Re: [PATCH v3 27/33] drm/xe: Add IOVA-based xe_res_cursor variant

[Niranjana Vishwanathapura <niranjana.vishwanathapura@intel.com>]

On Fri, Jul 10, 2026 at 07:56:13PM -0700, Matthew Brost wrote:
>Introduce xe_res_first_iova() to walk a buffer that was DMA mapped using
>the IOVA-based DMA API (dma_iova_try_alloc() and friends). Such a mapping
>is described by a struct dma_iova_state and is a single contiguous IOVA
>range, so the cursor walks it as one segment.
>
>Add an is_iova flag to struct xe_res_cursor to distinguish this case.
>When set, xe_res_dma() returns the IOVA backing the current position
>(state->addr + cursor offset), xe_res_next() advances within the single
>contiguous segment, and xe_res_is_vram() returns false since IOVA
>mappings never point to same-device VRAM.
>
>The flag is explicitly cleared in the existing xe_res_first(),
>xe_res_first_sg() and xe_res_first_dma() initializers so stale stack
>values cannot leak into the new path.
>
>Cc: Carlos Santa <carlos.santa@intel.com>
>Cc: Ryan Neph <ryanneph@google.com>
>Cc: Christian Koenig <christian.koenig@amd.com>
>Cc: Huang Rui <ray.huang@amd.com>
>Cc: Matthew Auld <matthew.auld@intel.com>
>Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
>Cc: Maxime Ripard <mripard@kernel.org>
>Cc: Thomas Zimmermann <tzimmermann@suse.de>
>Cc: David Airlie <airlied@gmail.com>
>Cc: Simona Vetter <simona@ffwll.ch>
>Cc: dri-devel@lists.freedesktop.org
>Cc: linux-kernel@vger.kernel.org
>Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
>Assisted-by: GitHub_Copilot:claude-opus-4.8
>Signed-off-by: Matthew Brost <matthew.brost@intel.com>
>---
> drivers/gpu/drm/xe/xe_res_cursor.h | 56 +++++++++++++++++++++++++++++-
> 1 file changed, 55 insertions(+), 1 deletion(-)
>
>diff --git a/drivers/gpu/drm/xe/xe_res_cursor.h b/drivers/gpu/drm/xe/xe_res_cursor.h
>index 0522caafd89d..f0f1d699e16e 100644
>--- a/drivers/gpu/drm/xe/xe_res_cursor.h
>+++ b/drivers/gpu/drm/xe/xe_res_cursor.h
>@@ -24,6 +24,7 @@
> #ifndef _XE_RES_CURSOR_H_
> #define _XE_RES_CURSOR_H_
>
>+#include <linux/dma-mapping.h>
> #include <linux/scatterlist.h>
>
> #include <drm/drm_pagemap.h>
>@@ -67,6 +68,12 @@ struct xe_res_cursor {
> 	u64 dma_start;
> 	/** @dma_seg_size: Size of the current DMA segment. */
> 	u64 dma_seg_size;
>+	/**
>+	 * @is_iova: Whether the cursor walks a single contiguous IOVA
>+	 * mapping described by a struct dma_iova_state. When true, the
>+	 * DMA address is @dma_start + @start and never points to VRAM.
>+	 */
>+	bool is_iova;
> };
>
> static struct gpu_buddy *xe_res_get_buddy(struct ttm_resource *res)
>@@ -93,6 +100,7 @@ static inline void xe_res_first(struct ttm_resource *res,
> {
> 	cur->sgl = NULL;
> 	cur->dma_addr = NULL;
>+	cur->is_iova = false;
> 	if (!res)
> 		goto fallback;
>
>@@ -224,6 +232,7 @@ static inline void xe_res_first_sg(const struct sg_table *sg,
> 	cur->dma_addr = NULL;
> 	cur->sgl = sg->sgl;
> 	cur->mem_type = XE_PL_TT;
>+	cur->is_iova = false;
> 	__xe_res_sg_next(cur);
> }
>
>@@ -255,6 +264,41 @@ static inline void xe_res_first_dma(const struct drm_pagemap_addr *dma_addr,
> 	__xe_res_dma_next(cur);
> 	cur->sgl = NULL;
> 	cur->mem_type = XE_PL_TT;
>+	cur->is_iova = false;
>+}

Assisted-by: LLM

NIT..Instead of defining and clearing is_iova flag, would it be better
to define enum defitions for different cursor types?

>+
>+/**
>+ * xe_res_first_iova() - initialize a xe_res_cursor with a dma_iova_state
>+ *
>+ * @state: struct dma_iova_state describing a single contiguous IOVA mapping
>+ * @start: Start of the range
>+ * @size: Size of the range
>+ * @cur: cursor object to initialize
>+ *
>+ * Start walking over the range of an allocation that was DMA mapped using the
>+ * IOVA-based DMA API (see dma_iova_try_alloc()). Such a mapping is a single
>+ * contiguous IOVA range, so the cursor walks it as one segment. Subsequent
>+ * calls to xe_res_next() advance within that range and xe_res_dma() returns
>+ * the IOVA backing the current position. xe_res_is_vram() always returns false
>+ * for such a cursor.
>+ */
>+static inline void xe_res_first_iova(struct dma_iova_state *state,
>+				     u64 start, u64 size,
>+				     struct xe_res_cursor *cur)
>+{
>+	XE_WARN_ON(!state);
>+	XE_WARN_ON(start + size > dma_iova_size(state));
>+
>+	cur->node = NULL;
>+	cur->start = start;
>+	cur->remaining = size;
>+	cur->size = size;
>+	cur->dma_addr = NULL;
>+	cur->sgl = NULL;
>+	cur->dma_start = state->addr;
>+	cur->dma_seg_size = dma_iova_size(state);

NIT...looks like cur->dma_seg_size is never used, but should be fine.

>+	cur->mem_type = XE_PL_TT;
>+	cur->is_iova = true;
> }
>
> /**
>@@ -283,6 +327,13 @@ static inline void xe_res_next(struct xe_res_cursor *cur, u64 size)
> 		return;
> 	}
>
>+	if (cur->is_iova) {
>+		/* Single contiguous IOVA segment. */
>+		cur->start += size;
>+		cur->size = cur->remaining;
>+		return;
>+	}

This code never gets executed. Probably remove it and add an assert
if we reach here with cur->is_iova set (or add a comment)?

Niranjana

>+
> 	if (cur->dma_addr) {
> 		cur->start += size;
> 		__xe_res_dma_next(cur);
>@@ -334,7 +385,7 @@ static inline void xe_res_next(struct xe_res_cursor *cur, u64 size)
>  */
> static inline u64 xe_res_dma(const struct xe_res_cursor *cur)
> {
>-	if (cur->dma_addr)
>+	if (cur->is_iova || cur->dma_addr)
> 		return cur->dma_start + cur->start;
> 	else if (cur->sgl)
> 		return sg_dma_address(cur->sgl) + cur->start;
>@@ -351,6 +402,9 @@ static inline u64 xe_res_dma(const struct xe_res_cursor *cur)
>  */
> static inline bool xe_res_is_vram(const struct xe_res_cursor *cur)
> {
>+	if (cur->is_iova)
>+		return false;
>+
> 	if (cur->dma_addr)
> 		return cur->dma_addr->proto == XE_INTERCONNECT_VRAM;
>
>-- 
>2.34.1
>