linux-kernel - Re: [PATCH 03/11] clk: sunxi-ng: a83t: Support new timing mode for mmc2 clock

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Date:   Mon, 17 Jul 2017 11:14:02 +0200
From:   Maxime Ripard <maxime.ripard@...e-electrons.com>
To:     Chen-Yu Tsai <wens@...e.org>
Cc:     Ulf Hansson <ulf.hansson@...aro.org>,
        Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...eaurora.org>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        linux-arm-kernel@...ts.infradead.org, linux-mmc@...r.kernel.org,
        linux-clk@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-sunxi@...glegroups.com
Subject: Re: [PATCH 03/11] clk: sunxi-ng: a83t: Support new timing mode for
 mmc2 clock

Hi,

On Fri, Jul 14, 2017 at 02:42:54PM +0800, Chen-Yu Tsai wrote:
> The MMC2 clock supports a new timing mode. When the new mode is active,
> the output clock rate is halved.
> 
> This patch sets the feature flag for the new timing mode, and adds
> a pre-divider based on the mode bit.
> 
> Signed-off-by: Chen-Yu Tsai <wens@...e.org>
> ---
>  drivers/clk/sunxi-ng/ccu-sun8i-a83t.c | 38 +++++++++++++++++++++++++++--------
>  1 file changed, 30 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/clk/sunxi-ng/ccu-sun8i-a83t.c b/drivers/clk/sunxi-ng/ccu-sun8i-a83t.c
> index 947f9f6e05d2..ee6688e9b361 100644
> --- a/drivers/clk/sunxi-ng/ccu-sun8i-a83t.c
> +++ b/drivers/clk/sunxi-ng/ccu-sun8i-a83t.c
> @@ -418,14 +418,36 @@ static SUNXI_CCU_PHASE(mmc1_sample_clk, "mmc1-sample", "mmc1",
>  static SUNXI_CCU_PHASE(mmc1_output_clk, "mmc1-output", "mmc1",
>  		       0x08c, 8, 3, 0);
>  
> -/* TODO Support MMC2 clock's new timing mode. */
> -static SUNXI_CCU_MP_WITH_MUX_GATE(mmc2_clk, "mmc2", mod0_default_parents,
> -				  0x090,
> -				  0, 4,		/* M */
> -				  16, 2,	/* P */
> -				  24, 2,	/* mux */
> -				  BIT(31),	/* gate */
> -				  0);
> +/*
> + * MMC2 supports both old and new timing modes. When the new timing
> + * mode is active, the output clock rate is halved by two. Here we
> + * treat it as a variable pre-divider. Note that the pre-divider is
> + * _not_ included in the possible factors during a set clock rate
> + * operation. It is only read out.
> + */
> +static const struct ccu_mux_var_prediv mmc2_new_timing_predivs[] = {
> +	{ .index = 0, .shift = 30, .width = 1 },
> +	{ .index = 1, .shift = 30, .width = 1 },
> +};
> +static struct ccu_mp mmc2_clk = {
> +	.enable = BIT(31),
> +	.m      = _SUNXI_CCU_DIV(0, 4),
> +	.p      = _SUNXI_CCU_DIV(16, 2),
> +	.mux    = {
> +		.shift  = 24,
> +		.width  = 2,
> +		.var_predivs    = mmc2_new_timing_predivs,
> +		.n_var_predivs  = ARRAY_SIZE(mmc2_new_timing_predivs),
> +	},
> +	.common         = {
> +		.reg            = 0x090,
> +		.features	= CCU_FEATURE_MMC_TIMING_SWITCH,
> +		.hw.init        = CLK_HW_INIT_PARENTS("mmc2",
> +						      mod0_default_parents,
> +						      &ccu_mp_ops,
> +						      CLK_GET_RATE_NOCACHE),
> +	},
> +};

Treating the new bit seems a bit of a hack to me. It only works
because we're not evaluating the various pre-dividers during a
determine_rate (and set_rate), but it might change in the future, and
we will break all our eMMC controllers then.

Since they're quite special, I was thinking about creating a new MMC
clock type? We're going to use it on a number of SoCs, and we'll be
able to model it properly, without crippling the regular and generic
MP clocks.

Maxime

-- 
Maxime Ripard, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com

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