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

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Date:   Wed, 30 May 2018 10:42:25 +0300
From:   Peter De Schrijver <pdeschrijver@...dia.com>
To:     Dmitry Osipenko <digetx@...il.com>
CC:     Stefan Agner <stefan@...er.ch>, <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>,
        <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 Tue, May 29, 2018 at 03:19:47PM +0300, Dmitry Osipenko wrote:
> On 29.05.2018 15:12, Stefan Agner wrote:
> > 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.
> 
> I wasn't even aware of existence of the assigned-clock properties, probably just
> like others.

This feature seems to be little used indeed. Not sure why.

Peter.