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

[Appana Durga Kedareswara Rao]

Hi Marc,

> -----Original Message-----
> From: Marc Kleine-Budde [mailto:mkl@xxxxxxxxxxxxxx]
> Sent: Thursday, October 22, 2015 1:52 PM
> To: Arnd Bergmann; linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
> Cc: Appana Durga Kedareswara Rao; Anirudha Sarangi; wg@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 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@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.
> >
> > 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?

Yes During validation of this IP on ARM 64 bit processor with using iowrite32() and sending a lot of packets it requires barriers before triggering the send.
With using writel() barriers are not needed.

Regards,
Kedar.

> 
> Marc
> 
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