[PATCH v3 8/9] drm/amdgpu/ttm: Allocate/Free 4K MMIO_REMAP Singleton BO

[Srinivasan Shanmugam]

Add amdgpu_ttm_mmio_remap_bo_init()/fini() to manage the kernel-owned
one-page(4K) MMIO_REMAP BO. The allocator runs during TTM init when the
hardware exposes a remap base (adev->rmmio_base) and the host PAGE_SIZE
is <= AMDGPU_GPU_PAGE_SIZE (4K).

The helper is idempotent (returns 0 if already allocated) and only
returns an error when the actual allocation fails.

This keeps MMIO_REMAP lifetime handling localized and prepares for the
subsequent patch that exposes a userspace handle.

v2:
 - Check mmio_remap bus address (adev->rmmio_remap.bus_addr) instead of
   rmmio_base. (Alex)
 - Skip quietly if PAGE_SIZE > AMDGPU_GPU_PAGE_SIZE or no bus address
   (no warn). (Alex)
 - Use `amdgpu_bo_create()` (not *_kernel) - Only with this The object
   is stored in adev->mmio_remap.bo and will later be exposed to
   userspace via a GEM handle. (Christian)

Suggested-by: Christian König <christian.koenig at amd.com>
Suggested-by: Alex Deucher <alexander.deucher at amd.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam at amd.com>
---
 drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c | 64 +++++++++++++++++++++++++
 1 file changed, 64 insertions(+)

diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c
index 7822d8969c9f..0e301cab74e0 100644
--- a/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c
+++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c
@@ -1853,6 +1853,63 @@ static void amdgpu_ttm_pools_fini(struct amdgpu_device *adev)
 	adev->mman.ttm_pools = NULL;
 }
 
+/**
+ * amdgpu_ttm_mmio_remap_bo_init - allocate the singleton 4K MMIO_REMAP BO
+ * @adev: amdgpu device
+ *
+ * Allocates a one-page (4K) GEM BO in AMDGPU_GEM_DOMAIN_MMIO_REMAP when the
+ * hardware exposes a remap base (adev->rmmio_remap.bus_addr) and the host
+ * PAGE_SIZE is <= AMDGPU_GPU_PAGE_SIZE (4K). The BO is created as a regular
+ * GEM object (amdgpu_bo_create), then reserved and kmap’ed once to exercise
+ * the io-mem setup path. If prerequisites are not met, this is a no-op.
+ *
+ * Return:
+ *  * 0 on success or intentional skip (feature not present/unsupported)
+ *  * negative errno on allocation failure
+ */
+static int amdgpu_ttm_mmio_remap_bo_init(struct amdgpu_device *adev)
+{
+	int r;
+	struct amdgpu_bo_param bp = { 0 };
+	void *kptr;
+
+	/* Skip if HW doesn’t expose remap or system PAGE > GPU 4K */
+	if (!adev->rmmio_remap.bus_addr || PAGE_SIZE > AMDGPU_GPU_PAGE_SIZE)
+		return 0;
+
+	/* Hardware remap page is fixed 4K; skip on larger PAGE_SIZE. */
+	if (PAGE_SIZE > AMDGPU_GPU_PAGE_SIZE)
+		return 0;
+
+	/* Create exactly one GEM BO in the MMIO_REMAP domain. */
+	bp.type        = ttm_bo_type_device;          /* userspace-mappable GEM */
+	bp.size        = AMDGPU_GPU_PAGE_SIZE;        /* 4K */
+	bp.byte_align  = AMDGPU_GPU_PAGE_SIZE;
+	bp.domain      = AMDGPU_GEM_DOMAIN_MMIO_REMAP;
+	bp.flags       = 0;
+	bp.resv        = NULL;
+	bp.bo_ptr_size = sizeof(struct amdgpu_bo);
+
+	r = amdgpu_bo_create(adev, &bp, &adev->rmmio_remap.bo);
+	if (r)
+		return r;
+
+	return 0;
+}
+
+/**
+ * amdgpu_ttm_mmio_remap_bo_fini - free the singleton MMIO_REMAP BO
+ * @adev: amdgpu device
+ *
+ * Frees the kernel-owned MMIO_REMAP BO if it was allocated by
+ * amdgpu_ttm_mmio_remap_bo_init().
+ */
+static void amdgpu_ttm_mmio_remap_bo_fini(struct amdgpu_device *adev)
+{
+	amdgpu_bo_unref(&adev->rmmio_remap.bo);
+	adev->mmio_remap.bo = NULL;
+}
+
 /*
  * amdgpu_ttm_init - Init the memory management (ttm) as well as various
  * gtt/vram related fields.
@@ -2027,6 +2084,11 @@ int amdgpu_ttm_init(struct amdgpu_device *adev)
 		return r;
 	}
 
+	/* Allocate the singleton MMIO_REMAP BO (4K) if supported */
+	r = amdgpu_ttm_mmio_remap_bo_init(adev);
+	if (r)
+		return r;
+
 	/* Initialize preemptible memory pool */
 	r = amdgpu_preempt_mgr_init(adev);
 	if (r) {
@@ -2090,6 +2152,8 @@ void amdgpu_ttm_fini(struct amdgpu_device *adev)
 	amdgpu_bo_free_kernel(&adev->mman.sdma_access_bo, NULL,
 					&adev->mman.sdma_access_ptr);
 
+	/* Drop the singleton MMIO_REMAP BO (if allocated) */
+	amdgpu_ttm_mmio_remap_bo_fini(adev);
 	amdgpu_ttm_fw_reserve_vram_fini(adev);
 	amdgpu_ttm_drv_reserve_vram_fini(adev);
 
-- 
2.34.1