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>

1 files changed, 13 insertions, 1 deletions

diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
index 214984c814e8..fe863255dedc 100644
--- 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) |