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

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Message-ID: <56289CA6.1060809@pengutronix.de>
Date:	Thu, 22 Oct 2015 10:21:58 +0200
From:	Marc Kleine-Budde <mkl@...gutronix.de>
To:	Arnd Bergmann <arnd@...db.de>, linux-arm-kernel@...ts.infradead.org
Cc:	Kedareswara rao Appana <appana.durga.rao@...inx.com>,
	anirudh@...inx.com, wg@...ndegger.com, 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 10/22/2015 10:14 AM, Arnd Bergmann wrote:
> 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.

We don't have DMA in CAN drivers, but usually a certain write triggers
sending. Do we need a barrier before triggering the sending?

Marc

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