Re: [PATCH v7 00/36] drm/connector: Create HDMI Connector infrastructure

Maxime Ripard
2024-02-26T13:22:46Z

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Hi Dmitry,

On Sun, Feb 25, 2024 at 04:50:00PM +0200, Dmitry Baryshkov wrote:
> On Thu, 22 Feb 2024 at 20:14, Maxime Ripard <mripard@kernel.org> wrote:
> > Here's a series that creates some extra infrastructure specifically
> > targeted at HDMI controllers.
> >
> > The idea behind this series came from a recent discussion on IRC during
> > which we discussed infoframes generation of i915 vs everything else.
> >
> > Infoframes generation code still requires some decent boilerplate, with
> > each driver doing some variation of it.
> >
> > In parallel, while working on vc4, we ended up converting a lot of i915
> > logic (mostly around format / bpc selection, and scrambler setup) to
> > apply on top of a driver that relies only on helpers.
> >
> > While currently sitting in the vc4 driver, none of that logic actually
> > relies on any driver or hardware-specific behaviour.
> >
> > The only missing piece to make it shareable are a bunch of extra
> > variables stored in a state (current bpc, format, RGB range selection,
> > etc.).
> >
> > The initial implementation was relying on some generic subclass of
> > drm_connector to address HDMI connectors, with a bunch of helpers that
> > will take care of all the "HDMI Spec" related code. Scrambler setup is
> > missing at the moment but can easily be plugged in.
> >
> > The feedback was that creating a connector subclass like was done for
> > writeback would prevent the adoption of those helpers since it couldn't
> > be used in all situations (like when the connector driver can implement
> > multiple output) and required more churn to cast between the
> > drm_connector and its subclass. The decision was thus to provide a set
> > of helper and to store the required variables in drm_connector and
> > drm_connector_state. This what has been implemented now.
> >
> > Hans Verkuil also expressed interest in implementing a mechanism in v4l2
> > to retrieve infoframes from HDMI receiver and implementing a tool to
> > decode (and eventually check) infoframes. His current work on
> > edid-decode to enable that based on that series can be found here:
> > https://git.linuxtv.org/hverkuil/edid-decode.git/log/?h=hverkuil
> >
> > And some more context here:
> > https://lore.kernel.org/dri-devel/50db7366-cd3d-4675-aaad-b857202234de@xs4all.nl/
> >
> > This series thus leverages the infoframe generation code to expose it
> > through debugfs.
>
> [...]
>
> >
> > Let me know what you think,
> > Maxime
>
> The overall idea looks great. I've started checking how I can use that
> for our msm devices family, which makes use of bridges and
> drm_bridge_connector.

Yeah, I had that in mind for a while too. I think it would be valuable,
but it's difficult as it is already so I didn't tackle it at first :)

The format negociation especially seems to divert quite a lot from what
i915 and vc4 have been doing, and that's user facing to some extent.

> My current idea is to extend the drm_bridge_funcs with the new
> callback to be called once the drm_connector has been instantiated.
> This way all the bridges can influence new connector.
> Another possibility is to follow drm_bridge_connector design closely
> and let it call into drm_connector_hdmi code if it detects that the
> last bridge is the HDMI one.
> WDYT?

I had the latter in mind, but I haven't really tried to reconcile the
connector state output_format with the
drm_atomic_helper_bridge_propagate_bus_fmt, and plug in the output_bpc
count too.

We would have to create the max_bpc properties from the HDMI connector,
and then propagate that upstream along the bridges I guess (or we can
just ignore it for now).

So, yeah, I had the latter in mind but it might turn out that the former
is actually easier. Both make sense to me at least.

Maxime

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