Re: [PATCH v3 4/7] of: configure the platform device dma parameters

[Arnd Bergmann <arnd@arndb.de>]

On Monday 05 May 2014 17:47:32 Santosh Shilimkar wrote:

> +       dev->coherent_dma_mask = DMA_BIT_MASK(32);
> +       if (!dev->dma_mask)
> +               dev->dma_mask = &dev->coherent_dma_mask;
> +
> +       /*
> +        * if dma-ranges property doesn't exist - just return else
> +        * setup the dma offset
> +        */
> +       ret = of_dma_get_range(dev->of_node, &dma_addr, &paddr, &size);
> +       if (ret < 0) {
> +               dev_dbg(dev, "no dma range information to setup\n");
> +               return;
> +       }
> +
> +       /* DMA ranges found. Calculate and set dma_pfn_offset */
> +       dev->dma_pfn_offset = PFN_DOWN(paddr - dma_addr);
> +       dev_dbg(dev, "dma_pfn_offset(%#08lx)\n", dev->dma_pfn_offset);
> +}

I think there should at least be a comment about why we are computing
the correct DMA mask here and then ignore that and just use DMA_BIT_MASK(32)
instead. I understand that Russell and Rob prefer it that way and I'm not
going to argue, but I find it counterintuitive and I think it deserves
an explanation in the source code for anybody who is trying to figure
out how things fit together.

	Arnd