linux-kernel - Re: [PATCH] arm: dts: gr-peach: Reduce extal

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Date:   Thu, 5 Oct 2017 10:32:08 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     jacopo mondi <jacopo@...ndi.org>
Cc:     Jacopo Mondi <jacopo+renesas@...ndi.org>,
        Simon Horman <horms@...ge.net.au>,
        Magnus Damm <magnus.damm@...il.com>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Russell King <linux@...linux.org.uk>,
        Linux-Renesas <linux-renesas-soc@...r.kernel.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>,
        Chris Brandt <Chris.Brandt@...esas.com>
Subject: Re: [PATCH] arm: dts: gr-peach: Reduce extal_clk resolution

Hi Jacopo,

On Thu, Oct 5, 2017 at 9:48 AM, jacopo mondi <jacopo@...ndi.org> wrote:
> On Wed, Oct 04, 2017 at 05:54:46PM +0200, Geert Uytterhoeven wrote:
>> On Wed, Oct 4, 2017 at 5:40 PM, Jacopo Mondi <jacopo+renesas@...ndi.org> wrote:
>> > The system clock described by extal_clk is reported to have a frequency
>> > of 13.333 Mhz and is correctly described by gr-peach device tree.
>> >
>> > However, when enabling a RIIC device the following error is reported by
>> > drivers/i2c/busses/i2c-riic.c
>> >
>> > "invalid parent clk (33332500). Must be 33325000Hz"
>> >
>> > As RIIC devices have a clock source obtained by dividing by 12 the
>> > system clock, the resulting value is not accepted by the driver
>> > (which clearly states not to support any frequency except the reported
>> > 33325000Hz one).
>> >
>> > Hence, reduce the system clock accuracy to a value which makes
>> > frequencies obtained through division accepted by RIIC driver.
>> >
>> > Please note that other r7s72100 boards, such as Genmai, report the same
>> > "reduced accuracy" frequency, even if their external clock sources are
>> > effectively 13.333Mhz as gr-peach one.
>> >
>> > Signed-off-by: Jacopo Mondi <jacopo+renesas@...ndi.org>
>>
>> There's no need to do this, Chris already proposed a fix, cfr.
>> "[PATCH v2] i2c: riic: remove fixed clock restriction"
>> (https://www.spinics.net/lists/linux-renesas-soc/msg18573.html).
>
> Oh that's even better!
> Do you see any value in augmenting the frequency resolution in Genmai
> DTS then?

The Genmai documentation says EXTAL is 13.33 MHz (which matches
r7s72100-genmai.dts) everywhere, except in the schematics, where it says
13.3333 MHz. Sigh...

The difference is slightly larger than 100 ppm, so still insignificant, I think.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds