pci_alloc_consistent from interrupt == BAD

[Troy Benjegerdes]

Somehow the docs in DMA-mappings.txt say pci_alloc_consistent is allowed from 
interrupt, but this is a "bad thing" on at least arm and PPC non-cache 
coherent cpus.

On these cpus we have to allocate page tables for consistent_alloc.. I really 
dont' think we want to be doing this during interrupt context.

This also causes the sym53c8xx_2 driver to not work on some embedded ppc 4xx 
boards, and in general, seems to be a 'bad thing' to allow.

For example, in the arch/arm/consistent.c:

/*
 * This allocates one page of cache-coherent memory space and returns
 * both the virtual and a "dma" address to that space.  It is not clear
 * whether this could be called from an interrupt context or not.  For
 * now, we expressly forbid it, especially as some of the stuff we do
 * here is not interrupt context safe.
 *
 * Note that this does *not* zero the allocated area!
 */
void *consistent_alloc(int gfp, size_t size, dma_addr_t *dma_handle)

(arm's pci_alloc_consistent always calls consistent_alloc).

The PPC version calls a similiar function when CONFIG_NOT_COHERENT_CACHE is 
defined. On 'regular' ppc machines, it's just a __get_free_pages, which is 
why no one from the pmac crowd has screamed.

-- 
Troy Benjegerdes | master of mispeeling | 'da hozer' |  [e-mail]
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