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

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Message-Id: <201403111140.44044.marex@denx.de>
Date:	Tue, 11 Mar 2014 11:40:43 +0100
From:	Marek Vasut <marex@...x.de>
To:	Yao Yuan <yao.yuan@...escale.com>
Cc:	Shawn Guo <shawn.guo@...aro.org>,
	"wsa@...-dreams.de" <wsa@...-dreams.de>,
	"mark.rutland@....com" <mark.rutland@....com>,
	"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 Tuesday, March 11, 2014 at 06:27:39 AM, Yao Yuan wrote:
> On Thu, Mar 10, 2014 at 10:01:42 AM, Marek Vasut wrote:
> > On Thu, Mar 06, 2014 at 12:57:42PM +0100, Marek Vasut wrote:
> > > On Thursday, March 06, 2014 at 06:02:03 AM, Yao Yuan wrote:
> > > > On Thu, March 06, 2014 at 12:44:14 PM, Marek Vasut wrote:
> > > > > 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
> > > > > > resource->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 ?
> > > > 
> > > > Yes, It's a register area. But I don't know why I should ioremap()
> > > > it? It's a bus address and DMA can use it directly. Is there some
> > > > problem for my understanding ?
> > > 
> > > I am not too sure here, thus I am poking someone who can clearly answer
> > 
> > this.
> > 
> > There is already a devm_ioremap_resource() call in the existing code for
> > CPU to access registers in virtual address.  And my understanding on
> > Yuan's patch is that he needs the physical address of I2C DATA register
> > for DMA from/to the controller.
> > 
> > Shawn
> 
> Thanks you Shawn. Yes, it is. DMA need the physical address which
> resource->start is.
> 
> And Hi Marek, How about you? Is there someone who can clearly answer this
> some and should I send the next version?

While I don't quite understand what Shawn wanted to say by the above, please go 
ahead and send V2 .
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