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SubjectRe: [PATCH] mtd: fix mtd_blkdevs problem with caches on some architectures (2.6.31)
On Sat, 21 Nov 2009 17:04:37 +0300 Ilya Loginov <isloginov@gmail.com> wrote:

> On Fri, 20 Nov 2009 16:37:51 -0800
> Andrew Morton <akpm@linux-foundation.org> wrote:
>
> > Your patch fixes bascially the same problem in MTD as we have in AOE.
> > And it introduces the same problem as well - pointless empty
> > cache-thrashing loops on architectures for which flush_dcache_page() is
> > a no-op.
> >
> > What would be better here would be for block core to add a new
> > rq_flush_dcache_pages() and bio_flush_dcache_pages() which the drivers
> > can call. Those functions would be a no-op on architectures which
> > don't need them.
>
> Do you mean something like this?
>
> in include/linux/bio.h
>
> #ifdef SOMETHING_LIKE_CONFIG_CPU_HAS_DCACHE_ALIAS
> extern void bio_flush_dcache_pages(struct bio *bi);
> #else
> static inline void bio_flush_dcache_pages(struct bio *bi)
> {
> }
> #endif /* SOMETHING_LIKE_CONFIG_CPU_HAS_DCACHE_ALIAS */
>
> in fs/bio.c
>
> void bio_flush_dcache_pages(struct bio *bi)
> {
> int i;
> struct bio_vec* bvec;
>
> rq_for_each_segment(bvec, bio, i)
> flush_dcache_page(bvec->bv_page);
> }
>
> in include/linux/blkdev.h
>
> #ifdef SOMETHING_LIKE_CONFIG_CPU_HAS_DCACHE_ALIAS
> extern void rq_flush_dcache_pages(struct request *rq);
> #else
> static inline void rq_flush_dcache_pages(struct request *rq)
> {
> }
> #endif
>
> in block/blk-core.c
>
> void rq_flush_dcache_pages(struct request *rq)
> {
> struct req_iterator iter;
> struct bio_vec* bvec;
>
> rq_for_each_segment(bvec, bio, iter)
> flush_dcache_page(bvec->bv_page);
> }
>
> And SOMETHING_LIKE_CONFIG_CPU_HAS_DCACHE_ALIAS should be defined
> in Kconfigs for each architecture that requires this fix.

yep, that would work.

The one somewhat fragile thing is that we'll end up in a situation
where an architecture could implement a real flush_dcache_page(), but
would forget to set SOMETHING_LIKE_CONFIG_CPU_HAS_DCACHE_ALIAS. Or
vice versa. To make things reliable it would be good to cause a
compilation failure in that case.

One way of addressing this is to

- change every arch/*/include/asm/cacheflush.h to include asm-generic/cacheflush.h
- put #ifndef SOMETHING_LIKE_CONFIG_CPU_HAS_DCACHE_ALIAS wrappers around all content
in asm-generic/cacheflush

So it the above mistake happens, we'll get lots of duplicated
definitions, or no definitions at all (I think).

> I think
> this is good solution and if you think the same I can create the
> patch.

Thanks.


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