RE: Patch added to scsi-rc-fixes-2.6: [SCSI] arcmsr: fix message allocation

James Bottomley
2008-02-25T14:38:42Z

On Mon, 2008-02-25 at 18:47 +0800, nickcheng wrote:
> Sorry, maybe I did not ask distinctly enough.
> I mean if I would like to allocate a memory space from ZONE_DMA for atomic
> context, why can I not use kmalloc(1032, GFP_ATOMIC|GFP_DMA)?
> In case of lack of GFP_DMA, kmalloc would grab the memory from ZONE_HIGH or
> ZONE_HIGHMEM, isn't it?(I read it from the textbook of Linux Kernel
> Development by Robert Love)

Um, no that's not true at all. GFP_DMA only allocates memory from
ZONE_DMA and fails otherwise. You only need memory from ZONE_DMA if you
cannot address physical memory above 24 bits (the old ISA restriction).
This only applies if you're a standard ISA device and you're going
actually to DMA to the memory in question. Neither of which applies in
the arcmsr case, since you're only using the memory as a copy buffer
within the kernel (it never sees an actual DMA transfer), and arcmsr
doesn't have the ISA restrictions.

> Or the basic is that you don't think it is necessary to allocate a memory
> space from DMA area?
> Please give me some comments.

It's unnecessary because you never DMA to it, but even if you did, since
arcmsr is a non-ISA device (with 64 bit DMA mask falling back to 32) you
can just use ordinary kmalloc'd memory for that (provided you obey the
coherence requirements).

James