Messages in this thread | | | Subject | Re: [patch][cft] zero-copy dma cd writing and ripping | From | "David S. Miller" <> | Date | 28 Oct 2002 08:03:37 -0800 |
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On Mon, 2002-10-28 at 07:08, Jens Axboe wrote: > Ok what I make of this is that from bio_map_user() (which does a > get_user_pages() I need to do a > > if (write_to_vm) > flush_dcache_page(page); > > and in bio_unmap_user() I do > > if (!write_to_vm) > flush_dcache_page(page); > > is that correct?
Ho hum, it is tricky :-)))
At bio_map_user() you need to see the user's most recent write to the page if you are going "user --> device". So if "user --> device" bio_map_user() must flush_dcache_page().
I find the write_to_vm condition confusion which is probably why I am sitting here spelling this out :-)
At bio_unmap_user(), if we are going "device --> user" you have to flush_dcache_page(). And actually, this flush could just as legitimately occur at bio_map_user() time.
Therefore, the easiest thing to do is always flush_dcache_page() at bio_map_user().
All the other cases are going to be like this, so we might as well cut to the chase and flush_dcache_page() for all the pages inside of get_user_pages().
Whoever made get_user_pages() and didn't carry over the flush_dcache_page calls from the mechanism it is meant to replace should be spanked :-)
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