[RFC PATCH 4/4] drm/amdgpu/dma-buf: Treat MMIO_REMAP as fixed I/O; don't move it

[Srinivasan Shanmugam]

The HDP flush page (AMDGPU_PL_MMIO_REMAP) is an MMIO window, not RAM.
It must not be migrated to GTT/VRAM by dma-buf paths.

This change makes pin/unpin no-ops for MMIO_REMAP and skips CPU-access
migration, keeping the object fixed.

Cc: Christian König <christian.koe...@amd.com>
Cc: Alex Deucher <alexander.deuc...@amd.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmu...@amd.com>
---
 drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c 
b/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
index 31d78561ab95..4c75e296b57f 100644
--- a/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
+++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c
@@ -109,6 +109,11 @@ static int amdgpu_dma_buf_pin(struct dma_buf_attachment 
*attach)
        struct amdgpu_bo *bo = gem_to_amdgpu_bo(dmabuf->priv);
        u32 domains = bo->allowed_domains;
 
+       /* Already pinned at export; don't try to move I/O apertures */
+       if (bo->tbo.resource &&
+           bo->tbo.resource->mem_type == AMDGPU_PL_MMIO_REMAP)
+               return 0;
+
        dma_resv_assert_held(dmabuf->resv);
 
        /* Try pinning into VRAM to allow P2P with RDMA NICs without ODP
@@ -148,6 +153,10 @@ static void amdgpu_dma_buf_unpin(struct dma_buf_attachment 
*attach)
        struct drm_gem_object *obj = attach->dmabuf->priv;
        struct amdgpu_bo *bo = gem_to_amdgpu_bo(obj);
 
+       if (bo->tbo.resource &&
+           bo->tbo.resource->mem_type == AMDGPU_PL_MMIO_REMAP)
+               return;
+
        amdgpu_bo_unpin(bo);
 }
 
@@ -354,6 +363,11 @@ static int amdgpu_dma_buf_begin_cpu_access(struct dma_buf 
*dma_buf,
        bool reads = (direction == DMA_BIDIRECTIONAL ||
                      direction == DMA_FROM_DEVICE);
 
+       /* Never migrate MMIO_REMAP for CPU access heuristics */
+       if (bo->tbo.resource &&
+           bo->tbo.resource->mem_type == AMDGPU_PL_MMIO_REMAP)
+               return 0;
+
        if (!reads || !(domain & AMDGPU_GEM_DOMAIN_GTT))
                return 0;
 
-- 
2.34.1