linux-kernel - Re: [PATCH 08/15] drm/vc4: hdmi: Introduce a CEC clock

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Message-ID: <20201218122334.tvdeslyyuus4hp4y@gilmour>
Date:   Fri, 18 Dec 2020 13:23:34 +0100
From:   Maxime Ripard <maxime@...no.tech>
To:     Dave Stevenson <dave.stevenson@...pberrypi.com>
Cc:     Eric Anholt <eric@...olt.net>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Daniel Vetter <daniel.vetter@...el.com>,
        David Airlie <airlied@...ux.ie>,
        Jason Cooper <jason@...edaemon.net>,
        bcm-kernel-feedback-list@...adcom.com,
        linux-arm-kernel@...ts.infradead.org,
        Marc Zyngier <maz@...nel.org>,
        Linux Media Mailing List <linux-media@...r.kernel.org>,
        Hans Verkuil <hverkuil-cisco@...all.nl>,
        LKML <linux-kernel@...r.kernel.org>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-rpi-kernel@...ts.infradead.org,
        DRI Development <dri-devel@...ts.freedesktop.org>
Subject: Re: [PATCH 08/15] drm/vc4: hdmi: Introduce a CEC clock

Hi Dave,

On Fri, Dec 18, 2020 at 11:37:50AM +0000, Dave Stevenson wrote:
> Hi Maxime
> 
> On Thu, 10 Dec 2020 at 13:47, Maxime Ripard <maxime@...no.tech> wrote:
> >
> > While the BCM2835 had the CEC clock derived from the HSM clock, the
> > BCM2711 has a dedicated parent clock for it.
> >
> > Let's introduce a separate clock for it so that we can handle both
> > cases.
> >
> > Signed-off-by: Maxime Ripard <maxime@...no.tech>
> > ---
> >  drivers/gpu/drm/vc4/vc4_hdmi.c | 9 ++++++++-
> >  drivers/gpu/drm/vc4/vc4_hdmi.h | 1 +
> >  2 files changed, 9 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/gpu/drm/vc4/vc4_hdmi.c b/drivers/gpu/drm/vc4/vc4_hdmi.c
> > index b93ee3e26e2b..0debd22bc992 100644
> > --- a/drivers/gpu/drm/vc4/vc4_hdmi.c
> > +++ b/drivers/gpu/drm/vc4/vc4_hdmi.c
> > @@ -145,7 +145,7 @@ static void vc4_hdmi_cec_update_clk_div(struct vc4_hdmi *vc4_hdmi)
> >          * Set the clock divider: the hsm_clock rate and this divider
> >          * setting will give a 40 kHz CEC clock.
> >          */
> > -       clk_cnt = clk_get_rate(vc4_hdmi->hsm_clock) / CEC_CLOCK_FREQ;
> > +       clk_cnt = clk_get_rate(vc4_hdmi->cec_clock) / CEC_CLOCK_FREQ;
> >         value |= clk_cnt << VC4_HDMI_CEC_DIV_CLK_CNT_SHIFT;
> >         HDMI_WRITE(HDMI_CEC_CNTRL_1, value);
> >  }
> > @@ -1740,6 +1740,7 @@ static int vc4_hdmi_init_resources(struct vc4_hdmi *vc4_hdmi)
> >                 return PTR_ERR(vc4_hdmi->hsm_clock);
> >         }
> >         vc4_hdmi->audio_clock = vc4_hdmi->hsm_clock;
> > +       vc4_hdmi->cec_clock = vc4_hdmi->hsm_clock;
> >
> >         return 0;
> >  }
> > @@ -1833,6 +1834,12 @@ static int vc5_hdmi_init_resources(struct vc4_hdmi *vc4_hdmi)
> >                 return PTR_ERR(vc4_hdmi->audio_clock);
> >         }
> >
> > +       vc4_hdmi->cec_clock = devm_clk_get(dev, "cec");
> > +       if (IS_ERR(vc4_hdmi->cec_clock)) {
> > +               DRM_ERROR("Failed to get CEC clock\n");
> > +               return PTR_ERR(vc4_hdmi->cec_clock);
> > +       }
> 
> Aren't we adding to the DT binding here and breaking backwards compatibility?
> Admittedly CEC didn't work before (and was masked out) for vc5, but do
> we need to worry about those with existing DT files that currently
> work happily?

The DT compatibility is not a worry here: I made sure the CEC clock and
range were part of the binding since it's been introduced:

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=2e3725b05b785e73482a194b99bff3d5a1c85140

So we were not using it so far, but it was in the DT all along

Maxime

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