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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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