[RFC PATCH 7/7] drm/amdgpu: Return Handle to MMIO_REMAP Singleton for GEM Create

[Srinivasan Shanmugam]

When userspace requests a GEM in AMDGPU_GEM_DOMAIN_MMIO_REMAP, return a
handle to the kernel-owned singleton BO instead of allocating a new one.

Validate inputs (exact PAGE_SIZE, alignment PAGE_SIZE, no extra flags)
and zero the ioctl out-struct on success for a clean echo.

This puts the userspace-visible behavior last, after all internal kernel
plumbing and initialization are in place.

Cc: Christian König <christian.koenig at amd.com>
Cc: Alex Deucher <alexander.deucher at amd.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam at amd.com>
---
 drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c | 56 +++++++++++++++++++++++++
 1 file changed, 56 insertions(+)

diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c
index e3f65977eeee..1345e81214e8 100644
--- a/drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c
+++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c
@@ -424,6 +424,26 @@ const struct drm_gem_object_funcs amdgpu_gem_object_funcs = {
 	.vm_ops = &amdgpu_gem_vm_ops,
 };
 
+/* ========= MMIO remap (HDP flush) GEM handle helper ========= */
+static int amdgpu_gem_get_mmio_remap_handle(struct drm_file *file_priv,
+					    struct amdgpu_device *adev,
+					    u32 *handle)
+{
+	struct amdgpu_bo *bo = adev->mmio_remap_bo;
+	struct drm_gem_object *gobj;
+	int r;
+
+	if (!bo)
+		return -ENODEV;
+
+	/* Take a temporary ref; the handle creation will hold its own ref. */
+	bo = amdgpu_bo_ref(bo);
+	gobj = &bo->tbo.base;
+	r = drm_gem_handle_create(file_priv, gobj, handle);
+	amdgpu_bo_unref(&bo);  /* drops our temporary ref */
+	return r;
+}
+
 /*
  * GEM ioctls.
  */
@@ -465,6 +485,42 @@ int amdgpu_gem_create_ioctl(struct drm_device *dev, void *data,
 	/* always clear VRAM */
 	flags |= AMDGPU_GEM_CREATE_VRAM_CLEARED;
 
+	/*
+	 * === MMIO remap (HDP flush) fast-path ===
+	 * If userspace asks for the MMIO_REMAP domain, don't allocate a new BO.
+	 * Return a handle to the singleton BO created at device init.
+	 */
+	if (args->in.domains & AMDGPU_GEM_DOMAIN_MMIO_REMAP) {
+		u32 mmio_handle;
+		/* Enforce fixed size & alignment (exactly one page). */
+		if (size && size != PAGE_SIZE)
+			return -EINVAL;
+		if (args->in.alignment && args->in.alignment != PAGE_SIZE)
+			return -EINVAL;
+		/* No extra domain flags for this special object. */
+		if (args->in.domain_flags)
+			return -EINVAL;
+		/* Disallow flags that don't make sense for a fixed I/O page. */
+		if (flags & (AMDGPU_GEM_CREATE_CPU_GTT_USWC |
+			     AMDGPU_GEM_CREATE_ENCRYPTED |
+			     AMDGPU_GEM_CREATE_DISCARDABLE))
+			return -EINVAL;
+
+		/* Normalize inputs (optional, for user-visible echo/debug). */
+		args->in.bo_size     = PAGE_SIZE;
+		args->in.alignment   = PAGE_SIZE;
+		args->in.domains     = AMDGPU_GEM_DOMAIN_MMIO_REMAP;
+		args->in.domain_flags = 0;
+
+		r = amdgpu_gem_get_mmio_remap_handle(filp, adev, &mmio_handle);
+		if (r)
+			return r;
+
+		memset(args, 0, sizeof(*args));
+		args->out.handle = mmio_handle;
+		return 0;
+	}
+
 	/* create a gem object to contain this object in */
 	if (args->in.domains & (AMDGPU_GEM_DOMAIN_GDS |
 	    AMDGPU_GEM_DOMAIN_GWS | AMDGPU_GEM_DOMAIN_OA)) {
-- 
2.34.1