linux-kernel - Re: [PATCH v18 0/2] Add memory bandwidth management to NVIDIA Tegra DRM driver

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Date:   Sun, 18 Jul 2021 23:59:51 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Thierry Reding <thierry.reding@...il.com>
Cc:     linux-tegra@...r.kernel.org, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        Matt Merhar <mattmerhar@...tonmail.com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        Peter Geis <pgwipeout@...il.com>,
        Nicolas Chauvet <kwizart@...il.com>,
        Michał Mirosław <mirq-linux@...e.qmqm.pl>
Subject: Re: [PATCH v18 0/2] Add memory bandwidth management to NVIDIA Tegra
 DRM driver

21.06.2021 14:43, Dmitry Osipenko пишет:
> 21.06.2021 14:01, Thierry Reding пишет:
>> On Mon, Jun 21, 2021 at 07:19:15AM +0300, Dmitry Osipenko wrote:
>>> 07.06.2021 01:40, Dmitry Osipenko пишет:
>>>> 01.06.2021 07:21, Dmitry Osipenko пишет:
>>>>> This series adds memory bandwidth management to the NVIDIA Tegra DRM driver,
>>>>> which is done using interconnect framework. It fixes display corruption that
>>>>> happens due to insufficient memory bandwidth.
>>>>>
>>>>> Changelog:
>>>>>
>>>>> v18: - Moved total peak bandwidth from CRTC state to plane state and removed
>>>>>        dummy plane bandwidth state initialization from T186+ plane hub. This
>>>>>        was suggested by Thierry Reding to v17.
>>>>>
>>>>>      - I haven't done anything about the cursor's plane bandwidth which
>>>>>        doesn't contribute to overlapping bandwidths for a small sized
>>>>>        window because it works okay as-is.
>>>>
>>>> Thierry, will you take these patches for 5.14?
>>>>
>>>
>>> The display controller does _NOT_WORK_ properly without bandwidth
>>> management.
>>
>> That's surprising. So either it has never worked before (which I think
>> I'd know) or something has caused this regression recently. In the
>> latter case we need to identify what that was and revert (or fix) it.
> 
> The problem is caused by the support of dynamic memory frequency scaling
> which does a good job at keeping memory in a low power state during idle
> time. So display controller may not get enough bandwidth at the start of
> scanout if it won't request BW beforehand. This problem existed for many
> years on T124 and now T20/30 are also affected. The DC of T20 is the
> least tolerant to memory bandwidth troubles.
> 
> This problem is not critical, but it hurts user experience since high
> resolution modes may not work at all and display output may become
> distorted, requiring a DC reset.
> 
>>> Can we get this patch into 5.14? What is the problem?
>>
>> There was not enough time to review and test this, so I didn't feel
>> comfortable picking it up so close to the -rc6 cut-off. I plan to pick
>> this up early in the v5.14 release cycle and target v5.15.
> 
> Thank you for the explanation! It's not uncommon to forget about
> patches, so the silence worries me. I hoped that both the dynamic freq
> scaling and display BW support would be merged around the same time, but
> apparently we got a disconnect here.
> 

Reminder ping :)