Re: [PATCH RFC 00/13] drm/connector: Create HDMI Connector infrastructure

[Daniel Vetter <daniel@ffwll.ch>]

On Tue, Aug 22, 2023 at 04:35:55PM +0200, Maxime Ripard wrote:
> Hi,
> 
> On Tue, Aug 22, 2023 at 04:16:08PM +0200, Daniel Vetter wrote:
> > On Mon, Aug 14, 2023 at 03:56:12PM +0200, Maxime Ripard wrote:
> > > Here's a series that creates a subclass of drm_connector 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 piec to make it shareable are a bunch of extra
> > > variables stored in a state (current bpc, format, RGB range selection,
> > > etc.).
> > > 
> > > Thus, I decided to create 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.
> > > 
> > > Last week, Hans Verkuil also expressed interest in retrieving the
> > > infoframes generated from userspace to create an infoframe-decode tool.
> > > This series thus leverages the infoframe generation code to expose it
> > > through debugfs.
> > > 
> > > This entire series is only build-tested at the moment. Let me know what
> > > you think,
> >
> > I think the idea overall makes sense, we we probably need it to roll out
> > actual hdmi support to all the hdmi drivers we have. But there's the
> > eternal issue of "C sucks at multiple inheritance".
> > 
> > Which means if you have a driver that subclasses drm_connector already for
> > it's driver needs it defacto cannot, or only under some serious pains, use
> > this.
> 
> That's what vc4 is doing, and it went fine I think? it was mostly a
> matter of subclassing drm_hdmi_connector instead of drm_connector, and
> adjusting the various pointers and accessors here and there.
> 
> It does create a fairly big diffstat, but nothing too painful.

Yeah it's the massive churn that's the pain for refactoring existing
bigger drivers.

Plus what do you do when you both need a hdmi connector and a dp connector
(or a writeback connector).

> > Which is kinda why in practice we tend to not subclass, but stuff
> > subclass fields into a name sub-structure. So essentially struct
> > drm_connector.hdmi and struct drm_connector_state.hdmi instead of
> > drm_hdmi_connector and drm_hdmi_connector_state. The helper functions to
> > set it all up would all still be the same roughly. It's less typesafe but
> > I think the gain in practical use (like you could make i915 use the
> > helpers probably, which with this approach here is practically
> > impossible).
> 
> Ack.
> 
> > The only other nit is that we probably want to put some of the hdmi
> > properties into struct drm_mode_config because there's no reason to have
> > per-connector valid values.
> 
> What property would you want to move?

The rgb broadcast property looked very much like it's connector invariant.
Just the one I noticed, I didn't check all the others.

> > Also, it might be really good if you can find a co-conspirator who also
> > wants to use this in their driver, then with some i915 extracting we'd
> > have three, which should ensure the helper api is solid.
> 
> I can convert sunxi (old) HDMI driver if needed. I'm not sure how
> helpful it would be since it doesn't support bpc > 8, but it could be a
> nice showcase still for "simple" HDMI controllers.

Yeah that might be good. Or perhaps poke Rob Clark whether msm is
interested and someone could do a conversion for dpu5 or so?

Cheers, Sima
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch