Re: pci_alloc_consistent from interrupt == BAD

David S. Miller
2002-01-18T20:34:07Z

From: Dan Malek <dan@embeddededge.com>
Date: Fri, 18 Jan 2002 14:22:03 -0500

> 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.

This isn't unique to PowerPC or ARM, and has nothing to do with allocating
page tables.

Yes it is in fact unique to those ports...

I don't understand how pci_alloc_consistent could ever be claimed to work
from an interrupt function because it actually allocates pages of memory
for all architectures. Anytime you call alloc_pages() (or friends) you could
potentially block or return an error (out of memory) condition.

If it specifies GFP_ATOMIC, there are no problems from interrupts.
You will see that every port other than the mentioned two above use
GFP_ATOMIC in their pci_alloc_consistent implementation, for this very
reason.

The ARM and PPC ports could set __GFP_HIGH in their page table
allocation calls when invoked via pci_alloc_consistent, and this is
the change I suggest they make. It is a trivial fix whereas backing
out this ability to call pci_alloc_consistent from interrupts is not
a trivial change at all.