netdev - Re: [PATCH 1/2] can: xilinx: use readl/writel instead of ioread/iowrite

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Date:	Thu, 22 Oct 2015 10:14:30 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	linux-arm-kernel@...ts.infradead.org
Cc:	Kedareswara rao Appana <appana.durga.rao@...inx.com>,
	anirudh@...inx.com, wg@...ndegger.com, mkl@...gutronix.de,
	michal.simek@...inx.com, soren.brinkmann@...inx.com,
	appanad@...inx.com, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-can@...r.kernel.org
Subject: Re: [PATCH 1/2] can: xilinx: use readl/writel instead of ioread/iowrite

On Thursday 22 October 2015 10:16:02 Kedareswara rao Appana wrote:
> The driver only supports memory-mapped I/O [by ioremap()],
> so readl/writel is actually the right thing to do, IMO.
> During the validation of this driver or IP on ARM 64-bit processor
> while sending lot of packets observed that the tx packet drop with iowrite
> Putting the barriers for each tx fifo register write fixes this issue
> Instead of barriers using writel also fixed this issue.
> 
> Signed-off-by: Kedareswara rao Appana <appanad@...inx.com>

The two should really do the same thing: iowrite32() is just a static inline
calling writel() on both ARM32 and ARM64. On which kernel version did you
observe the difference? It's possible that an older version used
CONFIG_GENERIC_IOMAP, which made this slightly more expensive.

If there are barriers that you want to get rid of for performance reasons,
you should use writel_relaxed(), but be careful to synchronize them correctly
with regard to DMA. It should be fine in this driver, as it does not
perform any DMA, but be aware that there is no big-endian version of
writel_relaxed() at the moment.

	Arnd
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