linux-kernel - Re: RFC: DSI host capabilities (was: [PATCH RFC 03/10] drm/panel: Add LGD panel driver for Sony Xperia XZ3)

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Date:   Wed, 5 Jul 2023 23:09:40 +0300
From:   Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To:     Maxime Ripard <mripard@...nel.org>
Cc:     Neil Armstrong <neil.armstrong@...aro.org>,
        AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@...labora.com>,
        "open list:DRM DRIVER FOR MSM ADRENO GPU" 
        <dri-devel@...ts.freedesktop.org>,
        Caleb Connolly <caleb@...nolly.tech>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@...ainline.org>,
        Marijn Suijten <marijn.suijten@...ainline.org>,
        Sam Ravnborg <sam@...nborg.org>,
        Kuogee Hsieh <quic_khsieh@...cinc.com>,
        Andy Gross <agross@...nel.org>,
        Jessica Zhang <quic_jesszhan@...cinc.com>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>, Conor Dooley <conor+dt@...nel.org>,
        "open list:DRM DRIVER FOR MSM ADRENO GPU" 
        <linux-arm-msm@...r.kernel.org>,
        Abhinav Kumar <quic_abhinavk@...cinc.com>,
        Rob Herring <robh+dt@...nel.org>,
        Martin Botka <martin.botka@...ainline.org>,
        ~postmarketos/upstreaming@...ts.sr.ht,
        Jami Kettunen <jami.kettunen@...ainline.org>,
        Bjorn Andersson <andersson@...nel.org>,
        open list <linux-kernel@...r.kernel.org>,
        Konrad Dybcio <konrad.dybcio@...aro.org>,
        freedreno <freedreno@...ts.freedesktop.org>
Subject: Re: RFC: DSI host capabilities (was: [PATCH RFC 03/10] drm/panel: Add
 LGD panel driver for Sony Xperia XZ3)

On 05/07/2023 19:53, Maxime Ripard wrote:
> On Wed, Jul 05, 2023 at 06:20:13PM +0300, Dmitry Baryshkov wrote:
>> On Wed, 5 Jul 2023 at 17:24, Maxime Ripard <mripard@...nel.org> wrote:
>>>
>>> On Wed, Jul 05, 2023 at 04:37:57PM +0300, Dmitry Baryshkov wrote:
>>>>>>>
>>>>>>> Either way, I'm not really sure it's a good idea to multiply the
>>>>>>> capabilities flags of the DSI host, and we should just stick to the
>>>>>>> spec. If the spec says that we have to support DSC while video is
>>>>>>> output, then that's what the panels should expect.
>>>>>>
>>>>>> Except some panels supports DSC & non-DSC, Video and Command mode, and
>>>>>> all that is runtime configurable. How do you handle that ?
>>>>>
>>>>> In this case, most of the constraints are going to be on the encoder
>>>>> still so it should be the one driving it. The panel will only care about
>>>>> which mode has been selected, but it shouldn't be the one driving it,
>>>>> and thus we still don't really need to expose the host capabilities.
>>>>
>>>> This is an interesting perspective. This means that we can and actually have
>>>> to extend the drm_display_mode with the DSI data and compression
>>>> information.
>>>
>>> I wouldn't extend drm_display_mode, but extending one of the state
>>> structures definitely.
>>>
>>> We already have some extra variables in drm_connector_state for HDMI,
>>> I don't think it would be a big deal to add a few for MIPI-DSI.
>>>
>>> We also floated the idea for a while to create bus-specific states, with
>>> helpers to match. Maybe it would be a good occasion to start doing it?
>>>
>>>> For example, the panel that supports all four types for the 1080p should
>>>> export several modes:
>>>>
>>>> 1920x1080-command
>>>> 1920x1080-command-DSC
>>>> 1920x1080-video
>>>> 1920x1080-video-DSC
>>>>
>>>> where video/command and DSC are some kinds of flags and/or information in
>>>> the drm_display_mode? Ideally DSC also has several sub-flags, which denote
>>>> what kind of configuration is supported by the DSC sink (e.g. bpp, yuv,
>>>> etc).
>>>
>>> So we have two things to do, right? We need to expose what the panel can
>>> take (ie, EDID for HDMI), and then we need to tell it what we picked
>>> (infoframes).
>>>
>>> We already express the former in mipi_dsi_device, so we could extend the
>>> flags stored there.
>>>
>>> And then, we need to tie what the DSI host chose to a given atomic state
>>> so the panel knows what was picked and how it should set everything up.
>>
>> This is definitely something we need. Marijn has been stuck with the
>> panels that support different models ([1]).
>>
>> Would you prefer a separate API for this kind of information or
>> abusing atomic_enable() is fine from your point of view?
>>
>> My vote would be for going with existing operations, with the slight
>> fear of ending up with another DSI-specific hack (like
>> pre_enable_prev_first).
> 
> I don't think we can get away without getting access to the atomic_state
> from the panel at least.
> 
> Choosing one setup over another is likely going to depend on the mode,
> and that's only available in the state.
> 
> We don't have to go the whole way though and create the sub-classes of
> drm_connector_state, but I think we should at least provide it to the
> panel.
> 
> What do you think of creating a new set of atomic_* callbacks for
> panels, call that new set of functions from msm and start from there?

We are (somewhat) bound by the panel_bridge, but yeah, it seems possible.

-- 
With best wishes
Dmitry