pci_alloc_consistent from interrupt == BAD

Troy Benjegerdes
2002-01-18T19:03:04Z

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' | hozer@drgw.net
-----"If this message isn't misspelled, I didn't write it" -- Me -----
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