On Fri, Jan 04, 2019 at 08:22:13AM +0100, Christoph Hellwig wrote:
> On Fri, Jan 04, 2019 at 08:18:52AM +0100, Christoph Hellwig wrote:
> > > CH: Does sg_page_iter_dma_address() even make any sense??
> >
> > No, it doesn't and should never have been edited. It's one of these
> > typical scatterlist abuses in the media code :(
>
> Actually I take this back. Due to the sg_page_iter structure iterating
> over segments first and then pages inside the segment it actually
> is ok. It just looks very awkward.
I'm looking at __sg_page_iter_next which does this:
static int sg_page_count(struct scatterlist *sg)
{
return PAGE_ALIGN(sg->offset + sg->length) >> PAGE_SHIFT;
}
bool __sg_page_iter_next(struct sg_page_iter *piter)
{
[..]
while (piter->sg_pgoffset >= sg_page_count(piter->sg)) {
piter->sg_pgoffset -= sg_page_count(piter->sg);
Which isn't going to yield the right math if
sg->length != sg_dma_len(sg)
Surely we need a version of sg_page_iter_next() that uses sg_dma_len
to compute page_count if the caller intents to call
sg_page_iter_dma_address() ?
I assume it works for media because they have arranged things so that
sg_dma_len == sg->length, but in the general case with dma_map_sg that
can't be assumed.
Jason
