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 15:05:33 +0200
From:   Neil Armstrong <neil.armstrong@...aro.org>
To:     Maxime Ripard <mripard@...nel.org>,
        Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
Cc:     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 14:04, Maxime Ripard wrote:
> Hi,
> 
> On Tue, May 30, 2023 at 03:36:04PM +0300, Dmitry Baryshkov wrote:
>> On 30/05/2023 15:15, AngeloGioacchino Del Regno wrote:
>>> Il 30/05/23 13:44, Dmitry Baryshkov ha scritto:
>>>> On Tue, 30 May 2023 at 10:24, Neil Armstrong
>>>> <neil.armstrong@...aro.org> wrote:
>>>>>
>>>>> Hi Marijn, Dmitry, Caleb, Jessica,
>>>>>
>>>>> On 29/05/2023 23:11, Marijn Suijten wrote:
>>>>>> On 2023-05-22 04:16:20, Dmitry Baryshkov wrote:
>>>>>> <snip>
>>>>>>>> +   if (ctx->dsi->dsc) {
>>>>>>>
>>>>>>> dsi->dsc is always set, thus this condition can be dropped.
>>>>>>
>>>>>> I want to leave room for possibly running the panel without DSC (at a
>>>>>> lower resolution/refresh rate, or at higher power consumption if there
>>>>>> is enough BW) by not assigning the pointer, if we get access to panel
>>>>>> documentation: probably one of the magic commands sent in this driver
>>>>>> controls it but we don't know which.
>>>>>
>>>>> I'd like to investigate if DSC should perhaps only be enabled if we
>>>>> run non certain platforms/socs ?
>>>>>
>>>>> I mean, we don't know if the controller supports DSC and those
>>>>> particular
>>>>> DSC parameters so we should probably start adding something like :
>>>>>
>>>>> static drm_dsc_config dsc_params_qcom = {}
>>>>>
>>>>> static const struct of_device_id panel_of_dsc_params[] = {
>>>>>           { .compatible = "qcom,sm8150", , .data = &dsc_params_qcom },
>>>>>           { .compatible = "qcom,sm8250", , .data = &dsc_params_qcom },
>>>>>           { .compatible = "qcom,sm8350", , .data = &dsc_params_qcom },
>>>>>           { .compatible = "qcom,sm8450", , .data = &dsc_params_qcom },
>>>>> };
>>>>
>>>> I think this would damage the reusability of the drivers. The panel
>>>> driver does not actually care if the SoC is SM8350, sunxi-something or
>>>> RCar.
>>>> Instead it cares about host capabilities.
>>>>
>>>> I think instead we should extend mipi_dsi_host:
>>>>
>>>> #define MIPI_DSI_HOST_MODE_VIDEO BIT(0)
>>>> #define MIPI_DSI_HOST_MODE_CMD  BIT(1)
>>>> #define MIPI_DSI_HOST_VIDEO_SUPPORTS_COMMANDS BIT(2)
>>>> // FIXME: do we need to provide additional caps here ?
>>>>
>>>> #define MIPI_DSI_DSC_1_1 BIT(0)
>>>> #define MIPI_DSI_DSC_1_2 BIT(1)
>>>> #define MIPI_DSI_DSC_NATIVE_422 BIT(2)
>>>> #define MIPI_DSI_DSC_NATIVE_420 BIT(3)
>>>> #define MIPI_DSI_DSC_FRAC_BPP BIT(4)
>>>> // etc.
>>>>
>>>> struct mipi_dsi_host {
>>>>    // new fields only
>>>>     unsigned long mode_flags;
>>>>     unsigned long dsc_flags;
>>>> };
>>>>
>>>> Then the panel driver can adapt itself to the host capabilities and
>>>> (possibly) select one of the internally supported DSC profiles.
>>>>
>>>
>>> I completely agree about extending mipi_dsi_host, other SoCs could reuse
>>> that and
>>> support for DSC panels would become a lot cleaner.
>>
>> Sounds good. I will wait for one or two more days (to get the possible
>> feedback on fields/flags/etc) and post an RFC patch to dri-devel.
> 
> I just came across that discussion, and couldn't find those patches, did
> you ever send them?
> 
> 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 ?

> 
> If a host isn't able to provide that, it's a bug and we should fix the
> controller driver instead of creating a workaround in the core for
> broken drivers.
> 
> Another concern I have is that, those broken drivers are usually the
> undocumented ones that already have trouble supporting the most trivial
> setup. Creating more combinations both at the controller and panel level
> will just make it harder for those drivers.
> 
> Maxime