drm/xe: Apply upper limit to sg element size

The iommu_dma_map_sg() function ensures iova allocation doesn't
cross dma segment boundary. It does so by padding some sg elements.
This can cause overflow, ending up with sg->length being set to 0.
Avoid this by halving the maximum segment size (rounded down to
PAGE_SIZE).

Specify maximum segment size for sg elements by using
sg_alloc_table_from_pages_segment() to allocate sg_table.

v2: Use correct max segment size in dma_set_max_seg_size() call
Signed-off-by: Niranjana Vishwanathapura <niranjana.vishwanathapura@intel.com>
Reviewed-by: Bruce Chang <yu.bruce.chang@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>

drivers/gpu/drm/xe/xe_bo.c

@@ -251,9 +251,11 @@ static int xe_tt_map_sg(struct ttm_tt *tt)
 	if (xe_tt->sg)
 		return 0;
 
-	ret = sg_alloc_table_from_pages(&xe_tt->sgt, tt->pages, num_pages,
-					0, (u64)num_pages << PAGE_SHIFT,
-					GFP_KERNEL);
+	ret = sg_alloc_table_from_pages_segment(&xe_tt->sgt, tt->pages,
+						num_pages, 0,
+						(u64)num_pages << PAGE_SHIFT,
+						xe_sg_segment_size(xe_tt->dev),
+						GFP_KERNEL);
 	if (ret)
 		return ret;
 

drivers/gpu/drm/xe/xe_bo.h

@@ -296,6 +296,30 @@ void xe_bo_put_commit(struct llist_head *deferred);
 
 struct sg_table *xe_bo_get_sg(struct xe_bo *bo);
 
+/*
+ * xe_sg_segment_size() - Provides upper limit for sg segment size.
+ * @dev: device pointer
+ *
+ * Returns the maximum segment size for the 'struct scatterlist'
+ * elements.
+ */
+static inline unsigned int xe_sg_segment_size(struct device *dev)
+{
+	struct scatterlist __maybe_unused sg;
+	size_t max = BIT_ULL(sizeof(sg.length) * 8) - 1;
+
+	max = min_t(size_t, max, dma_max_mapping_size(dev));
+
+	/*
+	 * The iommu_dma_map_sg() function ensures iova allocation doesn't
+	 * cross dma segment boundary. It does so by padding some sg elements.
+	 * This can cause overflow, ending up with sg->length being set to 0.
+	 * Avoid this by ensuring maximum segment size is half of 'max'
+	 * rounded down to PAGE_SIZE.
+	 */
+	return round_down(max / 2, PAGE_SIZE);
+}
+
 #if IS_ENABLED(CONFIG_DRM_XE_KUNIT_TEST)
 /**
  * xe_bo_is_mem_type - Whether the bo currently resides in the given

drivers/gpu/drm/xe/xe_mmio.c

@@ -11,6 +11,7 @@
 #include "regs/xe_engine_regs.h"
 #include "regs/xe_gt_regs.h"
 #include "regs/xe_regs.h"
+#include "xe_bo.h"
 #include "xe_device.h"
 #include "xe_gt.h"
 #include "xe_gt_mcr.h"
@@ -26,11 +27,7 @@ static int xe_set_dma_info(struct xe_device *xe)
 	unsigned int mask_size = xe->info.dma_mask_size;
 	int err;
 
-	/*
-	 * We don't have a max segment size, so set it to the max so sg's
-	 * debugging layer doesn't complain
-	 */
-	dma_set_max_seg_size(xe->drm.dev, UINT_MAX);
+	dma_set_max_seg_size(xe->drm.dev, xe_sg_segment_size(xe->drm.dev));
 
 	err = dma_set_mask(xe->drm.dev, DMA_BIT_MASK(mask_size));
 	if (err)

drivers/gpu/drm/xe/xe_vm.c

@@ -117,9 +117,11 @@ int xe_vma_userptr_pin_pages(struct xe_vma *vma)
 	if (ret)
 		goto out;
 
-	ret = sg_alloc_table_from_pages(&vma->userptr.sgt, pages, pinned,
-					0, (u64)pinned << PAGE_SHIFT,
-					GFP_KERNEL);
+	ret = sg_alloc_table_from_pages_segment(&vma->userptr.sgt, pages,
+						pinned, 0,
+						(u64)pinned << PAGE_SHIFT,
+						xe_sg_segment_size(xe->drm.dev),
+						GFP_KERNEL);
 	if (ret) {
 		vma->userptr.sg = NULL;
 		goto out;