[RFC v3 2/8] drm/xe/bo: Add purgeable bo state tracking and field madv to xe_bo

[Arvind Yadav <arvind.yadav@intel.com>]

Add infrastructure for tracking purgeable state of buffer objects.
This includes:

Introduce enum xe_madv_purgeable_state with three states:
   - XE_MADV_PURGEABLE_WILLNEED (0): BO is needed and should not be
     purged. This is the default state for all BOs.

   - XE_MADV_PURGEABLE_DONTNEED (1): BO is not currently needed and
     can be purged by the kernel under memory pressure to reclaim
     resources. Only non-shared BOs can be marked as DONTNEED.

   - XE_MADV_PURGEABLE_PURGED (2): BO has been purged by the kernel.
     Accessing a purged BO results in error. Follows i915 semantics
     where once purged, the BO remains permanently invalid ("once
     purged, always purged").

Add atomic_t madv field to struct xe_bo for state tracking
  of purgeable state across concurrent access paths

v2:
  - Add xe_bo_is_purged() helper, improve state documentation

V3:
  - Add the kernel doc(Matthew Brost)
  - Add the new helpers xe_bo_madv_is_dontneed(Matthew Brost)

Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>
Signed-off-by: Arvind Yadav <arvind.yadav@intel.com>
---
 drivers/gpu/drm/xe/xe_bo.h       | 56 ++++++++++++++++++++++++++++++++
 drivers/gpu/drm/xe/xe_bo_types.h |  3 ++
 2 files changed, 59 insertions(+)

diff --git a/drivers/gpu/drm/xe/xe_bo.h b/drivers/gpu/drm/xe/xe_bo.h
index 8ab4474129c3..1090a60f6ef6 100644
--- a/drivers/gpu/drm/xe/xe_bo.h
+++ b/drivers/gpu/drm/xe/xe_bo.h
@@ -86,6 +86,28 @@
 
 #define XE_PCI_BARRIER_MMAP_OFFSET	(0x50 << XE_PTE_SHIFT)
 
+/**
+ * enum xe_madv_purgeable_state - Buffer object purgeable state enumeration
+ *
+ * This enum defines the possible purgeable states for a buffer object,
+ * allowing userspace to provide memory usage hints to the kernel for
+ * better memory management under pressure.
+ *
+ * @XE_MADV_PURGEABLE_WILLNEED: The buffer object is needed and should not be purged.
+ * This is the default state.
+ * @XE_MADV_PURGEABLE_DONTNEED: The buffer object is not currently needed and can be
+ * purged by the kernel under memory pressure.
+ * @XE_MADV_PURGEABLE_PURGED: The buffer object has been purged by the kernel.
+ *
+ * Accessing a purged buffer will result in an error. Per i915 semantics,
+ * once purged, a BO remains permanently invalid and must be destroyed and recreated.
+ */
+enum xe_madv_purgeable_state {
+	XE_MADV_PURGEABLE_WILLNEED,
+	XE_MADV_PURGEABLE_DONTNEED,
+	XE_MADV_PURGEABLE_PURGED,
+};
+
 struct sg_table;
 
 struct xe_bo *xe_bo_alloc(void);
@@ -214,6 +236,40 @@ static inline bool xe_bo_is_protected(const struct xe_bo *bo)
 	return bo->pxp_key_instance;
 }
 
+/**
+ * xe_bo_is_purged() - Check if buffer object has been purged
+ * @bo: The buffer object to check
+ *
+ * Checks if the buffer object's backing store has been discarded by the
+ * kernel due to memory pressure after being marked as purgeable (DONTNEED).
+ * Once purged, the BO cannot be restored and any attempt to use it will fail.
+ *
+ * Context: Caller must hold the BO's dma-resv lock
+ * Return: true if the BO has been purged, false otherwise
+ */
+static inline bool xe_bo_is_purged(struct xe_bo *bo)
+{
+	xe_bo_assert_held(bo);
+	return atomic_read(&bo->madv_purgeable) == XE_MADV_PURGEABLE_PURGED;
+}
+
+/**
+ * xe_bo_madv_is_dontneed() - Check if BO is marked as DONTNEED
+ * @bo: The buffer object to check
+ *
+ * Checks if userspace has marked this BO as DONTNEED (i.e., its contents
+ * are not currently needed and can be discarded under memory pressure).
+ * This is used internally to decide whether a BO is eligible for purging.
+ *
+ * Context: Caller must hold the BO's dma-resv lock
+ * Return: true if the BO is marked DONTNEED, false otherwise
+ */
+static inline bool xe_bo_madv_is_dontneed(struct xe_bo *bo)
+{
+	xe_bo_assert_held(bo);
+	return atomic_read(&bo->madv_purgeable) == XE_MADV_PURGEABLE_DONTNEED;
+}
+
 static inline void xe_bo_unpin_map_no_vm(struct xe_bo *bo)
 {
 	if (likely(bo)) {
diff --git a/drivers/gpu/drm/xe/xe_bo_types.h b/drivers/gpu/drm/xe/xe_bo_types.h
index d4fe3c8dca5b..57b4dc7012e2 100644
--- a/drivers/gpu/drm/xe/xe_bo_types.h
+++ b/drivers/gpu/drm/xe/xe_bo_types.h
@@ -108,6 +108,9 @@ struct xe_bo {
 	 * from default
 	 */
 	u64 min_align;
+
+	/** @madv_purgeable: user space advise on BO purgeability */
+	atomic_t madv_purgeable;
 };
 
 #endif
-- 
2.43.0