[RFC PATCH v2 8/8] drm/amdgpu/gem: Return Handle to MMIO_REMAP Singleton in GEM_CREATE

[Srinivasan Shanmugam]

Enable userspace to obtain a handle to the kernel-owned MMIO_REMAP
singleton when AMDGPU_GEM_DOMAIN_MMIO_REMAP is requested via
amdgpu_gem_create_ioctl().

Validate the fixed 4K constraint: if PAGE_SIZE > AMDGPU_GPU_PAGE_SIZE
return -EINVAL; when provided, size and alignment must equal
AMDGPU_GPU_PAGE_SIZE.

If the singleton BO is not available, return -ENODEV.

Cc: 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_gem.c | 59 +++++++++++++++++++++++++
 1 file changed, 59 insertions(+)

diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c
index d8cffd26455b..655281f57a99 100644
--- a/drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c
+++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c
@@ -424,6 +424,38 @@ const struct drm_gem_object_funcs amdgpu_gem_object_funcs = {
 	.vm_ops = &amdgpu_gem_vm_ops,
 };
 
+/**
+ * amdgpu_gem_get_mmio_remap_handle - Create a GEM handle for the MMIO_REMAP BO
+ * @file_priv: DRM file corresponding to the calling process
+ * @adev: amdgpu device
+ * @handle: returned userspace GEM handle (out)
+ *
+ * Creates a GEM handle to the kernel-owned singleton MMIO_REMAP buffer object
+ * (adev->rmmio_remap.bo). The BO is expected to be allocated during TTM init
+ * when the hardware exposes a remap base and PAGE_SIZE <= 4K.
+ *
+ * drm_gem_handle_create() acquires the handle reference, which will be dropped
+ * by GEM_CLOSE in userspace.
+ *
+ * * Return:
+ * * 0 on success
+ * * -ENODEV if the MMIO_REMAP BO is not available
+ * * A negative errno from drm_gem_handle_create() on failure
+ *
+ */
+static int amdgpu_gem_get_mmio_remap_handle(struct drm_file *file_priv,
+					    struct amdgpu_device *adev,
+					    u32 *handle)
+{
+	struct amdgpu_bo *bo = READ_ONCE(adev->rmmio_remap.bo);
+
+	if (!bo)
+		return -ENODEV;
+
+	/* drm_gem_handle_create() gets the ref; GEM_CLOSE drops it */
+	return drm_gem_handle_create(file_priv, &bo->tbo.base, handle);
+}
+
 /*
  * GEM ioctls.
  */
@@ -468,6 +500,33 @@ int amdgpu_gem_create_ioctl(struct drm_device *dev, void *data,
 	if (args->in.domains & AMDGPU_GEM_DOMAIN_MMIO_REMAP)
 		return -EINVAL;
 
+	/*
+	 * === 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 ttm init.
+	 */
+	if (args->in.domains & AMDGPU_GEM_DOMAIN_MMIO_REMAP) {
+		/*
+		 * The MMIO remap page is fixed 4K on the GPU side. Do not
+		 * allow use if the system PAGE_SIZE is larger than the GPU
+		 * page size.
+		 */
+		if (PAGE_SIZE > AMDGPU_GPU_PAGE_SIZE)
+			return -EINVAL;
+
+		/* Enforce fixed size/alignment when provided by userspace. */
+		if (size && size != AMDGPU_GPU_PAGE_SIZE)
+			return -EINVAL;
+		if (args->in.alignment && args->in.alignment != AMDGPU_GPU_PAGE_SIZE)
+			return -EINVAL;
+
+		r = amdgpu_gem_get_mmio_remap_handle(filp, adev, &handle);
+		if (r)
+			return r;
+		args->out.handle = 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