[PATCH 4/4] ARM: keystone: Use dma-ranges for dma_pfn_offset configuration

[Arnd Bergmann]

On Tuesday 25 February 2014, Grygorii Strashko wrote:
> On 02/24/2014 11:38 PM, Santosh Shilimkar wrote:
> > On Monday 24 February 2014 04:11 PM, Arnd Bergmann wrote:
> >> On Monday 24 February 2014 15:53:55 Santosh Shilimkar wrote:
> >>> From: Grygorii Strashko <grygorii.strashko at ti.com>
>
> >> Hmm, isn't this function the same as of_translate_dma_address()?
> >>
> >> I think we should have the implementation in common code, not hidden
> >> in the keystone platform, to avoid duplication. If of_translate_dma_address
> >> doesn't work, what is the problem, and can you fix it there?
> >>
> > Will have a look at it and get back. At least it makes sense to
> > use/update the common function.
> 
> This function isn't the same as of_translate_dma_address(), more over
> it calls of_translate_dma_address() to get CPU address.
> 
> To calculate DMA bus offset we need to know the first DMA address explicitly,
> so then we can translate it to the corresponding CPU address.

Ah, got it. Sorry for being sloppy in my review. You are absolutely right.

> get_dma_pfn_offset() function does:
> - traverse DT and look for the first "dma-ranges" prop;
> - reads the first DMA address of the range;
> - translates it in CPU address using of_translate_dma_address();
> - calculates DMA bus offset as PFN_DOWN(cpu_addr - dma_addr);

I was confused about the first step here and thought you were actually
doing the translation there. After a more careful reading, I only
have comments on details:

> +static unsigned long get_dma_pfn_offset(struct device *dev)
> +{
> +       while (1) {
> +               naddr = of_n_addr_cells(node);
> +               nsize = of_n_size_cells(node);
> +               node = of_get_next_parent(node);
> +               if (!node)
> +                       break;
> +
> +               ranges = of_get_property(node, "dma-ranges", &len);
> +
> +               /* Ignore empty ranges, they imply no translation required */
> +               if (ranges && len > 0)
> +                       break;
> +       }

I think we should require an empty "dma-ranges" property to be
present to signify "no translation required" and interpret the
absence of the property as "no dma possible".

A special case is having no "dma-ranges" at all, which we have
on all existing systems, and I  would interpret that as "all
devices can do 32-bit DMA, no more but no less".


> +       /*
> +        * dma-ranges format:
> +        * DMA addr     : naddr cells
> +        * CPU addr     : pna cells
> +        * size         : nsize cells
> +        */
> +       len /= sizeof(u32);
> +       pna = of_n_addr_cells(node);
> +       dma_addr = of_read_number(ranges, nsize);

This should probably use naddr, not nsize.

	Arnd