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

[Bjorn Helgaas]

[+cc Ben, Chris]

On Fri, May 2, 2014 at 12:59 PM, Arnd Bergmann <arnd@xxxxxxxx> wrote:
> On Friday 02 May 2014 10:54:59 Bjorn Helgaas wrote:
>> > +static void of_dma_configure(struct device *dev)
>> > +{
>> > +     u64 dma_addr, paddr, size;
>> > +     int ret;
>> > +
>> > +     dev->coherent_dma_mask = DMA_BIT_MASK(32);
>> > +     if (!dev->dma_mask)
>> > +             dev->dma_mask = &dev->coherent_dma_mask;
>> > +
>> > +     /*
>> > +      * if dma-coherent property exist, call arch hook to setup
>> > +      * dma coherent operations.
>> > +      */
>> > +     if (of_dma_is_coherent(dev->of_node)) {
>> > +             set_arch_dma_coherent_ops(dev);
>> > +             dev_dbg(dev, "device is dma coherent\n");
>> > +     }
>> > +
>> > +     /*
>> > +      * 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 == -ENODEV) || (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);
>>
>> Is this effectively the same as an IOMMU that applies a constant offset
>> to the bus address?  Could or should this be done by adding a simple IOMMU
>> driver instead of adding dma_pfn_offset to struct device?
>
> We currently have two dma_map_ops variants on ARM (plus another set for
> coherent/noncoherent differences, but we can ignore that for the sake
> of this discussion): one that handles linear mappings and one that
> handles IOMMUs by calling into the linux/iommu.h APIs.
>
> I guess what you mean by 'a simple IOMMU driver' would be another
> dma_map_ops implementation that is separate from real IOMMUs, right?

I suppose so; it seems like the offset could be managed inside
arm_dma_ops.  My idea of an IOMMU is something that maps bus addresses
to physical memory addresses.  That's what we're doing here; it's just
that the mapping function is very simple.  So why add something new in
struct device for it?

I think powerpc and tile do something similar in dma_direct_map_page()
and tile_pci_dma_map_page() (they store the offset in struct
dev_archdata).

Bjorn
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