netdev - RE: [PATCH v5 3/5] bnx2x: Eliminate duplicate barriers on weakly-ordered archs

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Message-ID: <DM2PR07MB685FD0A0877865A41EE723889AF0@DM2PR07MB685.namprd07.prod.outlook.com>
Date:   Sat, 24 Mar 2018 14:30:17 +0000
From:   "Chopra, Manish" <Manish.Chopra@...ium.com>
To:     Sinan Kaya <okaya@...eaurora.org>,
        David Miller <davem@...emloft.net>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "timur@...eaurora.org" <timur@...eaurora.org>,
        "sulrich@...eaurora.org" <sulrich@...eaurora.org>,
        "linux-arm-msm@...r.kernel.org" <linux-arm-msm@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "Elior, Ariel" <Ariel.Elior@...ium.com>,
        Dept-Eng Everest Linux L2 <Dept-EngEverestLinuxL2@...ium.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v5 3/5] bnx2x: Eliminate duplicate barriers on
 weakly-ordered archs

> -----Original Message-----
> From: Sinan Kaya [mailto:okaya@...eaurora.org]
> Sent: Friday, March 23, 2018 10:44 PM
> To: David Miller <davem@...emloft.net>
> Cc: netdev@...r.kernel.org; timur@...eaurora.org; sulrich@...eaurora.org;
> linux-arm-msm@...r.kernel.org; linux-arm-kernel@...ts.infradead.org; Elior,
> Ariel <Ariel.Elior@...ium.com>; Dept-Eng Everest Linux L2 <Dept-
> EngEverestLinuxL2@...ium.com>; linux-kernel@...r.kernel.org
> Subject: Re: [PATCH v5 3/5] bnx2x: Eliminate duplicate barriers on weakly-
> ordered archs
> 
> On 3/23/2018 1:04 PM, David Miller wrote:
> > From: Sinan Kaya <okaya@...eaurora.org>
> > Date: Fri, 23 Mar 2018 12:51:47 -0400
> >
> >> It could if txdata->tx_db was not a union. There is a data dependency
> >> between txdata->tx_db.data.prod and txdata->tx_db.raw.
> >>
> >> So, no reordering.
> >
> > I don't see it that way, the code requires that:
> >
> >  	txdata->tx_db.data.prod += nbd;
> >
> > is visible before the doorbell update.>
> > barrier() doesn't provide that.
> >
> > Neither does writel_relaxed().  However plain writel() does.
> 
> Correct for some architectures including ARM but not correct universally.
> 
> writel() just guarantees register read/writes before and after to be ordered
> when HW observes it.
> 
> writel() doesn't guarantee that the memory update is visible to the HW on all
> architectures.
> 
> If you need memory update visibility, that barrier() should have been a
> wmb()
> 
> A correct multi-arch pattern is
> 
> wmb()
> writel_relaxed()
> mmiowb()
> 

Sinan,  Since you have mentioned the use of mmiowb() here after writel_relaxed().
I believe this is not always correct for all types of IO mapped memory [Specially if IO memory is mapped using write combined (for ex. Ioremap_wc())].
We have a current issue on our NIC (qede) driver on x86 for which the patch is already been sent more than a week ago [Still awaiting to hear from David on that].
where mmiowb() seems to be useless since we use write combined mapped doorbell and mmiowb() just seems to be a compiler barrier() there.
So in order to flush write combined buffer we really need writel_relaxed() followed by a wmb() to synchronize writes among CPU cores.
I think  the correct pattern in such cases (for write combined IO) would have been like below - 

wmb();
writel_relaxed();
wmb(); -> To flush the writes actually.

Thanks.