linux-kernel - Re: [PATCH 09/18] drm/sun4i: tcon: Adjust dotclock dividers range

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Message-ID: <20170720145500.kpwj7iscrwyjqi5q@flea>
Date:   Thu, 20 Jul 2017 16:55:00 +0200
From:   Maxime Ripard <maxime.ripard@...e-electrons.com>
To:     Chen-Yu Tsai <wens@...e.org>
Cc:     Mark Brown <broonie@...nel.org>,
        Thierry Reding <thierry.reding@...il.com>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Daniel Vetter <daniel.vetter@...el.com>,
        David Airlie <airlied@...ux.ie>,
        Mark Rutland <mark.rutland@....com>,
        Rob Herring <robh+dt@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        devicetree <devicetree@...r.kernel.org>,
        Boris Brezillon <boris.brezillon@...e-electrons.com>,
        Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>
Subject: Re: [PATCH 09/18] drm/sun4i: tcon: Adjust dotclock dividers range

On Fri, Jul 14, 2017 at 12:14:37PM +0800, Chen-Yu Tsai wrote:
> On Thu, Jul 13, 2017 at 10:13 PM, Maxime Ripard
> <maxime.ripard@...e-electrons.com> wrote:
> > It seems like the dotclock dividers are a bit less strict range, and can
> > operate even with a smaller than 6 divider. Loose the boundaries a bit.
> >
> > Signed-off-by: Maxime Ripard <maxime.ripard@...e-electrons.com>
> > ---
> >  drivers/gpu/drm/sun4i/sun4i_dotclock.c | 20 +++++++++++++++++++-
> >  1 file changed, 19 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/gpu/drm/sun4i/sun4i_dotclock.c b/drivers/gpu/drm/sun4i/sun4i_dotclock.c
> > index d401156490f3..0b844c0dd102 100644
> > --- a/drivers/gpu/drm/sun4i/sun4i_dotclock.c
> > +++ b/drivers/gpu/drm/sun4i/sun4i_dotclock.c
> > @@ -77,7 +77,25 @@ static long sun4i_dclk_round_rate(struct clk_hw *hw, unsigned long rate,
> >         u8 best_div = 1;
> >         int i;
> >
> > -       for (i = 6; i <= 127; i++) {
> > +       /*
> > +        * There's something odd here.
> > +        *
> > +        * In the A13 user manual, this is stated to be >= 6 when
> > +        * dclk1 and dclk2 are used (without any hint on how to use
> > +        * them), and >= 4 when only dclk is used.
> 
> You set it in TCON0_IO_POL_REG, which sets the clock phase delay.

oh, so it's d for delay? I assumed it was for dotclock.

> I think we were setting this before, but you removed it as part of
> the previous TCON clean up patches?

Hmmm, I might have.. :)

> In the A33, there are even more options, like DCLK / 2 (with 0 or 90
> degree phase delay).

Where did you find this documentation? I was under the impression that
all that DCLK stuff was in the higher bits of the DCLK register, but
apparently there's more to it.

> > +        *
> > +        * In the A33 user manual, when only dclk is used, it is set
> > +        * to be >= 6 in the former case, and >= 1 in the
> > +        * latter. There's also some (obscure) explanations about the
> > +        * dclk1 and dclk2 vs dclk that seems to be in the upper 4
> > +        * bits. What those clocks are and what bit does what is not
> > +        * really clear.
> 
> Looks like mux bits to me. How they differ from TCON0_IO_POL_REG is
> beyond me ATM.

It might be some additional dividers too.

Maxime

-- 
Maxime Ripard, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com

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