drm/atomic-helper: Again check modeset *before* plane states

This essentially reverts

commit 934ce1c23624526d9d784e0499190bb48113e6f4
Author: Rob Clark <robdclark@gmail.com>
Date:   Wed Nov 19 16:41:33 2014 -0500

    drm/atomic: check mode_changed *after* atomic_check

Depending upon the driver both orders (or maybe even interleaving) is
required:
- If ->atomic_check updates ->mode_changed then helper_check_modeset
  must be run afters.
- If ->atomic_check depends upon accurate adjusted dotclock values for
  e.g. watermarks, then helper_check_modeset must be run first.

The failure mode in the first case is usually a totally angry hw
because the pixel format switching doesn't happen. The failure mode in
the later case is usually nothing, since in most cases the old
adjusted mode from the previous modeset wont be too far off to be a
problem. So just underruns and perhaps even just suboptimal (from a
power consumption) watermarks.

Furthermore in the transitional helpers we only call ->atomic_check
after the new modeset state has been fully set up (and hence
computed).

Given that asymmetry in expected failure modes I think it's safer to
go back to the older order. So do that and give msm a special check
function to compensate.

Also update kerneldoc to explain this a bit.

v2: Actually add the missing hunk Rob spotted.

v3: Move msm_atomic_check into msm_atomic.c, requested by Rob.

Cc: Rob Clark <robdclark@gmail.com>
Reviewed-by: Rob Clark <robdclark@gmail.com>
Tested-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>

1 files changed, 8 insertions, 2 deletions

diff --git a/drivers/gpu/drm/drm_atomic_helper.c b/drivers/gpu/drm/drm_atomic_helper.c
index 0cd71dac394e..eb380ee2dc58 100644
--- a/drivers/gpu/drm/drm_atomic_helper.c
+++ b/drivers/gpu/drm/drm_atomic_helper.c
@@ -508,6 +508,12 @@ EXPORT_SYMBOL(drm_atomic_helper_check_planes);
  * Drivers without such needs can directly use this as their ->atomic_check()
  * callback.
  *
+ * This just wraps the two parts of the state checking for planes and modeset
+ * state in the default order: First it calls drm_atomic_helper_check_modeset()
+ * and then drm_atomic_helper_check_planes(). The assumption is that the
+ * ->atomic_check functions depend upon an updated adjusted_mode.clock to
+ * e.g. properly compute watermarks.
+ *
  * RETURNS
  * Zero for success or -errno
  */
@@ -516,11 +522,11 @@ int drm_atomic_helper_check(struct drm_device *dev,
 {
 	int ret;
 
-	ret = drm_atomic_helper_check_planes(dev, state);
+	ret = drm_atomic_helper_check_modeset(dev, state);
 	if (ret)
 		return ret;
 
-	ret = drm_atomic_helper_check_modeset(dev, state);
+	ret = drm_atomic_helper_check_planes(dev, state);
 	if (ret)
 		return ret;