Re: [patch][cft] zero-copy dma cd writing and ripping

[Jens Axboe <axboe@suse.de>]

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