linux-kernel - Re: [PATCH v2] loongarch/mm: disable WUC for pgprot_writecombine as same as ioremap

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Message-ID: <73ddfc81-3f94-4403-8bb2-d537742a7042@loongson.cn>
Date:   Fri, 13 Oct 2023 20:51:46 +0800
From:   Sui Jingfeng <suijingfeng@...ngson.cn>
To:     Xi Ruoyao <xry111@...111.site>, WANG Xuerui <kernel@...0n.name>,
        Icenowy Zheng <uwu@...nowy.me>,
        Huacai Chen <chenhuacai@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Weihao Li <liweihao@...ngson.cn>,
        "Mike Rapoport (IBM)" <rppt@...nel.org>,
        Jun Yi <yijun@...ngson.cn>, Baoquan He <bhe@...hat.com>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        David Hildenbrand <david@...hat.com>,
        Hongchen Zhang <zhanghongchen@...ngson.cn>,
        Binbin Zhou <zhoubinbin@...ngson.cn>,
        Zhen Lei <thunder.leizhen@...wei.com>,
        Tiezhu Yang <yangtiezhu@...ngson.cn>,
        Thomas Gleixner <tglx@...utronix.de>,
        Zhihong Dong <donmor3000@...mail.com>,
        loongarch@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] loongarch/mm: disable WUC for pgprot_writecombine as
 same as ioremap_wc

Hi,


On 2023/10/10 20:26, Xi Ruoyao wrote:
>> - For buffers at VRAM(device memory), we replace the WC mappings with uncached mappings.
>> - For buffers reside in RAM, we replace the WC mappings with cached mappings.
>>
>> By this way, we were able to minimum the side effects, and meet the usable requirements
>> for all of the GPU drivers.
> AFAIK there has been some clear NAK from DRM maintainers towards this
> "approach".  So it's not possible to be applied upstream.


Yeah, domain specific reviewers are really hard to persuade to accept our solution.
Probably because they are not know our hardware very well. But the side effects of
this patch is really too hurt to accept. In this case, if you really want to make
a progress by workaround. I think you could try scan the PCI device in the whole
system on boot time. Turn off the WC mappings when there is a AMD or ATI GPUs found.
Leave the the WC mappings open at the rest of cases. But this is yet another ugly
workaround. Perhaps it is a bit of better than this because there is no way left.