linux-kernel - Re: [PATCH v17 2/4] drm/msm/dpu: Defer SSPP allocation until CRTC check

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Message-ID: <CABymUCP6ZDGtEJeQSZ48x8TZyJ4gKVDC+JzJRz-tZ0ksCUCqsA@mail.gmail.com>
Date: Mon, 26 Jan 2026 18:06:24 +0800
From: Jun Nie <jun.nie@...aro.org>
To: Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>
Cc: Abhinav Kumar <abhinav.kumar@...ux.dev>, Dmitry Baryshkov <lumag@...nel.org>, 
	Sean Paul <sean@...rly.run>, Marijn Suijten <marijn.suijten@...ainline.org>, 
	David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>, 
	Rob Clark <robin.clark@....qualcomm.com>, Neil Armstrong <neil.armstrong@...aro.org>, 
	linux-arm-msm@...r.kernel.org, dri-devel@...ts.freedesktop.org, 
	freedreno@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v17 2/4] drm/msm/dpu: Defer SSPP allocation until CRTC check

Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com> 于2026年1月22日周四 18:22写道：
>
> On Thu, Jan 22, 2026 at 02:03:25PM +0800, Jun Nie wrote:
> > Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com> 于2026年1月21日周三 17:30写道：
> > >
> > > On Wed, Jan 21, 2026 at 04:01:51PM +0800, Jun Nie wrote:
> > > > Currently, plane splitting and SSPP allocation occur during the plane
> > > > check phase. Defer these operations until dpu_assign_plane_resources()
> > > > is called from the CRTC side to ensure the topology information from
> > > > the CRTC check is available.
> > >
> > > Why is it important? What is broken otherwise?
> >
> > I see. Thanks! Will add below lines in next version.
> >
> > By default, the plane check occurs before the CRTC check.
> > Without topology information from the CRTC, plane splitting
> > cannot be properly executed. Consequently, the SSPP
> > engine starts without a valid memory address, which triggers
> > an IOMMU warning.
>
> What is plane splitting? Write commit message for somebody who doesn't
> exactly know what is going on.

Thanks for the suggestion! Any more revise is needed?

Currently, splitting plane into SSPP rectangles the allocation occur
during the plane check phase, so that a plane can be supported by
multiple hardware pipe. While pipe topology is decided in CRTC check.
By default, the plane check occurs before the CRTC check in DRM
framework. Without topology information from the CRTC, plane splitting
cannot be properly executed. Consequently, the SSPP engine starts
without a valid memory address, which triggers IOMMU warning.

Defer above plane operations until dpu_assign_plane_resources()
is called from the CRTC side to ensure the topology information from
the CRTC check is available.


Regards,
Jun
>
>
> --
> With best wishes
> Dmitry