linux-kernel - Re: [RFC PATCH 1/8] drm: writeback: Refactor drm_writeback

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Message-ID: <aJ4LQvqli36TlETu@e110455-lin.cambridge.arm.com>
Date: Thu, 14 Aug 2025 17:13:54 +0100
From: "liviu.dudau@....com" <liviu.dudau@....com>
To: "Kandpal, Suraj" <suraj.kandpal@...el.com>
Cc: Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>,
	Laurent Pinchart <laurent.pinchart@...asonboard.com>,
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	"louis.chauvet@...tlin.com" <louis.chauvet@...tlin.com>
Subject: Re: [RFC PATCH 1/8] drm: writeback: Refactor drm_writeback_connector
 structure

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.

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.

Best regards,
Liviu


> 
> Regards,
> Suraj Kandpal
> 
> > > > I plan to add drm_connector_dp in a similar way, covering DP needs
> > > > (currently WIP).
> > 
> > --
> > With best wishes
> > Dmitry

-- 
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