[Intel-gfx] [PATCH] drm/i915: Deny wrapping an userptr into a framebuffer

[Chris Wilson]

Pinning a userptr onto the hardware raises interesting questions about
the lifetime of such a surface as the framebuffer extends that life
beyond the client's address space. That is the hardware will need to
keep scanning out from the backing storage even after the client wants
to remap its address space. As the hardware pins the backing storage,
the userptr becomes invalid and this raises a WARN when the clients
tries to unmap its address space. The situation can be even more
complicated when the buffer is passed between processes, between a
client and display server, where the lifetime and hardware access is
even more confusing. Deny it.

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
Cc: Michał Winiarski <michal.winiarski at intel.com>
Cc: stable at vger.kernel.org
---
 drivers/gpu/drm/i915/i915_gem_userptr.c | 5 ++++-
 drivers/gpu/drm/i915/intel_display.c    | 5 +++++
 2 files changed, 9 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/i915_gem_userptr.c b/drivers/gpu/drm/i915/i915_gem_userptr.c
index 2dd911ab3019..3ce1b557f7c4 100644
--- a/drivers/gpu/drm/i915/i915_gem_userptr.c
+++ b/drivers/gpu/drm/i915/i915_gem_userptr.c
@@ -974,7 +974,10 @@ out:
  * Also note, that the object created here is not currently a "first class"
  * object, in that several ioctls are banned. These are the CPU access
  * ioctls: mmap(), pwrite and pread. In practice, you are expected to use
- * direct access via your pointer rather than use those ioctls.
+ * direct access via your pointer rather than use those ioctls. Another
+ * restriction is that we do not allow userptr surfaces to be pinned to the
+ * hardware and so we reject any attempt to create a framebuffer out of a
+ * userptr.
  *
  * If you think this is a good interface to use to pass GPU memory between
  * drivers, please use dma-buf instead. In fact, wherever possible use
diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
index b89131654a0e..d1deaedcc4ce 100644
--- a/drivers/gpu/drm/i915/intel_display.c
+++ b/drivers/gpu/drm/i915/intel_display.c
@@ -14116,6 +14116,11 @@ static int intel_user_framebuffer_create_handle(struct drm_framebuffer *fb,
 	struct intel_framebuffer *intel_fb = to_intel_framebuffer(fb);
 	struct drm_i915_gem_object *obj = intel_fb->obj;
 
+	if (obj->userptr.mm) {
+		DRM_DEBUG("attempting to use a userptr for a framebuffer, denied\n");
+		return -EINVAL;
+	}
+
 	return drm_gem_handle_create(file, &obj->base, handle);
 }
 
-- 
2.6.1