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Date:   Thu, 25 Jan 2018 15:14:07 +0100
From:   jacopo mondi <jacopo@jmondi.org>
To:     Geert Uytterhoeven <geert@linux-m68k.org>
Cc:     Jacopo Mondi <jacopo+renesas@jmondi.org>,
        Magnus Damm <magnus.damm@gmail.com>,
        Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>,
        Laurent Pinchart <laurent.pinchart@ideasonboard.com>,
        Yoshinori Sato <ysato@users.sourceforge.jp>,
        Rich Felker <dalias@libc.org>,
        Linux-sh list <linux-sh@vger.kernel.org>,
        Linux-Renesas <linux-renesas-soc@vger.kernel.org>,
        Linux Media Mailing List <linux-media@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-clk <linux-clk@vger.kernel.org>
Subject: Re: [PATCH] sh: clk: Relax clk rate match test
Message-ID: <20180125141407.GH17416@w540>
References: <1516879493-24637-1-git-send-email-jacopo+renesas@jmondi.org>
 <CAMuHMdVZpAB+Xu4trs0Eaiygk1WD7DPzKr3ehF-kugB5Fbps2g@mail.gmail.com>
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Hi Geert,

On Thu, Jan 25, 2018 at 02:53:41PM +0100, Geert Uytterhoeven wrote:
> Hi Jacopo,
>
> CC linux-clk (yes I know this is about the legacy SH clock framework, but
> the public API is/should be the same)
>
> On Thu, Jan 25, 2018 at 12:24 PM, Jacopo Mondi
> <jacopo+renesas@jmondi.org> wrote:
> > When asking for a clk rate to be set, the sh core clock matches only
> > exact rate values against the calculated frequency table entries. If the
> > rate does not match exactly the test fails, and the whole frequency
> > table is walked, resulting in selection of the last entry, corresponding to
> > the lowest available clock rate.
>
> IIUIC, the code does not select the last entry, but returns an error code,
> which is propagated all the way up?
>
> > Ie. when asking for a 10MHz clock rate on div6 clocks (ie. "video_clk" line),
> > the calculated clock frequency 10088572 Hz gets ignored, and the clock is
> > actually set to 5201920 Hz, which is the last available entry of the frequencies
> > table.
>
> Perhaps 5201920 is just the default (or old value)?
>
> > Relax the clock frequency match test, allowing selection of clock rates
> > immediately slower than the required one.
> >
> > Signed-off-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
> >
> > ---
> > Hello renesas lists,
> >
> > I'm now working on handling frame rate for the ov7720 image sensor to have that
> > driver accepted as part of v4l2. The sensor is installed on on Migo-R board.
> > In order to properly calculate pixel clock and the framerate I noticed the
> > clock signal fed to the sensor from the SH7722 chip was always the lowest
> > available one.
> >
> > This patch fixes the issues and allows me to properly select which clock
> > frequency supply to the sensor, which according to datasheet does not support
> > input clock frequencies slower than 10MHz (but works anyhow).
> >
> > As all patches for SH architecture I wonder where they should be picked up from,
> > as SH seems not maintained at the moment.
> >
> > Thanks
> >    j
> >
> > ---
> >  drivers/sh/clk/core.c | 9 ++++++---
> >  1 file changed, 6 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/sh/clk/core.c b/drivers/sh/clk/core.c
> > index 92863e3..d2cb94c 100644
> > --- a/drivers/sh/clk/core.c
> > +++ b/drivers/sh/clk/core.c
> > @@ -198,9 +198,12 @@ int clk_rate_table_find(struct clk *clk,
> >  {
> >         struct cpufreq_frequency_table *pos;
> >
> > -       cpufreq_for_each_valid_entry(pos, freq_table)
> > -               if (pos->frequency == rate)
> > -                       return pos - freq_table;
> > +       cpufreq_for_each_valid_entry(pos, freq_table) {
> > +               if (pos->frequency > rate)
> > +                       continue;
>
> This assumes all frequency tables are sorted.
>
> Shouldn't you pick the closest frequency?
>
> However, that's what clk_rate_table_round() does, which is called from
> sh_clk_div_round_rate(), and thus already used as .round_rate:
>
>     static struct sh_clk_ops sh_clk_div_enable_clk_ops = {
>             .recalc         = sh_clk_div_recalc,
>             .set_rate       = sh_clk_div_set_rate,
>             .round_rate     = sh_clk_div_round_rate,
>             .enable         = sh_clk_div_enable,
>             .disable        = sh_clk_div_disable,
>     };

Does this implies clock rates should be set using clk_round_rate() and
not clk_set_rate() if I understand this right?

>
> (clk_rate_table_find() is called from sh_clk_div_set_rate())
>
> Or are you supposed to ask for the exact clock rate? Where does the 10 MHz
> come from?
>

From board initialization code, in order to provide a valid input
clock to OV7720 sensor.

The 10MHz seems like a workaround put in place to remove some
image quality issues, according to the comment:
https://elixir.free-electrons.com/linux/v4.15-rc9/source/arch/sh/boards/mach-migor/setup.c#L311

Thanks
   j

> > +
> > +               return pos - freq_table;
> > +       }
> >
> >         return -ENOENT;
> >  }
>
> Gr{oetje,eeting}s,
>
>                         Geert
>
> --
> Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-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