Re: How to map high memory for block io

[Russell King <rmk+lkml@arm.linux.org.uk>]

On Fri, Jan 27, 2006 at 09:04:33PM +0100, Pierre Ossman wrote:
> Russell King wrote:
> > I don't see what the problem is.  A sg entry is a list of struct page
> > pointers, an offset, and a size.  As such, it can't describe a transfer
> > which crosses a page because such a structure does not imply that one
> > struct page follows another struct page.
> 
> If the pages do not strictly follow each other then there is a lot of
> broken code in the kernel. drivers/mmc/mmci.c and drivers/block/ub.c
> being two occurences since both assume they can access the entire entry
> through a single mapping.

We don't make that assumption.  What we do is:

- map the current sg using kmap_atomic()
- copy up to sg->length into or out of that mapping
- unmap current sg
- if we have reached the end of this sg, move on to the next

What this means is that we assume sg->offset + sg->length <= PAGE_SIZE
in all cases, which is the same assumption architecture DMA code makes.
If that's invalid, there's likely to be a lot of architecture DMA support
which is broken.

-- 
Russell King
 Linux kernel    2.6 ARM Linux   - http://www.arm.linux.org.uk/
 maintainer of:  2.6 Serial core