linux-kernel - Re: [PATCH V2 net 1/9] net: hns3: default enable tx bounce buffer when smmu enabled

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Message-ID: <20241024160404.GC1202098@kernel.org>
Date: Thu, 24 Oct 2024 17:04:04 +0100
From: Simon Horman <horms@...nel.org>
To: Jijie Shao <shaojijie@...wei.com>
Cc: Paolo Abeni <pabeni@...hat.com>, davem@...emloft.net,
	edumazet@...gle.com, kuba@...nel.org, shenjian15@...wei.com,
	salil.mehta@...wei.com, liuyonglong@...wei.com,
	wangpeiyang1@...wei.com, lanhao@...wei.com, chenhao418@...wei.com,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2 net 1/9] net: hns3: default enable tx bounce buffer
 when smmu enabled

On Thu, Oct 24, 2024 at 04:31:46PM +0800, Jijie Shao wrote:
> 
> on 2024/10/24 16:26, Paolo Abeni wrote:
> > On 10/18/24 12:10, Jijie Shao wrote:
> > > From: Peiyang Wang <wangpeiyang1@...wei.com>
> > > 
> > > The SMMU engine on HIP09 chip has a hardware issue.
> > > SMMU pagetable prefetch features may prefetch and use a invalid PTE
> > > even the PTE is valid at that time. This will cause the device trigger
> > > fake pagefaults. The solution is to avoid prefetching by adding a
> > > SYNC command when smmu mapping a iova. But the performance of nic has a
> > > sharp drop. Then we do this workaround, always enable tx bounce buffer,
> > > avoid mapping/unmapping on TX path.
> > > 
> > > This issue only affects HNS3, so we always enable
> > > tx bounce buffer when smmu enabled to improve performance.
> > > 
> > > Signed-off-by: Peiyang Wang <wangpeiyang1@...wei.com>
> > > Signed-off-by: Jian Shen <shenjian15@...wei.com>
> > > Signed-off-by: Jijie Shao <shaojijie@...wei.com>
> > I'm sorry to nick pick on somewhat small details, but we really need a
> > fixes tag here to make 110% clear is a bugfix. I guess it could be the
> > commit introducing the support for the buggy H/W.
> > 
> > Thanks,
> > 
> > Paolo
> 
> I have a little doubt that this patch is about H/W problem,
> so how can we write the the fixes tag?

Hi Jijie,

That is a good point. But the much point of the Fixes tag is to indicate how
far back the fix should be backported. So I would say the ID of the patch
where the user would have first seen this problem - possibly the patch that
added the driver.