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

[Appana Durga Kedareswara Rao]

Hi Arnd,	

> -----Original Message-----
> From: Arnd Bergmann [mailto:arnd@xxxxxxxx]
> Sent: Thursday, October 22, 2015 1:45 PM
> To: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
> Cc: Appana Durga Kedareswara Rao; Anirudha Sarangi; wg@xxxxxxxxxxxxxx;
> mkl@xxxxxxxxxxxxxx; Michal Simek; Soren Brinkmann; Appana Durga
> Kedareswara Rao; netdev@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; linux-
> can@xxxxxxxxxxxxxxx
> 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@xxxxxxxxxx>
> 
> 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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