linux-kernel - Re: [PATCH 3/5] drm/bridge: samsung-dsim: update PLL reference clock

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Message-ID: <20230829074952.GF17387@pengutronix.de>
Date:   Tue, 29 Aug 2023 09:49:52 +0200
From:   Michael Tretter <m.tretter@...gutronix.de>
To:     Adam Ford <aford173@...il.com>
Cc:     Marco Felsch <m.felsch@...gutronix.de>,
        Neil Armstrong <neil.armstrong@...aro.org>,
        Robert Foss <rfoss@...nel.org>,
        Jonas Karlman <jonas@...boo.se>,
        dri-devel@...ts.freedesktop.org,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
        linux-kernel@...r.kernel.org,
        Jernej Skrabec <jernej.skrabec@...il.com>,
        Jagan Teki <jagan@...rulasolutions.com>,
        Andrzej Hajda <andrzej.hajda@...el.com>, kernel@...gutronix.de,
        Marek Szyprowski <m.szyprowski@...sung.com>
Subject: Re: [PATCH 3/5] drm/bridge: samsung-dsim: update PLL reference clock

On Mon, 28 Aug 2023 13:17:44 -0500, Adam Ford wrote:
> On Mon, Aug 28, 2023 at 11:42 AM Marco Felsch <m.felsch@...gutronix.de> wrote:
> >
> > On 23-08-28, Michael Tretter wrote:
> > > The PLL requires a clock between 2 MHz and 30 MHz after the pre-divider.
> > > The reference clock for the PLL may change due to changes to it's parent
> > > clock. Thus, the frequency may be out of range or unsuited for
> > > generating the high speed clock for MIPI DSI.
> > >
> > > Try to keep the pre-devider small, and set the reference clock close to
> > > 30 MHz before recalculating the PLL configuration. Use a divider with a
> > > power of two for the reference clock as this seems to work best in
> > > my tests.
> > >
> > > Signed-off-by: Michael Tretter <m.tretter@...gutronix.de>
> > > ---
> > >  drivers/gpu/drm/bridge/samsung-dsim.c | 15 +++++++++++++--
> > >  1 file changed, 13 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/drivers/gpu/drm/bridge/samsung-dsim.c b/drivers/gpu/drm/bridge/samsung-dsim.c
> > > index da90c2038042..4de6e4f116db 100644
> > > --- a/drivers/gpu/drm/bridge/samsung-dsim.c
> > > +++ b/drivers/gpu/drm/bridge/samsung-dsim.c
> > > @@ -611,10 +611,21 @@ static unsigned long samsung_dsim_set_pll(struct samsung_dsim *dsi,
> > >       u16 m;
> > >       u32 reg;
> > >
> > > -     if (dsi->pll_clk)
> > > +     if (dsi->pll_clk) {
> > > +             /*
> > > +              * Ensure that the reference clock is generated with a power of
> > > +              * two divider from its parent, but close to the PLLs upper
> > > +              * limit of the valid range of 2 MHz to 30 MHz.
> > > +              */
> > > +             fin = clk_get_rate(clk_get_parent(dsi->pll_clk));
> > > +             while (fin > 30 * MHZ)
> > > +                     fin = fin / 2;
> >
> > Really just a cosmetic nit: fin /= 2;
> >
> > Reviewed-by: Marco Felsch <m.felsch@...gutronix.de>
> >
> > > +             clk_set_rate(dsi->pll_clk, fin);
> > > +
> > >               fin = clk_get_rate(dsi->pll_clk);
> > > -     else
> > > +     } else {
> > >               fin = dsi->pll_clk_rate;
> > > +     }
> 
> I don't have all the code style rules memorized.  Did you run it
> through checkpatch?  I wonder if the braces here are appropriate for a
> 1-line after the else.  If checkpatch is happy, I am fine with it.

checkpatch is happy: The coding style states that if one of the branches needs
braces because it has multiple statements, all branches should have braces.

Michael

> 
> adam
> > >       dev_dbg(dsi->dev, "PLL ref clock freq %lu\n", fin);
> > >
> > >       fout = samsung_dsim_pll_find_pms(dsi, fin, freq, &p, &m, &s);
> > >
> > > --
> > > 2.39.2
> > >
> > >
> > >
>