linux-kernel - Re: [PATCH v4 6/8] clk: tegra20: Turn EMC clock gate into divider

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Date:   Sun, 22 Jul 2018 14:39:37 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Thierry Reding <thierry.reding@...il.com>,
        Peter De Schrijver <pdeschrijver@...dia.com>,
        Stephen Boyd <sboyd@...nel.org>
Cc:     Jonathan Hunter <jonathanh@...dia.com>,
        Prashant Gaikwad <pgaikwad@...dia.com>,
        Michael Turquette <mturquette@...libre.com>,
        Rob Herring <robh+dt@...nel.org>, linux-tegra@...r.kernel.org,
        linux-clk@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 6/8] clk: tegra20: Turn EMC clock gate into divider

On Thursday, 19 July 2018 16:21:30 MSK Dmitry Osipenko wrote:
> Kernel should never gate the EMC clock as it causes immediate lockup, so
> removing clk-gate functionality doesn't affect anything. Turning EMC clk
> gate into divider allows to implement glitch-less EMC scaling, avoiding
> reparenting to a backup clock.
> 
> Signed-off-by: Dmitry Osipenko <digetx@...il.com>
> Acked-by: Peter De Schrijver <pdeschrijver@...dia.com>
> ---
>  drivers/clk/tegra/clk-tegra20.c | 36 ++++++++++++++++++++++++---------
>  1 file changed, 26 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/clk/tegra/clk-tegra20.c
> b/drivers/clk/tegra/clk-tegra20.c index cc857d4d4a86..ebea97016d58 100644
> --- a/drivers/clk/tegra/clk-tegra20.c
> +++ b/drivers/clk/tegra/clk-tegra20.c
> @@ -578,7 +578,6 @@ static struct tegra_clk tegra20_clks[tegra_clk_max]
> __initdata = { [tegra_clk_afi] = { .dt_id = TEGRA20_CLK_AFI, .present =
> true }, [tegra_clk_fuse] = { .dt_id = TEGRA20_CLK_FUSE, .present = true },
> [tegra_clk_kfuse] = { .dt_id = TEGRA20_CLK_KFUSE, .present = true },
> -	[tegra_clk_emc] = { .dt_id = TEGRA20_CLK_EMC, .present = true }, };
> 
>  static unsigned long tegra20_clk_measure_input_freq(void)
> @@ -799,6 +798,31 @@ static struct tegra_periph_init_data
> tegra_periph_nodiv_clk_list[] = {
> TEGRA_INIT_DATA_NODIV("disp2",	mux_pllpdc_clkm, CLK_SOURCE_DISP2, 30, 2,
> 26,  0, TEGRA20_CLK_DISP2), };
> 
> +static void __init tegra20_emc_clk_init(void)
> +{
> +	struct clk *clk;
> +
> +	clk = clk_register_mux(NULL, "emc_mux", mux_pllmcp_clkm,
> +			       ARRAY_SIZE(mux_pllmcp_clkm),
> +			       CLK_SET_RATE_NO_REPARENT,
> +			       clk_base + CLK_SOURCE_EMC,
> +			       30, 2, 0, &emc_lock);
> +
> +	clk = tegra_clk_register_mc("mc", "emc_mux", clk_base + CLK_SOURCE_EMC,
> +				    &emc_lock);
> +	clks[TEGRA20_CLK_MC] = clk;
> +
> +	/*
> +	 * Note that 'emc_mux' source and 'emc' rate shouldn't be changed at
> +	 * the same time due to a HW bug, this won't happen because we're
> +	 * defining 'emc_mux' and 'emc' as distinct clocks.
> +	 */
> +	clk = clk_register_divider(NULL, "emc", "emc_mux", CLK_IS_CRITICAL,
> +				   clk_base + CLK_SOURCE_EMC, 0, 7,
> +				   0, &emc_lock);

Actually this is wrong. The divisor is 8 bits-wide and "lsb denote 0.5x", so 
this is a 7.1 divider. I'll prepare v5.