RE: [PATCH] drm: xlnx: zynqmp_dpsub: Enable plane in atomic update

[Klymenko, Anatoliy]

Hi Laurent,

Thanks a lot for the review.

> -----Original Message-----
> From: Laurent Pinchart <laurent.pinchart@xxxxxxxxxxxxxxxx>
> Sent: Wednesday, May 22, 2024 8:32 AM
> To: Klymenko, Anatoliy <Anatoliy.Klymenko@xxxxxxx>
> Cc: Tomi Valkeinen <tomi.valkeinen@xxxxxxxxxxxxxxxx>; Maarten
> Lankhorst <maarten.lankhorst@xxxxxxxxxxxxxxx>; Maxime Ripard
> <mripard@xxxxxxxxxx>; Thomas Zimmermann <tzimmermann@xxxxxxx>;
> David Airlie <airlied@xxxxxxxxx>; Daniel Vetter <daniel@xxxxxxxx>;
> Simek, Michal <michal.simek@xxxxxxx>; dri-
> devel@xxxxxxxxxxxxxxxxxxxxx; linux-arm-kernel@xxxxxxxxxxxxxxxxxxx; linux-
> kernel@xxxxxxxxxxxxxxx
> Subject: Re: [PATCH] drm: xlnx: zynqmp_dpsub: Enable plane in atomic
> update
> 
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> 
> 
> Hi Anatoliy,
> 
> Thank you for the patch.
> 
> On Mon, May 20, 2024 at 08:22:31PM -0700, Anatoliy Klymenko wrote:
> > Unconditionally enable the DPSUB layer in the corresponding atomic
> plane
> > update callback. Setting the new display mode may require disabling
> and
> > re-enabling the CRTC. This effectively resets DPSUB to the default state
> > with all layers disabled.
> 
> Ah, I went through the code and I see that. Oops.
> 
> > The original implementation of the plane atomic
> > update enables the corresponding DPSUB layer only if the framebuffer
> > format has changed. This would leave the layer disabled after switching
> to
> > a different display mode with the same framebuffer format.
> 
> Do we need a Fixes: tag or has this issue been there since the beginning
> ?

Yes, this was introduced in the initial driver commit.

> 
> > Signed-off-by: Anatoliy Klymenko <anatoliy.klymenko@xxxxxxx>
> > ---
> >  drivers/gpu/drm/xlnx/zynqmp_kms.c | 5 ++---
> >  1 file changed, 2 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/xlnx/zynqmp_kms.c
> b/drivers/gpu/drm/xlnx/zynqmp_kms.c
> > index 43bf416b33d5..c4f038e34814 100644
> > --- a/drivers/gpu/drm/xlnx/zynqmp_kms.c
> > +++ b/drivers/gpu/drm/xlnx/zynqmp_kms.c
> > @@ -120,9 +120,8 @@ static void
> zynqmp_dpsub_plane_atomic_update(struct drm_plane *plane,
> >               zynqmp_disp_blend_set_global_alpha(dpsub->disp, true,
> >                                                  plane->state->alpha >> 8);
> >
> > -     /* Enable or re-enable the plane if the format has changed. */
> > -     if (format_changed)
> > -             zynqmp_disp_layer_enable(layer);
> > +     /* Enable or re-enable the plane. */
> > +     zynqmp_disp_layer_enable(layer);
> 
> This should be safe for now, as the function will just write the plane
> registers with identical values. The waste of CPU cycles may not be a
> big issue, even if it would be best to avoid it.
> 

The CPU time wasted on doubling down layer enablement is neglectable
compared to DP link training time.

> What bothers me more is that we may be working around a larger
> problem.
> Resetting the CRTC when disabling it affects the hardware state of the
> whole device, and thus the state of all software DRM objects. The
> hardware and software states lose sync. Maybe this patch is all that is
> needed for now, but other similar issues could pop up in the future.
> 

I had similar thoughts about proper HW state tracking, but that would be
rather large rework.

> Would it be possible, at atomic check time, to detect the objects whose
> hardware state will need to be synced, and marked that in their state ?
> Then in zynqmp_dpsub_plane_atomic_update() you could re-enable the
> layer
> based on that. You may need to subclass the drm_plane_state if there's
> no field in that structure that is suitable to store the information.
> The format_changed local variable would move there.
> 

Thank you for the idea! I'll check it out.

> >  }
> >
> >  static const struct drm_plane_helper_funcs
> zynqmp_dpsub_plane_helper_funcs = {
> >
> > ---
> > base-commit: 673087d8b023faf34b84e8faf63bbeea3da87bab
> > change-id: 20240520-dp-layer-enable-7b561af29ca8
> 
> --
> Regards,
> 
> Laurent Pinchart
--
Thank you,
Anatoliy