Re: [PATCH] MIPS: CPU_LOONGSON2EF need software to maintain cache consistency

From: Jiaxun Yang <jiaxun.yang@flygoat.com>
To: Lichao Liu <liulichao@loongson.cn>
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
	Paul Burton <paulburton@kernel.org>,
	Robin Murphy <robin.murphy@arm.com>,
	Geert Uytterhoeven <geert@linux-m68k.org>,
	Max Filippov <jcmvbkbc@gmail.com>,
	yuanjunqing@loongson.cn, linux-mips@vger.kernel.org
Subject: Re: [PATCH] MIPS: CPU_LOONGSON2EF need software to maintain cache consistency
Date: Tue, 26 May 2020 21:25:44 +0800	[thread overview]
Message-ID: <20200526212544.1737411f@halation.net.flygoat.com> (raw)
In-Reply-To: <e9c015c2-b979-27c8-5f43-7af8d43174c5@loongson.cn>

On Tue, 26 May 2020 20:40:28 +0800
Lichao Liu <liulichao@loongson.cn> wrote:

> On 2020/5/26 下午7:38, Jiaxun Yang wrote:
> > On Tue, 26 May 2020 19:14:38 +0800
> > Lichao Liu <liulichao@loongson.cn> wrote:
> >  
> >> CPU_LOONGSON2EF need software to maintain cache consistency,
> >> so modify the 'cpu_needs_post_dma_flush' function to return true
> >> when the cpu type is CPU_LOONGSON2EF.  
> >
> > Hi Lichao,
> >
> > I don't think that's required for Loongson-2EF,
> >
> > According to the comment in code:
> >
> > The affected CPUs below in 'cpu_needs_post_dma_flush()' can
> > speculatively
> > fill random cachelines with stale data at any time, requiring an
> > extra flush post-DMA.
> >
> > And according to my understanding that's not going to happen on
> > Loongson-2EF. We're always allocating coherent DMA memory in
> > uncached range, Loongson-2EF's writeback policy will ensure it
> > won't writeback random lines to the memory but only modified dirty
> > lines.
> >
> > We've been fine without post flush for almost 10 years, there is no
> > stability issue revealed.
> >
> > Btw: Please keep me CCed for Loongson-2EF patches. I'm not very
> > active on 2EF development but I'll still review patches.
> >
> > Thanks.
> >  
> Hi Jiaxun,
> 
> Loongson-2EF need software maintain cache consistency, So when using 
> streaming DMA, software needs to maintain consistency.
> 
> dma_map_single() is correct, but dma_unmap_single is wrong. 
> 
> The function call path:
> 'dma_unmap_single->dma_unmap_page_attrs->dma_direct_unmap_page->
>  dma_direct_sync_single_for_cpu->arch_sync_dma_for_cpu->
>  cpu_needs_post_dma_flush'
> 
> In current version, 'cpu_needs_post_dma_flush' will return false 
> at Loongon-2EF platform, and dma_unmap_single will not invalidate
> cache, driver may access wrong dma data.
> 
> I don't know what's the exact meaning of "fill random cachelines with 
> stale data at any time". I always think 'cpu_needs_post_dma_flush()' 
> means whether this platform needs software to maintain cache
> consistency.
> 
> I found this problem in 4.19.90 kernel's ethernet driver, 
> and this patch can fix this problem.

Which machine with which ethernet card?

My Fuloong 2F is still serving as printer server and looks like there
is no stability issue.

Thanks.
--
Jiaxun Yang