg(mmc_dev(host->mmc), "calibration started\n");
> -
> -       timeout = jiffies + HZ * SDXC_CAL_TIMEOUT;
> -
> -       while (!((reg = readl(host->reg_base + reg_off)) & SDXC_CAL_DONE)) {
> -               if (time_before(jiffies, timeout))
> -                       cpu_relax();
> -               else {
> -                       reg &= ~SDXC_CAL_START;
> -                       writel(reg, host->reg_base + reg_off);
> -
> -                       return -ETIMEDOUT;
> -               }
> -       }
> -
> -       delay = (reg >> SDXC_CAL_DL_SHIFT) & SDXC_CAL_DL_MASK;
> -
> -       reg &= ~SDXC_CAL_START;
> -       reg |= (delay << SDXC_CAL_DL_SW_SHIFT) | SDXC_CAL_DL_SW_EN;
> -
> -       writel(reg, host->reg_base + reg_off);
> -
> -       dev_dbg(mmc_dev(host->mmc), "calibration ended, reg is 0x%x\n", reg);
> +       /*
> +        * FIXME:
> +        * This is not clear how the calibration is supposed to work
> +        * yet. The best rate have been obtained by simply setting the
> +        * delay to 0, as Allwinner does in its BSP.
> +        *
> +        * The only mode that doesn't have such a delay is HS400, that
> +        * is in itself a TODO.
> +        */
> +       writel(SDXC_CAL_DL_SW_EN, host->reg_base + reg_off);
>
>         return 0;
>  }
> @@ -809,7 +787,13 @@ static int sunxi_mmc_clk_set_rate(struct sunxi_mmc_host *host,
>         if (ret)
>                 return ret;
>
> -       /* TODO: enable calibrate on sdc2 SDXC_REG_DS_DL_REG of A64 */
> +       /*
> +        * FIXME:
> +        *
> +        * In HS400 we'll also need to calibrate the data strobe
> +        * signal. This should only happen on the MMC2 controller (at
> +        * least on the A64).
> +        */
>
>         return sunxi_mmc_oclk_onoff(host, 1);
>  }
> --
> git-series 0.8.11