linux-kernel - Re: [PATCH net-next] net: stmmac: remove superfluous wmb() memory barriers

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Message-ID: <20180309104855.GA17998@amd>
Date:   Fri, 9 Mar 2018 11:48:56 +0100
From:   Pavel Machek <pavel@....cz>
To:     Jose Abreu <Jose.Abreu@...opsys.com>
Cc:     Niklas Cassel <niklas.cassel@...s.com>,
        Giuseppe Cavallaro <peppe.cavallaro@...com>,
        Alexandre Torgue <alexandre.torgue@...com>,
        Niklas Cassel <niklass@...s.com>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next] net: stmmac: remove superfluous wmb() memory
 barriers

On Fri 2018-03-09 10:26:11, Jose Abreu wrote:
> Hi Niklas,
> 
> On 08-03-2018 10:30, Niklas Cassel wrote:
> > These wmb() memory barriers are performed after the last descriptor write,
> > and they are followed by enable_dma_transmission()/set_tx_tail_ptr(),
> > i.e. a writel() to MMIO register space.
> > Since writel() itself performs the equivalent of a wmb() 
> 
> Sorry but I know at least two architectures which don't do a
> wmb() upon an writel [1] [2]. This can be critical if if we are
> accessing the device through some slow or filled bus which will
> delay accesses to the device IO. Notice that writel and then
> readl to the same address will force CPU to wait for writel
> completion before readl, but in this case we are using DMA and
> then writel so I think a wmb() before the writel is a safe measure.

This also matches documentation, as I tried to point out.

-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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