drm/i915: Document that mmap forwarding is discouraged

[firefly-linux-kernel-4.4.55.git] / drivers / gpu / drm / i915 / i915_gem.c

diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index e9c783d5561239f7689ad4e3e690d27f104ae1b6..895f9881f0aa899b4850888606dbd1e3d2879564 100644 (file)
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -1466,6 +1466,16 @@ unlock:
  *
  * While the mapping holds a reference on the contents of the object, it doesn't
  * imply a ref on the object itself.
+ *
+ * IMPORTANT:
+ *
+ * DRM driver writers who look a this function as an example for how to do GEM
+ * mmap support, please don't implement mmap support like here. The modern way
+ * to implement DRM mmap support is with an mmap offset ioctl (like
+ * i915_gem_mmap_gtt) and then using the mmap syscall on the DRM fd directly.
+ * That way debug tooling like valgrind will understand what's going on, hiding
+ * the mmap call in a driver private ioctl will break that. The i915 driver only
+ * does cpu mmaps this way because we didn't know better.
  */
 int
 i915_gem_mmap_ioctl(struct drm_device *dev, void *data,