linux-kernel - Re: [PATCH] i2c: designware: use {readl|writel}

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Message-ID: <20150113143654.GG7660@katana>
Date:	Tue, 13 Jan 2015 15:36:54 +0100
From:	Wolfram Sang <wsa@...-dreams.de>
To:	Jisheng Zhang <jszhang@...vell.com>
Cc:	linux-i2c@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] i2c: designware: use {readl|writel}_relaxed instead of
 readl/writel


On Thu, Dec 11, 2014 at 02:26:41PM +0800, Jisheng Zhang wrote:
> readl/writel is too expensive especially on Cortex A9 w/ outer L2 cache.
> This introduces i2c read/write errors on Marvell BG2/BG2Q SoCs when there
> are heavy L2 cache maintenance operations at the same time.

Reading this again, I got a question:

Really read/write errors? I would think that there is a performance
penalty because of the memory barriers. But errors?

> The driver does not perform DMA, so it's safe to use the relaxed version.
> From another side, the relaxed io accessor macros are available on all
> architectures now, so we can use the relaxed versions instead.

Can the designware core make use of DMA in theory?


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