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

Hi Arnd,	

-----Original Message-----
From: Arnd Bergmann [mailto:arnd@arndb.de]
Sent: Thursday, October 22, 2015 1:45 PM
To: linux-arm-kernel@lists.infradead.org
Cc: Appana Durga Kedareswara Rao; Anirudha Sarangi; wg@grandegger.com;
mkl@pengutronix.de; Michal Simek; Soren Brinkmann; Appana Durga
Kedareswara Rao; netdev@vger.kernel.org; linux-kernel@vger.kernel.org; linux-
can@vger.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:

quoted

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 <redacted>

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.

I observed this issue with the 4.0.0 kernel version

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.

There is no DMA in CAN for this IP.

Regards,
Kedar.

	Arnd