linux-kernel - Re: [PATCH v2 1/2] i2c: add DMA support for freescale i2c driver

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Message-Id: <201403060541.21665.marex@denx.de>
Date:	Thu, 6 Mar 2014 05:41:21 +0100
From:	Marek Vasut <marex@...x.de>
To:	Yao Yuan <yao.yuan@...escale.com>
Cc:	"wsa@...-dreams.de" <wsa@...-dreams.de>,
	"mark.rutland@....com" <mark.rutland@....com>,
	"shawn.guo@...aro.org" <shawn.guo@...aro.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-i2c@...r.kernel.org" <linux-i2c@...r.kernel.org>
Subject: Re: [PATCH v2 1/2] i2c: add DMA support for freescale i2c driver

On Thursday, March 06, 2014 at 05:36:14 AM, Yao Yuan wrote:
> On Thu, March 06, 2014 at 11:23:50 AM, Marek Vasut wrote:
> > On Wednesday, March 05, 2014 at 07:52:31 AM, Yuan Yao wrote:
> > > Add dma support for i2c. This function depend on DMA driver.
> > > You can turn on it by write both the dmas and dma-name properties in
> > > dts node.
> > > 
> > > Signed-off-by: Yuan Yao <yao.yuan@...escale.com>
> > > ---
> > 
> > [...]
> > 
> > > @@ -601,6 +826,7 @@ static int i2c_imx_probe(struct platform_device
> > 
> > *pdev)
> > 
> > >  	void __iomem *base;
> > >  	int irq, ret;
> > >  	u32 bitrate;
> > > 
> > > +	u32 phy_addr;
> > > 
> > >  	dev_dbg(&pdev->dev, "<%s>\n", __func__);
> > > 
> > > @@ -611,6 +837,7 @@ static int i2c_imx_probe(struct platform_device
> > 
> > *pdev)
> > 
> > >  	}
> > >  	
> > >  	res = platform_get_resource(pdev, IORESOURCE_MEM, 0);
> > > 
> > > +	phy_addr = res->start;
> > 
> > Uh ... Shawn, I really think I am lost here. Don't you need to map this
> > memory before you can use it for DMA ? The DMA mapping function should
> > give you the physical address and is the right way to go about this
> > instead of pulling the address from here, no ?
> > 
> > I might be wrong here, I am rather uncertain, so please help me out.
> > Thanks!
> 
> Hi, Marek, Thanks for your suggestion.
> Here you can review the code in include/linux/ioport.h
> The resource->start describes the entity on the CPU bus as a starting
> physical address. So I thinks it can used for dma directly.

This doesn't feel right for some reason. If this is a register area, you should 
ioremap() it. If it's a memory area you do DMA to/from, you need to make sure 
you correctly flush/invalidate caches and properly handle the effects the write 
buffer might have. But I have a feeling you actually do DMA to/from register 
space here ?

Best regards,
Marek Vasut
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