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

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Message-Id: <20180308.215053.1594215295032419362.davem@davemloft.net>
Date:   Thu, 08 Mar 2018 21:50:53 -0500 (EST)
From:   David Miller <davem@...emloft.net>
To:     niklas.cassel@...s.com
Cc:     peppe.cavallaro@...com, alexandre.torgue@...com,
        Jose.Abreu@...opsys.com, pavel@....cz, niklass@...s.com,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next] net: stmmac: remove superfluous wmb() memory
 barriers

From: Niklas Cassel <niklas.cassel@...s.com>
Date: Thu,  8 Mar 2018 11:30:05 +0100

> 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() before doing the
> actual write, these barriers are superfluous, and removing them should
> thus not change any existing behavior.
> 
> Ordering within the descriptor writes is already ensured with dma_wmb()
> barriers inside prepare_tx_desc(first, ..)/prepare_tso_tx_desc(first, ..).
> 
> Signed-off-by: Niklas Cassel <niklas.cassel@...s.com>

Please allow me some time to consider this issue a little bit more
before applying this patch.

Thank you Niklas.