[PATCH] drm/hyperv: wait for flip_done instead of vblanks in commit tail

[Hamza Mahfooz <hamzamahfooz@linux.microsoft.com>]

Since commit 52e6b1988334 ("drm/hypervdrm: Use vblank timer"), the
driver's vblanks are generated by an hrtimer, and the default commit
tail waits in drm_atomic_helper_wait_for_vblanks() for that timer to
fire. In a VM, a timer can fire arbitrarily late, for instance when the
host does not run the vCPU that the timer is queued on while the
committing task keeps running on another vCPU. If the delay exceeds the
helper's timeout, it warns, even though the display works fine:

  [CRTC:35:crtc-0] vblank wait timed out
  WARNING: drivers/gpu/drm/drm_atomic_helper.c:1921 at drm_atomic_helper_wait_for_vblanks.part.0+0x212/0x230, CPU#30: setfont/559
  Call Trace:
   drm_atomic_helper_commit_tail
   commit_tail
   drm_atomic_helper_commit
   drm_atomic_commit
   drm_client_modeset_commit_atomic
   drm_client_modeset_commit_locked
   drm_fb_helper_pan_display
   fb_pan_display
   bit_update_start
   fbcon_switch
   redraw_screen
   fbcon_do_set_font
   fbcon_set_font
   con_font_op
   vt_ioctl

This was seen intermittently on Azure VMs during boot, when setfont
redraws the console right after hyperv_drm has taken it over.

Cc: stable@vger.kernel.org
Fixes: 52e6b1988334 ("drm/hypervdrm: Use vblank timer")
Signed-off-by: Hamza Mahfooz <hamzamahfooz@linux.microsoft.com>
---
This is inspired by VKMS (which also uses DRM's vblank timer), wait for
the commit's flip_done instead. It completes once the vblank event that
drm_crtc_vblank_atomic_flush() armed has been sent on the next timer
tick.
---
 drivers/gpu/drm/hyperv/hyperv_drm_modeset.c | 24 +++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/drivers/gpu/drm/hyperv/hyperv_drm_modeset.c b/drivers/gpu/drm/hyperv/hyperv_drm_modeset.c
index cc5c5fbc8..8e5036051 100644
--- a/drivers/gpu/drm/hyperv/hyperv_drm_modeset.c
+++ b/drivers/gpu/drm/hyperv/hyperv_drm_modeset.c
@@ -504,6 +504,29 @@ static const struct drm_mode_config_funcs hv_drm_mode_config_funcs = {
 	.atomic_commit = drm_atomic_helper_commit,
 };
 
+/*
+ * Like drm_atomic_helper_commit_tail(), but wait for the commit's vblank
+ * events rather than for the vblank counter to advance. The host can delay
+ * the vblank timer for longer than drm_atomic_helper_wait_for_vblanks()
+ * waits before warning.
+ */
+static void hv_drm_atomic_commit_tail(struct drm_atomic_commit *state)
+{
+	struct drm_device *dev = state->dev;
+
+	drm_atomic_helper_commit_modeset_disables(dev, state);
+	drm_atomic_helper_commit_planes(dev, state, 0);
+	drm_atomic_helper_commit_modeset_enables(dev, state);
+	drm_atomic_helper_fake_vblank(state);
+	drm_atomic_helper_commit_hw_done(state);
+	drm_atomic_helper_wait_for_flip_done(dev, state);
+	drm_atomic_helper_cleanup_planes(dev, state);
+}
+
+static const struct drm_mode_config_helper_funcs hv_drm_mode_config_helper_funcs = {
+	.atomic_commit_tail = hv_drm_atomic_commit_tail,
+};
+
 int hv_drm_mode_config_init(struct hv_drm_device *hv)
 {
 	struct drm_device *dev = &hv->dev;
@@ -526,6 +549,7 @@ int hv_drm_mode_config_init(struct hv_drm_device *hv)
 	dev->mode_config.prefer_shadow = 0;
 
 	dev->mode_config.funcs = &hv_drm_mode_config_funcs;
+	dev->mode_config.helper_private = &hv_drm_mode_config_helper_funcs;
 
 	ret = hv_drm_pipe_init(hv);
 	if (ret) {
-- 
2.56.0