lib/scatterlist: sg_page_iter: support sg lists w/o backing pages

The i915 driver uses sg lists for memory without backing 'struct page'
pages, similarly to other IO memory regions, setting only the DMA
address for these. It does this, so that it can program the HW MMU
tables in a uniform way both for sg lists with and without backing pages.

Without a valid page pointer we can't call nth_page to get the current
page in __sg_page_iter_next, so add a helper that relevant users can
call separately. Also add a helper to get the DMA address of the current
page (idea from Daniel).

Convert all places in i915, to use the new API.

Signed-off-by: Imre Deak <[e-mail]>
Reviewed-by: Damien Lespiau <[e-mail]>
Signed-off-by: Daniel Vetter <[e-mail]>

drivers/gpu/drm/drm_cache.c

···
 
 		mb();
 		for_each_sg_page(st->sgl, &sg_iter, st->nents, 0)
-			drm_clflush_page(sg_iter.page);
+			drm_clflush_page(sg_page_iter_page(&sg_iter));
 		mb();
 
 		return;

drivers/gpu/drm/i915/i915_drv.h

···
 	struct sg_page_iter sg_iter;
 
 	for_each_sg_page(obj->pages->sgl, &sg_iter, obj->pages->nents, n)
-		return sg_iter.page;
+		return sg_page_iter_page(&sg_iter);
 
 	return NULL;
 }

drivers/gpu/drm/i915/i915_gem.c

···
 
 	for_each_sg_page(obj->pages->sgl, &sg_iter, obj->pages->nents,
 			 offset >> PAGE_SHIFT) {
-		struct page *page = sg_iter.page;
+		struct page *page = sg_page_iter_page(&sg_iter);
 
 		if (remain <= 0)
 			break;
···
 
 	for_each_sg_page(obj->pages->sgl, &sg_iter, obj->pages->nents,
 			 offset >> PAGE_SHIFT) {
-		struct page *page = sg_iter.page;
+		struct page *page = sg_page_iter_page(&sg_iter);
 		int partial_cacheline_write;
 
 		if (remain <= 0)
···
 		obj->dirty = 0;
 
 	for_each_sg_page(obj->pages->sgl, &sg_iter, obj->pages->nents, 0) {
-		struct page *page = sg_iter.page;
+		struct page *page = sg_page_iter_page(&sg_iter);
 
 		if (obj->dirty)
 			set_page_dirty(page);
···
 err_pages:
 	sg_mark_end(sg);
 	for_each_sg_page(st->sgl, &sg_iter, st->nents, 0)
-		page_cache_release(sg_iter.page);
+		page_cache_release(sg_page_iter_page(&sg_iter));
 	sg_free_table(st);
 	kfree(st);
 	return PTR_ERR(page);

drivers/gpu/drm/i915/i915_gem_dmabuf.c

···
 
 	i = 0;
 	for_each_sg_page(obj->pages->sgl, &sg_iter, obj->pages->nents, 0);
-		pages[i++] = sg_iter.page;
+		pages[i++] = sg_page_iter_page(&sg_iter);
 
 	obj->dma_buf_vmapping = vmap(pages, i, 0, PAGE_KERNEL);
 	drm_free_large(pages);

drivers/gpu/drm/i915/i915_gem_gtt.c

···
 	for_each_sg_page(pages->sgl, &sg_iter, pages->nents, 0) {
 		dma_addr_t page_addr;
 
