linux-kernel - Re: [PATCH 1/4] csky: Fixup dma_rmb/wmb synchronization problem

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Date:   Tue, 30 Jul 2019 15:29:38 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Guo Ren <guoren@...nel.org>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        linux-csky@...r.kernel.org, feng_shizhu@...uatech.com,
        zhang_jian5@...uatech.com, zheng_xingjian@...uatech.com,
        zhu_peng@...uatech.com, Guo Ren <ren_guo@...ky.com>
Subject: Re: [PATCH 1/4] csky: Fixup dma_rmb/wmb synchronization problem

On Tue, Jul 30, 2019 at 2:15 PM <guoren@...nel.org> wrote:
>
> From: Guo Ren <ren_guo@...ky.com>
>
> If arch didn't define dma_r/wmb(), linux will use w/rmb instead. Csky
> use bar.xxx to implement mb() and that will cause problem when sync data
> with dma device, becasue bar.xxx couldn't guarantee bus transactions
> finished at outside bus level. We must use sync.s instead of bar.xxx for
> dma data synchronization and it will guarantee retirement after getting
> the bus bresponse.
>
> Signed-off-by: Guo Ren <ren_guo@...ky.com>

This change looks good to me, but I think your regular barriers
(mb, rmb, wmb) are still wrong: These are meant to be the superset
of dma_{r,w}mb and smp_{,r,w}mb, and synchronize
against both SMP and DMA interactions.

I suspect you can drop the '.s' for non-SMP builds. What I don't
see is whether you might need to add '.i' for dma_wmb() or
dma_rmb().

       Arnd