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

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Message-ID: <91d590de-fa00-4df3-923f-b49ad00cd9da@oss.qualcomm.com>
Date: Mon, 26 Jan 2026 12:13:27 +0200
From: Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>
To: Jun Nie <jun.nie@...aro.org>
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

On 26/01/2026 12:06, Jun Nie wrote:
> 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?

Sadly enough the text below is not a significant improvement.

> 
> 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.

What does this mean? Without virtual planes in place, there are no 
multiple hardware pipes.

> While pipe topology is decided in CRTC check.

?? What does it mean here?

> 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.

What does 'properly' mean here? How is it executed? What happens?

> Consequently, the SSPP engine starts
> without a valid memory address, which triggers IOMMU warning.

IOMMU faults. There are no "warnings".

> 
> 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.



-- 
With best wishes
Dmitry