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Date:   Sun, 9 Aug 2020 14:13:15 +0200
From:   Christian König <>
To:     Jiaxun Yang <>,
        Thomas Bogendoerfer <>,
        Tiezhu Yang <>
Cc:     Alex Deucher <>,
        Huacai Chen <>,,,
Subject: Re: [PATCH] gpu/drm: Remove TTM_PL_FLAG_WC of VRAM to fix
 writecombine issue for Loongson64

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 


> Thanks.
> - Jiaxun

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