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

[Neil Armstrong]

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@xxxxxxxxxx> 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