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Subject: Re: [PATCH] gpu/drm: Remove TTM_PL_FLAG_WC of VRAM to fix
 writecombine issue for Loongson64
To:     Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
        Tiezhu Yang <yangtiezhu@loongson.cn>
Cc:     Alex Deucher <alexander.deucher@amd.com>, christian.koenig@amd.com,
        Huacai Chen <chenhc@lemote.com>, linux-mips@vger.kernel.org,
        amd-gfx@lists.freedesktop.org, linux-kernel@vger.kernel.org
References: <1596871502-3432-1-git-send-email-yangtiezhu@loongson.cn>
 <20200808134147.GA5772@alpha.franken.de>
From:   Jiaxun Yang <jiaxun.yang@flygoat.com>
Message-ID: <b7b16df1-d661-d59a-005b-da594ce9fc95@flygoat.com>
Date:   Sat, 8 Aug 2020 21:50:59 +0800
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?? 2020/8/8 ????9:41, Thomas Bogendoerfer д??:
> 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).

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.

Thanks.

- Jiaxun