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

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Date:	Thu, 22 Oct 2015 11:02:38 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	linux-arm-kernel@...ts.infradead.org
Cc:	Appana Durga Kedareswara Rao <appana.durga.rao@...inx.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-can@...r.kernel.org" <linux-can@...r.kernel.org>,
	Michal Simek <michals@...inx.com>,
	Soren Brinkmann <sorenb@...inx.com>,
	Anirudha Sarangi <anirudh@...inx.com>,
	"mkl@...gutronix.de" <mkl@...gutronix.de>,
	"wg@...ndegger.com" <wg@...ndegger.com>
Subject: Re: [PATCH 1/2] can: xilinx: use readl/writel instead of ioread/iowrite

On Thursday 22 October 2015 08:34:53 Appana Durga Kedareswara Rao 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.
> 
> I observed this issue with the 4.0.0 kernel version

Is it possible that you have nonstandard patches on your kernel? If so, can
you send a diff against the mainline version?

I don't see CONFIG_GENERIC_IOMAP in 4.0.0, and writel() definitely has the
necessary barriers on arm64, the same way that iowrite() does.

> > 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.

Ok, good.

	Arnd
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