Messages in this thread | | | Date | Mon, 28 Oct 2002 16:54:01 +0100 | From | Jens Axboe <> | Subject | Re: [patch][cft] zero-copy dma cd writing and ripping |
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On Mon, Oct 28 2002, David S. Miller wrote: > 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().
Yes, that that is the
if (write_to_vm) flush_dcache_page(page);
> I find the write_to_vm condition confusion which is probably why > I am sitting here spelling this out :-)
Hehe, actually these are confusing, but write_to_vm is probably the least confusing wording I think. It means we are reading from the device, thus writing to vm pages.
> 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.
Thus
if (!write_to_vm) flush_dcache_page(page);
> 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 :-)
I agree then. I will remove references to flush_dcache_page() in bio_map_user() and bio_unmap_user(), and we'll just unconditionally do flush_dcache_page() on every page mapped in get_user_pages(). That sounds far better to me. Do you really expect users of get_user_pages() to get this right (remember, we are often talking about device drivers :-). I sure am not :)
-- Jens Axboe
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