drm/i915: Add a more detailed comment about the set_base() fastboot hack

Instead of it just being on the mailing list, let's put Jesse's
explanation next to the code in question.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
index 214984c814e830d00e0e2a701e08b9446deb05b1..fe863255dedcdb1fd2d20ff4ae7a96087c75aeb7 100644 (file)
--- a/drivers/gpu/drm/i915/intel_display.c
+++ b/drivers/gpu/drm/i915/intel_display.c
@@ -2320,7 +2320,19 @@ intel_pipe_set_base(struct drm_crtc *crtc, int x, int y,
                return ret;
        }
 
-       /* Update pipe size and adjust fitter if needed */
+       /*
+        * Update pipe size and adjust fitter if needed: the reason for this is
+        * that in compute_mode_changes we check the native mode (not the pfit
+        * mode) to see if we can flip rather than do a full mode set. In the
+        * fastboot case, we'll flip, but if we don't update the pipesrc and
+        * pfit state, we'll end up with a big fb scanned out into the wrong
+        * sized surface.
+        *
+        * To fix this properly, we need to hoist the checks up into
+        * compute_mode_changes (or above), check the actual pfit state and
+        * whether the platform allows pfit disable with pipe active, and only
+        * then update the pipesrc and pfit state, even on the flip path.
+        */
        if (i915_fastboot) {
                I915_WRITE(PIPESRC(intel_crtc->pipe),
                           ((crtc->mode.hdisplay - 1) << 16) |