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

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Date: Wed, 22 May 2024 18:14:37 +0000
From: "Klymenko, Anatoliy" <Anatoliy.Klymenko@....com>
To: Laurent Pinchart <laurent.pinchart@...asonboard.com>
CC: Tomi Valkeinen <tomi.valkeinen@...asonboard.com>, Maarten Lankhorst
	<maarten.lankhorst@...ux.intel.com>, Maxime Ripard <mripard@...nel.org>,
	Thomas Zimmermann <tzimmermann@...e.de>, David Airlie <airlied@...il.com>,
	Daniel Vetter <daniel@...ll.ch>, "Simek, Michal" <michal.simek@....com>,
	"dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
	"linux-arm-kernel@...ts.infradead.org"
	<linux-arm-kernel@...ts.infradead.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] drm: xlnx: zynqmp_dpsub: Enable plane in atomic update



> -----Original Message-----
> From: Laurent Pinchart <laurent.pinchart@...asonboard.com>
> Sent: Wednesday, May 22, 2024 11:11 AM
> To: Klymenko, Anatoliy <Anatoliy.Klymenko@....com>
> Cc: Tomi Valkeinen <tomi.valkeinen@...asonboard.com>; Maarten
> Lankhorst <maarten.lankhorst@...ux.intel.com>; Maxime Ripard
> <mripard@...nel.org>; Thomas Zimmermann <tzimmermann@...e.de>;
> David Airlie <airlied@...il.com>; Daniel Vetter <daniel@...ll.ch>;
> Simek, Michal <michal.simek@....com>; dri-
> devel@...ts.freedesktop.org; linux-arm-kernel@...ts.infradead.org; linux-
> kernel@...r.kernel.org
> Subject: Re: [PATCH] drm: xlnx: zynqmp_dpsub: Enable plane in atomic
> update
> 
> Caution: This message originated from an External Source. Use proper
> caution when opening attachments, clicking links, or responding.
> 
> 
> On Wed, May 22, 2024 at 05:52:56PM +0000, Klymenko, Anatoliy wrote:
> > On Wednesday, May 22, 2024 8:32 AM, Laurent Pinchart wrote:
> > > 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@....com>
> > > > ---
> > > >  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.
> 
> Good point.
> 
> > > 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.
> 
> If it ends up being overkill I think this patch is probably OK. A
> comment to explain the reasoning in the code would be nice though.
> 

Sure, I'll add this.

> > > >  }
> > > >
> > > >  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