[RFC PATCH 1/8] drm: writeback: Refactor drm_writeback_connector structure

[Dmitry Baryshkov]

On Thu, Aug 14, 2025 at 05:13:54PM +0100, liviu.dudau at arm.com wrote:
> Hi,
> 
> On Wed, Aug 13, 2025 at 10:04:22AM +0000, Kandpal, Suraj wrote:
> > > > > };
> > > >
> > > > I still don't like that. This really doesn't belong here. If anything,
> > > > the drm_connector for writeback belongs to drm_crtc.
> > > 
> > > Why? We already have generic HDMI field inside drm_connector. I am really
> > > hoping to be able to land DP parts next to it. In theory we can have a DVI-
> > > specific entry there (e.g. with the subconnector type).
> > > The idea is not to limit how the drivers subclass those structures.
> > > 
> > > I don't see a good case why WB should deviate from that design.
> > > 
> > > > If the issue is that some drivers need a custom drm_connector
> > > > subclass, then I'd rather turn the connector field of
> > > > drm_writeback_connector into a pointer.
> > > 
> > > Having a pointer requires additional ops in order to get drm_connector from
> > > WB code and vice versa. Having drm_connector_wb inside drm_connector
> > > saves us from those ops (which don't manifest for any other kind of structure).
> > > Nor will it take any more space since union will reuse space already taken up by
> > > HDMI part.
> > > 
> > > >
> > 
> > Seems like this thread has died. We need to get a conclusion on the design.
> > Laurent do you have any issue with the design given Dmitry's explanation as to why this
> > Design is good for drm_writeback_connector.
> 
> I'm with Laurent here. The idea for drm_connector (and a lot of drm structures) are to
> be used as base "classes" for extended structures. I don't know why HDMI connector ended
> up inside drm_connector as not all connectors have HDMI functionality, but that's a cleanup
> for another day.

Maybe Maxime can better comment on it, but I think it was made exactly
for the purpose of not limiting the driver's design. For example, a lot
of drivers subclass drm_connector via drm_bridge_connector. If
struct drm_connector_hdmi was a wrapper around struct drm_connector,
then it would have been impossible to use HDMI helpers for bridge
drivers, while current design freely allows any driver to utilize
corresponding library code.

> 
> drm_writeback_connector uses the 'base' drm_connector only for a few things, mostly in
> __drm_writeback_connector_init() and prepare_job()/cleanup_job(). In _init() we just setup
> the properties and the encoder after we disable interlacing. prepare_job()/cleanup_job()
> is another workaround to be to some custom ops some drivers might want for signalling. So
> we should be able to convert the 'base' drm_connector to a pointer relatively easy. We shouldn't
> need to get to the drm_connector from a drm_writeback_connector() outside drm_writeback.c.
> 
> Then it looks like what we need is a __drm_writeback_connector_init_with_connector() where we
> can pass a base pointer and remember it. Maybe an extra parameter to existing init functions,
> or a new one that skips the encoder initialisation entirely.

I've refactored out drm_encoder, that's not a big problem. The bigger
problem is the embedded 'drm_connector base' field. It's really use to
overlook that it's not initialized / not used.


-- 
With best wishes
Dmitry