Hi Christoph,
On 05.05.2020 13:09, Christoph Hellwig wrote:
On Tue, May 05, 2020 at 12:51:58PM +0200, Marek Szyprowski wrote:
On 05.05.2020 12:15, Christoph Hellwig wrote:
for_each_sg_page(st->sgl, &sg_iter, st->nents, 0)
for_each_sg_page(st->sgl, &sg_iter, st->orig_nents, 0)Would it make sense to also add a for_each_sgtable_page helper that hides the use of orig_nents? To be used like:
for_each_sgtable_page(st, &sg_iter, 0) {We would need two helpers:
for_each_sgtable_cpu_page() and for_each_sgtable_dma_page().
I considered them, but then I found that there are already for_each_sg_page(), for_each_sg_dma_page() and various special iterators like sg_page_iter, sg_dma_page_iter and sg_mapping_iter. Too bad that they are almost not used, at least in the DRM subsystem. I wonder if it make sense to apply them or simply provide the two above mentioned wrappers?
None of the helpers helps with passing the right parameters from the sg_table. So in doube we'd need wrappers for all of the above, or none..
I've played a bit with the code and the existing scatterlist iterators - for_each_sg_page() and for_each_sg_dma_page(). I've found them quite handy!
The biggest advantage of them is that they always iterate over scatterlist in PAGE_SIZE units, what should make the code much easier to understand. Here is example of their application to the function that started this thread:
int drm_prime_sg_to_page_addr_arrays(struct sg_table *sgt, struct page **pages, dma_addr_t *addrs, int max_entries) { struct sg_dma_page_iter dma_iter; struct sg_page_iter page_iter;
if (addrs) for_each_sgtable_dma_page(sgt, &dma_iter, 0) *addrs++ = sg_page_iter_dma_address(&dma_iter); if (pages) for_each_sgtable_page(sgt, &page_iter, 0) *pages++ = sg_page_iter_page(&page_iter); return 0; }
After applying those iterators where possible (they can be used only for reading the scatterlist), we would just need to add 2 trivial wrappers to use them with sg_table objects:
#define for_each_sgtable_page(sgt, piter, pgoffset) \ for_each_sg_page(sgt->sgl, piter, sgt->orig_nents, pgoffset)
#define for_each_sgtable_dma_page(sgt, dma_iter, pgoffset) \ for_each_sg_dma_page(sgt->sgl, dma_iter, sgt->nents, pgoffset)
Then we would just need one more helper to construct scatterlist, as the above two are read-only don't allow to modify scatterlist:
#define for_each_sgtable_sg(sgt, sg, i) \ for_each_sg(sgt->sgl, sg, sgt->orig_nents, i)
With the above 3 helpers we can probably get rid of all instances of sg_table->{nents,orig_nents} from the DRM code. I will prepare patches soon.
Best regards