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

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Message-ID: <CAJF2gTStGrrELOr-Dga98ueNQuSG5WBcYgzAPp_W_RjWnsGXgg@mail.gmail.com>
Date:   Tue, 30 Jul 2019 23:28:28 +0800
From:   Guo Ren <guoren@...nel.org>
To:     Arnd Bergmann <arnd@...db.de>
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

Thx Arnd,

On Tue, Jul 30, 2019 at 9:29 PM Arnd Bergmann <arnd@...db.de> wrote:
>
> 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.
Wow, thanks for correction, yes! mb, rmb, wmb is the superset of dma
and smp, and I should use sync.s for mb.

I check the arm64, it use dsb for mb and dmb for smp_mb and dma_mb.

I also check the drivers, you are right , a lot of them use mb() to
sync mem data with device.

>
> 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().
It's no need for sky, when there is non-SMP, the cpu is in reset mode,
and sync.s is equal to sync in efficient.

-- 
Best Regards
 Guo Ren

ML: https://lore.kernel.org/linux-csky/