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Subject: Re: [PATCH] MIPS: Introduce cmdline argument writecombine=
To:     Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
        Tiezhu Yang <yangtiezhu@loongson.cn>
Cc:     Huacai Chen <chenhc@lemote.com>, linux-mips@vger.kernel.org,
        linux-kernel@vger.kernel.org
References: <1596697741-3561-1-git-send-email-yangtiezhu@loongson.cn>
 <10e04885-b775-e7db-b927-6400382fd32d@flygoat.com>
 <cc0c8b89-748d-0d38-bcc8-1c2dbb0996bf@loongson.cn>
 <20200806101733.GA8136@alpha.franken.de>
From:   Jiaxun Yang <jiaxun.yang@flygoat.com>
Message-ID: <9e563c75-ffaa-45b4-0e99-25a8b1eff828@flygoat.com>
Date:   Thu, 6 Aug 2020 19:56:20 +0800
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在 2020/8/6 下午6:17, Thomas Bogendoerfer 写道:
> On Thu, Aug 06, 2020 at 04:32:13PM +0800, Tiezhu Yang wrote:
>> On 08/06/2020 03:39 PM, Jiaxun Yang wrote:
>>>
>>> 在 2020/8/6 下午3:09, Tiezhu Yang 写道:
>>>> 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.
>>>>
>>>> With this patch, disable writecombine by default for Loongson64 to
>>>> work well with ATI Radeon or AMD GPU, and it has no influence on the
>>>> other platforms due to writecombine is enabled by default.
>>>>
>>>> Additionally, if it is necessary, writecombine=on can be set manually
>>>> in the cmdline to enhance the performance for Loongson LS7A integrated
>>>> graphics in the future.
>>>>
>>>> [   60.958721] radeon 0000:03:00.0: ring 0 stalled for more than
>>>> 10079msec
>>>> [   60.965315] radeon 0000:03:00.0: GPU lockup (current fence id
>>>> 0x0000000000000112 last fence id 0x000000000000011d on ring 0)
>>>> [   60.976525] radeon 0000:03:00.0: ring 3 stalled for more than
>>>> 10086msec
>>>> [   60.983156] radeon 0000:03:00.0: GPU lockup (current fence id
>>>> 0x0000000000000374 last fence id 0x00000000000003a8 on ring 3)
>>> Hi Tiezhu,
>>>
>>> Thanks for your patch.
>>> Personally I didn't have any issue with writecombine on my test systems,
>>> but there
>>> are some complains about unstable graphic card from users. So generally a
>>> cmdline
>>> writecombine switch is necessary.
> no, if there is hardware which can't work with writecombining enabled
> the driver should disable it by it's own and not by some user switch.
> It might even be better to revert the patch enabling writecombining
> blindly and add code to enable it for hardware where it works.

Our current problem is Loongson's writecombine implementation seems buggy.
This is our platform issue rather than target hardware issue.
And we don't even know which hardware is known to be good. The same graphic
card became a different story on different user's hand.

However, turning off writecombine would cause a visible performance 
regression
on graphic.

I understood what Teizhu thought. For entry-level users, we don't want 
to trouble
them, so we have writecombine disabled by default. However, for advanced 
user
trying to tweak their system, we should leave a switch for them to get 
it back.

Thanks.

- Jiaxun

>
> Thomas.
>