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

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Date:   Fri, 28 Aug 2020 21:10:14 +0200
From:   Stefan Wahren <stefan.wahren@...e.com>
To:     Maxime Ripard <maxime@...no.tech>
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 Maxime,

Am 28.08.20 um 17:25 schrieb Maxime Ripard:
> 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.

you are right. Sorry for the noise.

Best regards

>
> Maxime