Linux 2.6.x: arch powerpc: PCI DMA allocation misunderstandings

["Laurent Lagrange" <lagrange@fr.oleane.com>]

Hello,

I use a 82xx or 85xx host platform with a Linux 2.6.24 or 2.2.31.

On this host, I want to write a PCI driver for a target PMC device
wich only supports 30bits (1GB) DMA addressing.
The PMC device is the master of the DMA transfers from/to the host memory.

In the host driver, I begin to set the two DMA masks with 
 - pci_set_dma_mask(pdev, DMA_BIT_MASK(30)) and
 - pci_set_consistent_dma_mask(pdev, DMA_BIT_MASK(30))

Then I allocate DMA buffers in the host memory with
 - pci_alloc_consistent or
 - pci_pool_create and pci_pool_alloc

My problem is that the alloc functions return physical addresses
which are coherent with 32bits but not with the wanted 30bits.

The allocations seem to work like a kmalloc with a GFP_DMA flag.
On powerpc architecture, GFP_DMA preserves the allocations on 32bits
unlike on x86 architecture which restrict the allocations on 24bits.

I don't understand why it is recommended to use the PCI DMA API
with the DMA masks if the final allocator is only able to 
use a GFP_DMA-like restriction and not a real provided mask.

Surely I missed something...
Any idea would be welcome.

Thanks
Laurent