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

[David S. Miller]

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 :-)