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

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Date:   Fri, 28 Aug 2020 11:11:19 +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, dave.stevenson@...pberrypi.com,
        sboyd@...nel.org, mturquette@...libre.com, kdasu.kdev@...il.com,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        linux-clk@...r.kernel.org, robh+dt@...nel.org,
        bcm-kernel-feedback-list@...adcom.com,
        linux-rpi-kernel@...ts.infradead.org, phil@...pberrypi.com,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 3/3] drm/vc4: hdmi: Add pixel bvb clock control

Hi Stefan,

On Thu, Aug 27, 2020 at 11:49:34AM +0200, 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?

It won't cause any issue on a non-RPi4 platform since the clock pointer
will be NULL and clk_set_rate can deal with NULL pointers just fine:
https://elixir.bootlin.com/linux/latest/source/drivers/clk/clk.c#L2221

For the older DTBs, it shouldn't be an issue either. We haven't merged
the binding yet, so we don't have an upstream DTB using it

Maxime