drm/amdgpu/ttm: Pin 4K MMIO_REMAP Singleton BO at Init v2

MMIO_REMAP (HDP flush page) is a hardware I/O window exposed via a PCI
BAR.  It must not migrate or be evicted.

Allocate a single 4 KB GEM BO in AMDGPU_GEM_DOMAIN_MMIO_REMAP during TTM
initialization when the hardware exposes a remap bus address and the
host page size is <= 4 KiB. Reserve the BO and pin it at the TTM level
so it remains fixed for its lifetime. No CPU mapping is established
here.

On teardown, reserve, unpin, and free the BO if present.

This prepares the object to be shared (e.g., via dma-buf) without
triggering placement changes or no CPU-access migration

v2: Added extra NULL checks

Suggested-by: Christian König <christian.koenig@amd.com>
Suggested-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

1 files changed, 32 insertions, 0 deletions

diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c
index 2b931e855abd..ac67886acaa2 100644
--- a/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c
+++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c
@@ -1820,6 +1820,10 @@ static void amdgpu_ttm_pools_fini(struct amdgpu_device *adev)
  * PAGE_SIZE is <= AMDGPU_GPU_PAGE_SIZE (4K). The BO is created as a regular
  * GEM object (amdgpu_bo_create).
  *
+ * The BO is created as a normal GEM object via amdgpu_bo_create(), then
+ * reserved and pinned at the TTM level (ttm_bo_pin()) so it can never be
+ * migrated or evicted. No CPU mapping is established here.
+ *
  * Return:
  *  * 0 on success or intentional skip (feature not present/unsupported)
  *  * negative errno on allocation failure
@@ -1848,7 +1852,26 @@ static int amdgpu_ttm_mmio_remap_bo_init(struct amdgpu_device *adev)
 	if (r)
 		return r;
 
+	r = amdgpu_bo_reserve(adev->rmmio_remap.bo, true);
+	if (r)
+		goto err_unref;
+
+	/*
+	 * MMIO_REMAP is a fixed I/O placement (AMDGPU_PL_MMIO_REMAP).
+	 * Use TTM-level pin so the BO cannot be evicted/migrated,
+	 * independent of GEM domains. This
+	 * enforces the “fixed I/O window”
+	 */
+	ttm_bo_pin(&adev->rmmio_remap.bo->tbo);
+
+	amdgpu_bo_unreserve(adev->rmmio_remap.bo);
 	return 0;
+
+err_unref:
+	if (adev->rmmio_remap.bo)
+		amdgpu_bo_unref(&adev->rmmio_remap.bo);
+	adev->rmmio_remap.bo = NULL;
+	return r;
 }
 
 /**
@@ -1860,6 +1883,15 @@ static int amdgpu_ttm_mmio_remap_bo_init(struct amdgpu_device *adev)
  */
 static void amdgpu_ttm_mmio_remap_bo_fini(struct amdgpu_device *adev)
 {
+	struct amdgpu_bo *bo = adev->rmmio_remap.bo;
+
+	if (!bo)
+		return;   /* <-- safest early exit */
+
+	if (!amdgpu_bo_reserve(adev->rmmio_remap.bo, true)) {
+		ttm_bo_unpin(&adev->rmmio_remap.bo->tbo);
+		amdgpu_bo_unreserve(adev->rmmio_remap.bo);
+	}
 	amdgpu_bo_unref(&adev->rmmio_remap.bo);
 	adev->rmmio_remap.bo = NULL;
 }