On Sat, 2013-02-09 at 19:56 +0100, Daniel Vetter wrote:
> Hi Imre!
>
> On Sat, Feb 09, 2013 at 05:27:33PM +0200, Imre Deak wrote:
> > Add a helper to walk through a scatter list a page at a time. Needed by
> > upcoming patches fixing the scatter list walking logic in the i915 driver.
>
> Nice patch, but I think this would make a rather nice addition to the
> common scatterlist api in scatterlist.h, maybe called sg_page_iter.
> There's already another helper which does cpu mappings, but it has a
> different use-case (gives you the page mapped, which we don't neeed and
> can cope with not page-aligned sg tables). With dma-buf using sg tables I
> expect more users of such a sg page iterator to pop up. Most possible
> users of this will hang around on linaro-mm-sig, so please also cc that
> besides the usual suspects.
Ok, I wanted to sneak this in to drm (and win some time) thinking there
isn't any other user of it atm. But I agree the ideal place for it is in
scatterlist.h, so will send it there.
> Cheers, Daniel
>
> >
> > Signed-off-by: Imre Deak <imre.deak at intel.com>
> > ---
> > include/drm/drmP.h | 44 ++++++++++++++++++++++++++++++++++++++++++++
> > 1 file changed, 44 insertions(+)
> >
> > diff --git a/include/drm/drmP.h b/include/drm/drmP.h
> > index fad21c9..0c0c213 100644
> > --- a/include/drm/drmP.h
> > +++ b/include/drm/drmP.h
> > @@ -1578,6 +1578,50 @@ extern int drm_sg_alloc_ioctl(struct drm_device *dev, void *data,
> > extern int drm_sg_alloc(struct drm_device *dev, struct drm_scatter_gather * request);
> > extern int drm_sg_free(struct drm_device *dev, void *data,
> > struct drm_file *file_priv);
> > +struct drm_sg_iter {
> > + struct scatterlist *sg;
> > + int sg_offset;
>
> Imo using sg_pfn_offset (i.e. sg_offset >> PAGE_SIZE) would make it
> clearer that this is all about iterating page-aligned sg tables.
>
> > + struct page *page;
> > +};
> > +
> > +static inline int
> > +__drm_sg_iter_seek(struct drm_sg_iter *iter)
> > +{
> > + while (iter->sg && iter->sg_offset >= iter->sg->length) {
> > + iter->sg_offset -= iter->sg->length;
> > + iter->sg = sg_next(iter->sg);
>
> And adding a WARN_ON(sg->legnth & ~PAGE_MASK); here would enforce that.
>
> > + }
> > +
> > + return iter->sg ? 0 : -1;
> > +}
> > +
> > +static inline struct page *
> > +drm_sg_iter_next(struct drm_sg_iter *iter)
> > +{
> > + struct page *page;
> > +
> > + if (__drm_sg_iter_seek(iter))
> > + return NULL;
> > +
> > + page = nth_page(sg_page(iter->sg), iter->sg_offset >> PAGE_SHIFT);
> > + iter->sg_offset = (iter->sg_offset + PAGE_SIZE) & PAGE_MASK;
> > +
> > + return page;
> > +}
> > +
> > +static inline struct page *
> > +drm_sg_iter_start(struct drm_sg_iter *iter, struct scatterlist *sg,
> > + unsigned long offset)
> > +{
> > + iter->sg = sg;
> > + iter->sg_offset = offset;
> > +
> > + return drm_sg_iter_next(iter);
> > +}
> > +
> > +#define drm_for_each_sg_page(iter, sg, pgoffset) \
> > + for ((iter)->page = drm_sg_iter_start((iter), (sg), (pgoffset));\
> > + (iter)->page; (iter)->page = drm_sg_iter_next(iter))
>
> Again, for the initialization I'd go with page numbers, not an offset in
> bytes.
Ok, can change it to use page numbers instead.
--Imre
