[Intel-gfx] [PATCH 5/6] drm/rcar_du: changes to rcar-du driver resulting from drm_writeback_connector structure changes

[Laurent Pinchart]

Hello,

On Mon, Feb 28, 2022 at 02:28:27PM +0200, Laurent Pinchart wrote:
> On Mon, Feb 28, 2022 at 02:09:15PM +0200, Jani Nikula wrote:
> > On Mon, 28 Feb 2022, Laurent Pinchart wrote:
> > > On Sat, Feb 26, 2022 at 10:27:59AM -0800, Rob Clark wrote:
> > >> On Wed, Feb 2, 2022 at 7:41 AM Jani Nikula wrote:
> > >> > On Wed, 02 Feb 2022, Laurent Pinchart wrote:
> > >> > > On Wed, Feb 02, 2022 at 03:15:03PM +0200, Jani Nikula wrote:
> > >> > >> On Wed, 02 Feb 2022, Laurent Pinchart wrote:
> > >> > >> > On Wed, Feb 02, 2022 at 02:24:28PM +0530, Kandpal Suraj wrote:
> > >> > >> >> Changing rcar_du driver to accomadate the change of
> > >> > >> >> drm_writeback_connector.base and drm_writeback_connector.encoder
> > >> > >> >> to a pointer the reason for which is explained in the
> > >> > >> >> Patch(drm: add writeback pointers to drm_connector).
> > >> > >> >>
> > >> > >> >> Signed-off-by: Kandpal Suraj <suraj.kandpal at intel.com>
> > >> > >> >> ---
> > >> > >> >>  drivers/gpu/drm/rcar-du/rcar_du_crtc.h      | 2 ++
> > >> > >> >>  drivers/gpu/drm/rcar-du/rcar_du_writeback.c | 8 +++++---
> > >> > >> >>  2 files changed, 7 insertions(+), 3 deletions(-)
> > >> > >> >>
> > >> > >> >> diff --git a/drivers/gpu/drm/rcar-du/rcar_du_crtc.h b/drivers/gpu/drm/rcar-du/rcar_du_crtc.h
> > >> > >> >> index 66e8839db708..68f387a04502 100644
> > >> > >> >> --- a/drivers/gpu/drm/rcar-du/rcar_du_crtc.h
> > >> > >> >> +++ b/drivers/gpu/drm/rcar-du/rcar_du_crtc.h
> > >> > >> >> @@ -72,6 +72,8 @@ struct rcar_du_crtc {
> > >> > >> >>   const char *const *sources;
> > >> > >> >>   unsigned int sources_count;
> > >> > >> >>
> > >> > >> >> + struct drm_connector connector;
> > >> > >> >> + struct drm_encoder encoder;
> > >> > >> >
> > >> > >> > Those fields are, at best, poorly named. Furthermore, there's no need in
> > >> > >> > this driver or in other drivers using drm_writeback_connector to create
> > >> > >> > an encoder or connector manually. Let's not polute all drivers because
> > >> > >> > i915 doesn't have its abstractions right.
> > >> > >>
> > >> > >> i915 uses the quite common model for struct inheritance:
> > >> > >>
> > >> > >>      struct intel_connector {
> > >> > >>              struct drm_connector base;
> > >> > >>              /* ... */
> > >> > >>      }
> > >> > >>
> > >> > >> Same with at least amd, ast, fsl-dcu, hisilicon, mga200, msm, nouveau,
> > >> > >> radeon, tilcdc, and vboxvideo.
> > >> > >>
> > >> > >> We could argue about the relative merits of that abstraction, but I
> > >> > >> think the bottom line is that it's popular and the drivers using it are
> > >> > >> not going to be persuaded to move away from it.
> > >> > >
> > >> > > Nobody said inheritance is bad.
> > >> > >
> > >> > >> It's no coincidence that the drivers who've implemented writeback so far
> > >> > >> (komeda, mali, rcar-du, vc4, and vkms) do not use the abstraction,
> > >> > >> because the drm_writeback_connector midlayer does, forcing the issue.
> > >> > >
> > >> > > Are you sure it's not a coincidence ? :-)
> > >> > >
> > >> > > The encoder and especially connector created by drm_writeback_connector
> > >> > > are there only because KMS requires a drm_encoder and a drm_connector to
> > >> > > be exposed to userspace (and I could argue that using a connector for
> > >> > > writeback is a hack, but that won't change). The connector is "virtual",
> > >> > > I still fail to see why i915 or any other driver would need to wrap it
> > >> > > into something else. The whole point of the drm_writeback_connector
> > >> > > abstraction is that drivers do not have to manage the writeback
> > >> > > drm_connector manually, they shouldn't touch it at all.
> > >> >
> > >> > The thing is, drm_writeback_connector_init() calling
> > >> > drm_connector_init() on the drm_connector embedded in
> > >> > drm_writeback_connector leads to that connector being added to the
