drm/i915/gem: Calculate object page offset for partial memory mapping

To enable partial memory mapping of GPU virtual memory, it's
necessary to introduce an offset to the object's memory
(obj->mm.pages) scatterlist. This adjustment compensates for
instances when userspace mappings do not start from the beginning
of the object.

Based on a patch by Chris Wilson.

Signed-off-by: Andi Shyti <[e-mail]>
Cc: Chris Wilson <[e-mail]>
Cc: Lionel Landwerlin <[e-mail]>
Reviewed-by: Nirmoy Das <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/i915/gem/i915_gem_mman.c

···
 	struct vm_area_struct *area = vmf->vma;
 	struct i915_mmap_offset *mmo = area->vm_private_data;
 	struct drm_i915_gem_object *obj = mmo->obj;
+	unsigned long obj_offset;
 	resource_size_t iomap;
 	int err;
 
···
 		iomap -= obj->mm.region->region.start;
 	}
 
+	obj_offset = area->vm_pgoff - drm_vma_node_start(&mmo->vma_node);
 	/* PTEs are revoked in obj->ops->put_pages() */
 	err = remap_io_sg(area,
 			  area->vm_start, area->vm_end - area->vm_start,
-			  obj->mm.pages->sgl, iomap);
+			  obj->mm.pages->sgl, obj_offset, iomap);
 
 	if (area->vm_flags & VM_WRITE) {
 		GEM_BUG_ON(!i915_gem_object_has_pinned_pages(obj));

drivers/gpu/drm/i915/i915_mm.c

···
  * @addr: target user address to start at
  * @size: size of map area
  * @sgl: Start sg entry
+ * @offset: offset from the start of the page
  * @iobase: Use stored dma address offset by this address or pfn if -1
  *
  *  Note: this is only safe if the mm semaphore is held when called.
  */
 int remap_io_sg(struct vm_area_struct *vma,
 		unsigned long addr, unsigned long size,
-		struct scatterlist *sgl, resource_size_t iobase)
+		struct scatterlist *sgl, unsigned long offset,
+		resource_size_t iobase)
 {
 	struct remap_pfn r = {
 		.mm = vma->vm_mm,
···
 
 	/* We rely on prevalidation of the io-mapping to skip track_pfn(). */
 	GEM_BUG_ON((vma->vm_flags & EXPECTED_FLAGS) != EXPECTED_FLAGS);
+
+	while (offset >= sg_dma_len(r.sgt.sgp) >> PAGE_SHIFT) {
+		offset -= sg_dma_len(r.sgt.sgp) >> PAGE_SHIFT;
+		r.sgt = __sgt_iter(__sg_next(r.sgt.sgp), use_dma(iobase));
+		if (!r.sgt.sgp)
+			return -EINVAL;
+	}
+	r.sgt.curr = offset << PAGE_SHIFT;
 
 	if (!use_dma(iobase))
 		flush_cache_range(vma, addr, size);

drivers/gpu/drm/i915/i915_mm.h

···
 
 int remap_io_sg(struct vm_area_struct *vma,
 		unsigned long addr, unsigned long size,
-		struct scatterlist *sgl, resource_size_t iobase);
+		struct scatterlist *sgl, unsigned long offset,
+		resource_size_t iobase);
 
 #endif /* __I915_MM_H__ */