linux-kernel - Re: [PATCH v3 1/13] mmc: sunxi: Fix clock frequency change sequence

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Date:   Tue, 24 Jan 2017 09:12:07 +0100
From:   Ulf Hansson <ulf.hansson@...aro.org>
To:     Maxime Ripard <maxime.ripard@...e-electrons.com>
Cc:     Chen-Yu Tsai <wens@...e.org>, Rob Herring <robh+dt@...nel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mmc@...r.kernel.org" <linux-mmc@...r.kernel.org>,
        Andre Przywara <andre.przywara@....com>
Subject: Re: [PATCH v3 1/13] mmc: sunxi: Fix clock frequency change sequence

On 16 January 2017 at 17:56, Maxime Ripard
<maxime.ripard@...e-electrons.com> wrote:
> The MMC and SD specifications documents that the clock frequency should
> only be changed once gated.

Where?

>
> The current code first modifies the parent clock, gates it and then
> modifies the internal divider. This means that since the parent clock rate
> might be changed, the bus clock might be changed as well before it is
> gated, which breaks the specification.
>
> Move the gating before the parent rate modification.

This all makes perfect sense to me, however I am not sure you need to
refer to the spec to justify these changes.

Kind regards
Uffe

>
> Signed-off-by: Maxime Ripard <maxime.ripard@...e-electrons.com>
> ---
>  drivers/mmc/host/sunxi-mmc.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/mmc/host/sunxi-mmc.c b/drivers/mmc/host/sunxi-mmc.c
> index b1d1303389a7..ab4324e6eb74 100644
> --- a/drivers/mmc/host/sunxi-mmc.c
> +++ b/drivers/mmc/host/sunxi-mmc.c
> @@ -761,6 +761,10 @@ static int sunxi_mmc_clk_set_rate(struct sunxi_mmc_host *host,
>         u32 rval, clock = ios->clock;
>         int ret;
>
> +       ret = sunxi_mmc_oclk_onoff(host, 0);
> +       if (ret)
> +               return ret;
> +
>         /* 8 bit DDR requires a higher module clock */
>         if (ios->timing == MMC_TIMING_MMC_DDR52 &&
>             ios->bus_width == MMC_BUS_WIDTH_8)
> @@ -783,10 +787,6 @@ static int sunxi_mmc_clk_set_rate(struct sunxi_mmc_host *host,
>                 return ret;
>         }
>
> -       ret = sunxi_mmc_oclk_onoff(host, 0);
> -       if (ret)
> -               return ret;
> -
>         /* clear internal divider */
>         rval = mmc_readl(host, REG_CLKCR);
>         rval &= ~0xff;
> --
> git-series 0.8.11