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

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Message-Id: <20180307.124249.619165601144161535.davem@davemloft.net>
Date:   Wed, 07 Mar 2018 12:42:49 -0500 (EST)
From:   David Miller <davem@...emloft.net>
To:     niklas.cassel@...s.com
Cc:     pavel@....cz, 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

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.