Re: [PATCH] gpu/drm: Remove TTM_PL_FLAG_WC of VRAM to fix writecombine issue for Loongson64

From: "Michel Dänzer" <michel@daenzer.net>
To: "Christian König" <christian.koenig@amd.com>,
	"Jiaxun Yang" <jiaxun.yang@flygoat.com>,
	"Thomas Bogendoerfer" <tsbogend@alpha.franken.de>,
	"Tiezhu Yang" <yangtiezhu@loongson.cn>
Cc: Alex Deucher <alexander.deucher@amd.com>,
	Huacai Chen <chenhc@lemote.com>,
	linux-mips@vger.kernel.org, amd-gfx@lists.freedesktop.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH] gpu/drm: Remove TTM_PL_FLAG_WC of VRAM to fix writecombine issue for Loongson64
Date: Mon, 10 Aug 2020 12:50:45 +0200	[thread overview]
Message-ID: <2f3fd325-8093-98e7-5bc8-75490258baac@daenzer.net> (raw)
In-Reply-To: <38857c24-25c4-cff3-569e-5bcb773bfae6@amd.com>

On 2020-08-09 2:13 p.m., 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.

To be fair though, amdgpu & radeon are already disabling write-combining
for system memory pages in 32-bit x86 kernels for similar reasons.

-- 
Earthling Michel Dänzer               |               https://redhat.com
Libre software enthusiast             |             Mesa and X developer
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