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

[Marek Vasut <marex-ynQEQJNshbs@public.gmane.org>]

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-KZfg59tc24xl57MIdRCFDg@public.gmane.org>
> > > > > ---
> > > > 
> > > > [...]
> > > > 
> > > > > @@ -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 ?
> 
> 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.

Best regards,
Marek Vasut