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

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Date:	Thu, 22 Oct 2015 08:34:53 +0000
From:	Appana Durga Kedareswara Rao <appana.durga.rao@...inx.com>
To:	Arnd Bergmann <arnd@...db.de>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
CC:	Anirudha Sarangi <anirudh@...inx.com>,
	"wg@...ndegger.com" <wg@...ndegger.com>,
	"mkl@...gutronix.de" <mkl@...gutronix.de>,
	Michal Simek <michals@...inx.com>,
	Soren Brinkmann <sorenb@...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>
Subject: RE: [PATCH 1/2] can: xilinx: use readl/writel instead of
 ioread/iowrite

Hi Arnd,	

> -----Original Message-----
> From: Arnd Bergmann [mailto:arnd@...db.de]
> Sent: Thursday, October 22, 2015 1:45 PM
> To: linux-arm-kernel@...ts.infradead.org
> Cc: Appana Durga Kedareswara Rao; Anirudha Sarangi; wg@...ndegger.com;
> mkl@...gutronix.de; Michal Simek; Soren Brinkmann; Appana Durga
> Kedareswara Rao; 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.

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
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