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Date:   Mon, 10 Aug 2020 08:58:13 +0800
From:   Tiezhu Yang <>
To:     Christian König <>,
        Jiaxun Yang <>,
        Thomas Bogendoerfer <>
Cc:     Alex Deucher <>,
        Huacai Chen <>,,,
Subject: Re: [PATCH] gpu/drm: Remove TTM_PL_FLAG_WC of VRAM to fix
 writecombine issue for Loongson64

On 08/09/2020 08:13 PM, Christian König wrote:
> Am 08.08.20 um 15:50 schrieb Jiaxun Yang:
>> 在 2020/8/8 下午9:41, Thomas Bogendoerfer 写道:
>>> On Sat, Aug 08, 2020 at 03:25:02PM +0800, Tiezhu Yang wrote:
>>>> Loongson processors have a writecombine issue that maybe failed to
>>>> write back framebuffer used with ATI Radeon or AMD GPU at times,
>>>> after commit 8a08e50cee66 ("drm: Permit video-buffers writecombine
>>>> mapping for MIPS"), there exists some errors such as blurred screen
>>>> and lockup, and so on.
>>>> Remove the flag TTM_PL_FLAG_WC of VRAM to fix writecombine issue for
>>>> Loongson64 to work well with ATI Radeon or AMD GPU, and it has no any
>>>> influence on the other platforms.
>>> well it's not my call to take or reject this patch, but I already
>>> indicated it might be better to disable writecombine on the CPU
>>> detection side (or do you have other devices where writecombining
>>> works ?). Something like below will disbale it for all loongson64 CPUs.
>>> If you now find out where it works and where it doesn't, you can even
>>> reduce it to the required minium of affected CPUs.
>> Hi Tiezhu, Thomas,
>> Yes, writecombine works well on LS7A's internal GPU....
>> And even works well with some AMD GPUs (in my case, RX550).
> In this case the patch is a clear NAK since you haven't root caused 
> the issue and are just working around it in a very questionable manner.
>> Tiezhu, is it possible to investigate the issue deeper in Loongson?
>> Probably we just need to add some barrier to maintain the data 
>> coherency,
>> or disable writecombine for AMD GPU's command buffer and leave 
>> texture/frame
>> buffer wc accelerated.
> Have you moved any buffer to VRAM and forgot to add an HDP 
> flush/invalidate?
> The acceleration is not much of a problem, but if WC doesn't work in 
> general you need to disable it for the whole CPU and not for 
> individual drivers.

Hi Thomas, Jiaxun and Christian,

Thank you very much for your suggestions.

Actually, this patch is a temporary solution to just make it work well,
it is not a proper and final solution.

I understand your opinions, it will take some time to find the root cause.


> Regards,
> Christian.
>> Thanks.
>> - Jiaxun

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