linux-kernel - Re: [PATCH v3 2/2] dma/imx-sdma: use writel to write register when enable a channel

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Message-ID: <20120105091904.GO2414@b20223-02.ap.freescale.net>
Date:	Thu, 5 Jan 2012 17:19:05 +0800
From:	Richard Zhao <richard.zhao@...escale.com>
To:	Shawn Guo <shawn.guo@...aro.org>
CC:	Richard Zhao <richard.zhao@...aro.org>,
	Arnd Bergmann <arnd@...db.de>, <patches@...aro.org>,
	<vinod.koul@...el.com>, <linux-kernel@...r.kernel.org>,
	<eric.miao@...aro.org>, <kernel@...gutronix.de>,
	<dan.j.williams@...el.com>, <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v3 2/2] dma/imx-sdma: use writel to write register when
 enable a channel

Hi Shawn,

On Thu, Jan 05, 2012 at 05:23:17PM +0800, Shawn Guo wrote:
> On Thu, Jan 05, 2012 at 03:59:22PM +0800, Richard Zhao wrote:
> > dma_alloc_coherent memory may be bufferable when set
> > CONFIG_ARM_DMA_MEM_BUFFERABLE. We need to add nececcary
> > memory barrier. writel implicitly call wmb in such case.
> > 
> > Signed-off-by: Richard Zhao <richard.zhao@...aro.org>
> > ---
> >  drivers/dma/imx-sdma.c |    2 +-
> >  1 files changed, 1 insertions(+), 1 deletions(-)
> > 
> > diff --git a/drivers/dma/imx-sdma.c b/drivers/dma/imx-sdma.c
> > index c2bc4f1..e987468 100644
> > --- a/drivers/dma/imx-sdma.c
> > +++ b/drivers/dma/imx-sdma.c
> > @@ -396,7 +396,7 @@ static int sdma_config_ownership(struct sdma_channel *sdmac,
> >  
> >  static void sdma_enable_channel(struct sdma_engine *sdma, int channel)
> >  {
> > -	__raw_writel(1 << channel, sdma->regs + SDMA_H_START);
> > +	writel(1 << channel, sdma->regs + SDMA_H_START);
> >  }
> >  
> 
> As educated by Arnd, generally it's safer to use pair of readl/writel
> than __raw_readl/__raw_writel in driver.  I'm wondering if it's a good
> opportunity for us to change the pair all over this driver.
Russel mentioned that too. But I cannot understand.
If CONFIG_ARM_DMA_MEM_BUFFERABLE, readl/writel always call rmb/wmb. Do
we realy need it? It might affect performance.

Du to the concern, I didn't convert all the readl/writel.

Thanks
Richard
> 
> -- 
> Regards,
> Shawn
> 
> _______________________________________________
> linux-arm-kernel mailing list
> linux-arm-kernel@...ts.infradead.org
> http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
> 

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