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

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Message-ID: <bbd8e584-4398-40da-9759-0c27f34214d3@oss.qualcomm.com>
Date: Mon, 26 Jan 2026 12:49:07 +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 12:29, Jun Nie wrote:
> Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com> 于2026年1月26日周一 18:13写道：
>>
>> 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.
>>
>>
> Thanks for the patience!
> 
> 
> Currently, splitting plane into SSPP rectangles and allocation occur
> during the plane check phase. When virtual plane is enabled to support
> quad-pipe topology later, 2 SSPPs with 4 rect will be needed, so that
> a plane can be supported by 4 hardware pipes. And pipe number is

number of pipes

> decided in CRTC check per interface number, resolution and hardware
> feature.

Okay, but IOMMU errors were reported with virtual planes being disabled. 
So how is it relevant?

> 
> By default, the plane check occurs before the CRTC check in DRM
> framework. Without topology information from the CRTC, plane splitting

WHat is plane splitting?

> will be skipped for the first time as pipe number is 0. Consequently,
> the SSPP engine starts without a valid memory address, which triggers
> IOMMU fault.
> 
> Defer above plane related 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


-- 
With best wishes
Dmitry