linux-kernel - Re: [PATCH v2 4/6] clk: tegra20: init NDFLASH clock to sensible rate

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Date:   Tue, 29 May 2018 14:12:35 +0200
From:   Stefan Agner <stefan@...er.ch>
To:     Peter De Schrijver <pdeschrijver@...dia.com>
Cc:     boris.brezillon@...tlin.com, dwmw2@...radead.org,
        computersforpeace@...il.com, marek.vasut@...il.com,
        robh+dt@...nel.org, mark.rutland@....com, thierry.reding@...il.com,
        mturquette@...libre.com, sboyd@...nel.org, dev@...xeye.de,
        miquel.raynal@...tlin.com, richard@....at, marcel@...wiler.com,
        krzk@...nel.org, digetx@...il.com, benjamin.lindqvist@...ian.se,
        jonathanh@...dia.com, pgaikwad@...dia.com, mirza.krak@...il.com,
        linux-mtd@...ts.infradead.org, linux-tegra@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-clk@...r.kernel.org
Subject: Re: [PATCH v2 4/6] clk: tegra20: init NDFLASH clock to sensible rate

On 29.05.2018 09:48, Peter De Schrijver wrote:
> On Mon, May 28, 2018 at 05:53:08PM +0200, Stefan Agner wrote:
>> On 28.05.2018 09:55, Peter De Schrijver wrote:
>> > On Sun, May 27, 2018 at 11:54:40PM +0200, Stefan Agner wrote:
>> >> From: Lucas Stach <dev@...xeye.de>
>> >>
>> >> Set up the NAND Flash controller clock to run at 150MHz
>> >> instead of the rate set by the bootloader. This is a
>> >> conservative rate which also yields good performance.
>> >>
>> >> Signed-off-by: Lucas Stach <dev@...xeye.de>
>> >> Signed-off-by: Stefan Agner <stefan@...er.ch>
>> >> ---
>> >>  drivers/clk/tegra/clk-tegra20.c | 1 +
>> >>  1 file changed, 1 insertion(+)
>> >>
>> >> diff --git a/drivers/clk/tegra/clk-tegra20.c b/drivers/clk/tegra/clk-tegra20.c
>> >> index 0ee56dd04cec..dff8c425cd28 100644
>> >> --- a/drivers/clk/tegra/clk-tegra20.c
>> >> +++ b/drivers/clk/tegra/clk-tegra20.c
>> >> @@ -1049,6 +1049,7 @@ static struct tegra_clk_init_table init_table[] __initdata = {
>> >>  	{ TEGRA20_CLK_GR2D, TEGRA20_CLK_PLL_C, 300000000, 0 },
>> >>  	{ TEGRA20_CLK_GR3D, TEGRA20_CLK_PLL_C, 300000000, 0 },
>> >>  	{ TEGRA20_CLK_VDE, TEGRA20_CLK_CLK_MAX, 300000000, 0 },
>> >> +	{ TEGRA20_CLK_NDFLASH, TEGRA20_CLK_PLL_P, 150000000, 0 },
>> >>  	/* must be the last entry */
>> >>  	{ TEGRA20_CLK_CLK_MAX, TEGRA20_CLK_CLK_MAX, 0, 0 },
>> >>  };
>> >> --
>> >> 2.17.0
>> >>
>> >
>> > Maybe better to specify this in the Tegra20 dtsi? See
>> > "Assigned clock parents and rates" in
>> > Documentation/devicetree/bindings/clock/clock-bindings.txt
>>
>> assigned-clocks indeed works just fine for this case. Thanks for
>> bringing this up, will drop this patch and add the device tree
>> properties in v3.
>>
>> Hm, interesting that none of the Tegra device tree make use of the
>> feature so far. I guess there would be other cases where this would be
>> useful as well (the one just above, VDE?).
>>
> 
> Yes, historically this feature wasn't available, so we used these init tables.
> Unfortunately it's not easy to get rid of them for parent and rate
> configuration, because new kernels should also work with existing DTBs, so we
> can't just add assigned-clock properties and remove the existing table
> entries. What we could do is use the CLK_IS_CRITICAL flag for all clocks which
> are only enabled by the init table. For not yet merged blocks, this is
> ofcourse not a concern.

Sure I understand.

Was just somewhat surprised that it isn't used at all yet (grep -r -e
assigned-clock arch/arm/boot/dts/tegra* returns nothing). After all,
assigned clocks bindings have been merged in 2014 :-)

At least "clk: tegra: Specify VDE clock rate" merged earlier this year
would have been a candidate already.

--
Stefan