Re: Bounce buffer usage

[Jens Axboe <axboe@suse.de>]

On Sun, Nov 03 2002, Mark Lobo wrote:
> Now in the initiator driver, all what I get is a
> request buffer address. To actually use the above API,
> I need the page address, and that I get from
> virt_to_page ( as described in the bounce buffer patch
> doc) So I dont need a virt_to_page call to get the
> page address? I dont understand how I can use 
> this --> "struct page *page = buffer->page;"? In that
> case, what gets passed down to the initiator in the
> "no scatter gather" case is some kind of structure?
> Dont I need to do "struct page *page =
> virt_to_page(buffer);"?

For drivers that deal with highmem, you are getting sg setup even for a
single segment. It would indeed be possible to send down a virtual
address for non-sg, if the original page didn't reside in highmem. In
that case, yes all you would have to do is

	page = virt_to_page(buf);
	offset = ((unsigned long) buf) & ~PAGE_MASK;

to get the remaining data you need. However, most drivers typically do

	if (command->use_sg)
		nr_entries = pci_map_page() sg list
	else
		pci_map_single() virtual address

-- 
Jens Axboe