linux-kernel - Re: [PATCH 3/3] drm/vc4: hdmi: Add pixel bvb clock control

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Message-ID: <20200828152510.jhhqvka6fmouozff@gilmour.lan>
Date:   Fri, 28 Aug 2020 17:25:10 +0200
From:   Maxime Ripard <maxime@...no.tech>
To:     Stefan Wahren <stefan.wahren@...e.com>
Cc:     Hoegeun Kwon <hoegeun.kwon@...sung.com>, nsaenzjulienne@...e.de,
        eric@...olt.net, devicetree@...r.kernel.org,
        tim.gover@...pberrypi.com, kdasu.kdev@...il.com, sboyd@...nel.org,
        mturquette@...libre.com, dave.stevenson@...pberrypi.com,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        phil@...pberrypi.com, robh+dt@...nel.org,
        bcm-kernel-feedback-list@...adcom.com,
        linux-rpi-kernel@...ts.infradead.org, linux-clk@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 3/3] drm/vc4: hdmi: Add pixel bvb clock control

Hi,

On Fri, Aug 28, 2020 at 02:45:49PM +0200, Stefan Wahren wrote:
> Am 28.08.20 um 08:30 schrieb Hoegeun Kwon:
> > On 8/27/20 6:49 PM, Stefan Wahren wrote:
> >> Am 27.08.20 um 06:35 schrieb Hoegeun Kwon:
> >>> Hi Stefan,
> >>>
> >>> Thank you for your review.
> >>>
> >>>
> >>> On 8/26/20 7:04 PM, Stefan Wahren wrote:
> >>>> Hi Hoeguen,
> >>>>
> >>>> Am 21.08.20 um 09:10 schrieb Hoegeun Kwon:
> >>>>> There is a problem that the output does not work at a resolution
> >>>>> exceeding FHD. To solve this, we need to adjust the bvb clock at a
> >>>>> resolution exceeding FHD.
> >>>> this patch introduces a mandatory clock, please update
> >>>> brcm,bcm2835-hdmi.yaml first.
> >>>>
> >>>> Is this clock physically available on BCM283x or only on BCM2711?
> >>> As far as I know, BCM2711 raspberry pi 4 supports 4k,
> >>>
> >>> don't supported on pi 3 and pi 3+.
> >>>
> >>> Since 4k is not supported in versions prior to Raspberry Pi 4,
> >>>
> >>> I don't think we need to modify the bvb clock.
> >>>
> >>>
> >>> So I think it is better to update 'brcm,bcm2711-hdmi.yaml'
> >>>
> >>> instead of 'brcm,bcm2835-hdmi.yaml'.
> >> You are correct please update only brcm,bcm2711-hdmi.yaml.
> >>
> >> My concern was that the function vc4_hdmi_encoder_pre_crtc_configure()
> >> is called on a non-bcm2711 platform or on a Raspberry Pi 4 with an older
> >> DTB. So making the BVB clock optional might be better?
> > You are right, if use old dtb, we have a problem with the hdmi driver.
> >
> > So how about modifying it like this?
> >
> > @@ -1614,8 +1614,8 @@ static int vc5_hdmi_init_resources(struct vc4_hdmi 
> > *vc4_hdmi)
> >
> >          vc4_hdmi->pixel_bvb_clock = devm_clk_get(dev, "bvb");
> >          if (IS_ERR(vc4_hdmi->pixel_bvb_clock)) {
> > -               DRM_ERROR("Failed to get pixel bvb clock\n");
> > -               return PTR_ERR(vc4_hdmi->pixel_bvb_clock);
> > +               DRM_WARN("Failed to get pixel bvb clock\n");
> > +               vc4_hdmi->pixel_bvb_clock = NULL;
> >          }
> 
> i think the better solution would be devm_clk_get_optional(), which
> return NULL in case the clock doesn't exist.

It's not really optional though. BCM2711 will require it in order to run
properly (as Hoegeun experienced), and the previous SoCs won't.

If we use clk_get_optional and that the DT is missing the clock on the
BCM2711, we will silently ignore it which doesn't sound great.

Maxime

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