[RFC/RFT 1/2] ARM: mm: introduce arch hooks for dma address translation routines

[arnd@arndb.de (Arnd Bergmann)]

On Tuesday 04 February 2014, Santosh Shilimkar wrote:
> Currently arch specific DMA address translation routines can be enabled
> using only defines which makes impossible to use them in with
> multi-platform builds.
> 
> Hence, introduce arch specific hooks for DMA address translations
> routines to be compatible with multi-platform builds:
> dma_addr_t (*arch_pfn_to_dma)(struct device *dev, unsigned long pfn);
> unsigned long (*arch_dma_to_pfn)(struct device *dev, dma_addr_t addr);
> void* (*arch_dma_to_virt)(struct device *dev, dma_addr_t addr);
> dma_addr_t (*arch_virt_to_dma)(struct device *dev, void *addr);
> 
> In case if architecture won't use it - DMA address translation routines
> will fall-back to existing implementation.
> v
> Also, modify machines omap1, ks8695, iop13xx to use new DMA hooks.

I think this is going into a wrong direction. DMA translation is not
at all a platform-specific thing, but rather bus specific. The most
common scenario is that you have some 64-bit capable buses and some
buses that are limited to 32-bit DMA (or less if you are unfortunate).

We also can't rely on {pfn,phys,virt}_to_{bus,dma} and the reverse
to work anywhere outside of the dma_map_ops implementation, because
of IOMMUs in-between.

Of course we do need a proper solution for this problem, but we
can't make it a per-platform decision, and whatever the solution is
needs to take into account both nontrivial linear mappings (offset
or cropped) and IOMMUs, and set the appropriate dma_map_ops for
the device.

I guess for the legacy cases (omap1, iop13xx, ks8695), we can
hardcode dma_map_ops for all devices to get this right. For everything
else, I'd suggest defaulting to the arm_dma_ops unless we get
other information from DT. This means we have to create standardized
properties to handle any combination of these:

1. DMA is coherent
2. DMA space is offset from phys space
3. DMA space is smaller than 32-bit
4. DMA space is larger than 32-bit
5. DMA goes through an IOMMU

The dma-ranges property can deal with 2-4. Highbank already introduced
a "dma-coherent" flag for 1, and we can decide to generalize that.
I don't know what the state of IOMMU support is, but we have to come
up with something better than what we had on PowerPC, because we now
have to deal with a combination of different IOMMUs in the same system,
whereas the most complex case on PowerPC was some devices all going
through one IOMMU and the other devices being linearly mapped.

	Arnd