linux-kernel - Re: [PATCH net-next 2/4] net: stmmac: use correct barrier between coherent memory and MMIO

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Message-ID: <20180308090535.GD17761@amd>
Date:   Thu, 8 Mar 2018 10:05:35 +0100
From:   Pavel Machek <pavel@....cz>
To:     David Miller <davem@...emloft.net>
Cc:     niklas.cassel@...s.com, peppe.cavallaro@...com,
        alexandre.torgue@...com, Jose.Abreu@...opsys.com,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next 2/4] net: stmmac: use correct barrier between
 coherent memory and MMIO

On Wed 2018-03-07 12:42:49, David Miller wrote:
> From: Niklas Cassel <niklas.cassel@...s.com>
> Date: Wed, 7 Mar 2018 18:21:57 +0100
> 
> > Considering this, you can drop/revert:
> > 95eb930a40a0 ("net: stmmac: use correct barrier between coherent memory and MMIO")
> > or perhaps you want me to send a revert?
> 
> You must submit explicit patches to do a revert or any other change.
> 
> > After reverting 95eb930a40a0, we will still have a dma_wmb() _after_ the
> > last descriptor word write. You just explained that nothing else is needed
> > after the last descriptor word write, so I actually think that this last
> > barrier is superfluous.
> 
> You don't need one after the last descriptor write.
> 
> Look, you're only concerned with ordering within the descriptor writes.
> 
> So it's only about:
> 
> 	desc->a = x;
> 
> 	/* Write to 'a' must be visible to the hardware before 'b'. */
> 	dma_wmb();
> 	desc->b = y;
> 
> 	writel();
> 
> That's all that you need.

We may need to fix the docs then, there's wmb() there in the docs:

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/memory-barriers.txt?h=v4.16-rc1#n1913

									Pavel

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

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