Re: is remap_pfn_range should align to 2(n) * (page size) ?

[zhuzhenhua <zzh.hust@gmail.com>]

On Wed, May 14, 2008 at 1:23 AM, Ralf Baechle <ralf@linux-mips.org> wrote:

On Tue, May 13, 2008 at 07:44:06PM +0800, zhuzhenhua wrote:

> thanks for your advice, i found in newest kernel version, in some arch , the
> dma_alloc_coherent will call split_page.
> because my kernel version is 2.6.14, so i first patch a split_page patch as
> follow:
> http://www.kernel.org/pub/linux/kernel/people/npiggin/patches/lockless/2.6.16-rc5/broken-out/mm-split-highorder.patch
>
> but it seemes that there is still no split_page in
> dma_alloc_coherent/dma_alloc_noncoherent
> so i copy from other arch code to arch/mips/mm/dma-noncoherent.c (attach at
> the end of mail)
> and now my driver just use dma_alloc_coherent malloc 3M directly, and it
> seemes ok.
> i just wonder why mips arch dma_alloc_coherent/dma_alloc_nocoherent do not
> call split_page while other arch calling.

I have not identified the waste of memory as a big problem for typical
MIPS systems yet.

The 3MB requirement of your device is sort of odd because it's not a power
of two.  Have you considered splitting the allocation into a 2MB and a 1MB
allocation or would that be undersirable?

 Ralf

Thanks for your reply.
Our board is for embedded system , It only have 32M sdram and we don't want to
 waste 1M sdram.  My sensor driver need about 2.5xM memory to capture a picture
 by DMA (our DMA controller do not support scatter/gather).

I also can use bootargs "mem=29M" to keep 3M sdram.  but it's not flexible as 
passing a param to driver module(calling dma_alloc_coherent). maybe my situation 
is not common for MIPS arch. so that's is no split_page in dma_alloc_coherent.
and now after patch,it seemes ok for me. 

Best Regards

zzh