Re: [PATCH] powerpc/mm: add ZONE_NORMAL zone for 64 bit kernel

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Fri, 2012-07-20 at 20:21 +0800, Shaohui Xie wrote:
> PowerPC platform only supports ZONE_DMA zone for 64bit kernel, so all the
> memory will be put into this zone. If the memory size is greater than
> the device's DMA capability and device uses dma_alloc_coherent to allocate
> memory, it will get an address which is over the device's DMA addressing,
> the device will fail.
> 
> So we split the memory to two zones by adding a zone ZONE_NORMAL, since
> we already allocate PCICSRBAR/PEXCSRBAR right below the 4G boundary (if the
> lowest PCI address is above 4G), so we constrain the DMA zone ZONE_DMA
> to 2GB, also, we clear the flag __GFP_DMA and set it only if the device's
> dma_mask < total memory size. By doing this, devices which cannot DMA all
> the memory will be limited to ZONE_DMA, but devices which can DMA all the
> memory will not be affected by this limitation.

This is wrong. Don't you have an iommu do deal with those devices
anyway ? What about swiotlb ?

If you *really* need to honor 32 (or 31 even) bit DMAs, what you -may-
want to do is create a ZONE_DMA32 like other architectures, do not
hijack the historical ZONE_DMA.

But even then, I'm dubious this is really needed.

Cheers,
Ben.