> > >> > drm_device's list of connectors. Ditto for the encoder.
> > >> >
> > >> > All the driver code that handles drm_connectors would need to take into
> > >> > account they might not be embedded in intel_connector. Throughout the
> > >> > driver. Ditto for the encoders.
> > >> 
> > >> The assumption that a connector is embedded in intel_connector doesn't
> > >> really play that well with how bridge and panel connectors work.. so
> > >> in general this seems like a good thing to unwind.
> > >> 
> > >> But as a point of practicality, i915 is a large driver covering a lot
> > >> of generations of hw with a lot of users.  So I can understand
> > >> changing this design isn't something that can happen quickly or
> > >> easily.  IMO we should allow i915 to create it's own connector for
> > >> writeback, and just document clearly that this isn't the approach new
> > >> drivers should take.  I mean, I understand idealism, but sometimes a
> > >> dose of pragmatism is needed. :-)
> > >
> > > i915 is big, but so is Intel. It's not fair to treat everybody else as a
> > > second class citizen and let Intel get away without doing its homework.
> > 
> > Laurent, as you accuse us of not doing our homework, I'll point out that
> > we've been embedding drm crtc, encoder and connector ever since
> > modesetting support was added to i915 in 2008, since before *any* of the
> > things you now use as a rationale for asking us to do a massive rewrite
> > of the driver existed.
> > 
> > It's been ok to embed those structures for well over ten years. It's a
> > common pattern, basically throughout the kernel. Other drivers do it
> > too, not just i915. There hasn't been the slightest hint this should not
> > be done until this very conversation.
> > 
> > > I want to see this refactoring effort moving forward in i915 (and moving
> > > to drm_bridge would then be a good idea too). If writeback support in
> > > i915 urgent, then we can discuss *temporary* pragmatic stopgap measures,
> > > but not without a real effort to fix the core issue.
> > 
> > I think the onus is on you to first convince everyone that embedding the
> > drm core kms structures is an antipattern that all drivers, not just
> > i915, should stop using. In OO terms, you're saying they are classes
> > that should be final and not extended.
> > 
> > And even then, to be totally honest, refactoring the structures is not
> > going to be anywhere near the top of our list of things to do, for a
> > very long time.
> 
> I may have not expressed myself correctly. There's nothing wrong as such
> in embedded those structures in driver-specific structures (a.k.a. C
> inheritance). That doesn't need to change (albeit for drm_encoder I
> think we should move away from that pattern, but that's an entirely
> different issue, and nothing that needs to be addressed soonà.
> 
> The issue here is assuming that every drm_connector instance can be
> up-casted to an i915-specific structure.

Thinking some more about this, I wonder a way forward could be to drop
the writeback connectors from the connectors list, or at least make them
invisible to drivers. The connectors list is used extensively for two
different purposes: tracking all drm_connector instances, and tracking
all real connectors. The former is mostly needed by the DRM core for
bookkeeping purposes and to expose all drm_connector instances to
userspace, while the latter is also used by drivers, in many cases in
locations that don't expect writeback connectors. Using a drm_connector
to implement writeback isn't something we can revisit, but we could
avoid exposing that to drivers by considering "real" connectors and
writeback connectors two different types of entities in the APIs the DRM
core exposes to drivers. What do you think, would it help for i915 ?

> > >> > The point is, you can't initialize a connector or an encoder for a
> > >> > drm_device in isolation of the rest of the driver, even if it were
> > >> > supposed to be hidden away.

-- 
Regards,

Laurent Pinchart