-		page_addr = sg_dma_address(sg_iter.sg) +
-				(sg_iter.sg_pgoffset << PAGE_SHIFT);
+		page_addr = sg_page_iter_dma_address(&sg_iter);
 		pt_vaddr[act_pte] = gen6_pte_encode(ppgtt->dev, page_addr,
 						    cache_level);
 		if (++act_pte == I915_PPGTT_PT_ENTRIES) {
···
 	dma_addr_t addr;
 
 	for_each_sg_page(st->sgl, &sg_iter, st->nents, 0) {
-		addr = sg_dma_address(sg_iter.sg) +
-			(sg_iter.sg_pgoffset << PAGE_SHIFT);
+		addr = sg_page_iter_dma_address(&sg_iter);
 		iowrite32(gen6_pte_encode(dev, addr, level), &gtt_entries[i]);
 		i++;
 	}

drivers/gpu/drm/i915/i915_gem_tiling.c

···
 
 	i = 0;
 	for_each_sg_page(obj->pages->sgl, &sg_iter, obj->pages->nents, 0) {
-		struct page *page = sg_iter.page;
+		struct page *page = sg_page_iter_page(&sg_iter);
 		char new_bit_17 = page_to_phys(page) >> 17;
 		if ((new_bit_17 & 0x1) !=
 		    (test_bit(i, obj->bit_17) != 0)) {
···
 
 	i = 0;
 	for_each_sg_page(obj->pages->sgl, &sg_iter, obj->pages->nents, 0) {
-		if (page_to_phys(sg_iter.page) & (1 << 17))
+		if (page_to_phys(sg_page_iter_page(&sg_iter)) & (1 << 17))
 			__set_bit(i, obj->bit_17);
 		else
 			__clear_bit(i, obj->bit_17);

include/linux/scatterlist.h

···
  * sg page iterator
  *
  * Iterates over sg entries page-by-page.  On each successful iteration,
- * @piter->page points to the current page, @piter->sg to the sg holding this
- * page and @piter->sg_pgoffset to the page's page offset within the sg. The
- * iteration will stop either when a maximum number of sg entries was reached
- * or a terminating sg (sg_last(sg) == true) was reached.
+ * you can call sg_page_iter_page(@piter) and sg_page_iter_dma_address(@piter)
+ * to get the current page and its dma address. @piter->sg will point to the
+ * sg holding this page and @piter->sg_pgoffset to the page's page offset
+ * within the sg. The iteration will stop either when a maximum number of sg
+ * entries was reached or a terminating sg (sg_last(sg) == true) was reached.
  */
 struct sg_page_iter {
-	struct page		*page;		/* current page */
 	struct scatterlist	*sg;		/* sg holding the page */
 	unsigned int		sg_pgoffset;	/* page offset within the sg */
 
···
 void __sg_page_iter_start(struct sg_page_iter *piter,
 			  struct scatterlist *sglist, unsigned int nents,
 			  unsigned long pgoffset);
+/**
+ * sg_page_iter_page - get the current page held by the page iterator
+ * @piter:	page iterator holding the page
+ */
+static inline struct page *sg_page_iter_page(struct sg_page_iter *piter)
+{
+	return nth_page(sg_page(piter->sg), piter->sg_pgoffset);
+}
+
+/**
+ * sg_page_iter_dma_address - get the dma address of the current page held by
+ * the page iterator.
+ * @piter:	page iterator holding the page
+ */
+static inline dma_addr_t sg_page_iter_dma_address(struct sg_page_iter *piter)
+{
+	return sg_dma_address(piter->sg) + (piter->sg_pgoffset << PAGE_SHIFT);
+}
 
 /**
  * for_each_sg_page - iterate over the pages of the given sg list

lib/scatterlist.c

···
 	piter->__pg_advance = 0;
 	piter->__nents = nents;
 
-	piter->page = NULL;
 	piter->sg = sglist;
 	piter->sg_pgoffset = pgoffset;
 }
···
 		if (!--piter->__nents || !piter->sg)
 			return false;
 	}
-	piter->page = nth_page(sg_page(piter->sg), piter->sg_pgoffset);
 
 	return true;
 }
···
 		miter->__remaining = min_t(unsigned long, miter->__remaining,
 					   PAGE_SIZE - miter->__offset);
 	}
-	miter->page = miter->piter.page;
+	miter->page = sg_page_iter_page(&miter->piter);
 	miter->consumed = miter->length = miter->__remaining;
 
 	if (miter->__flags & SG_MITER_ATOMIC)