[PATCH] drm/hyperv: add a parameter to disable the vblank timer

[Lanfeng Wei]

A performance regression has been observed on Hyper-V causing serious
frame rate drops since commit 52e6b1988334 ("drm/hypervdrm: Use vblank
timer").

The regression occurs when using Xorg with a software rendered cursor.
The offending commit uses the DRM vblank hrtimer to pace atomic update
completion events, making each event wait for the next simulated vblank.
However, cursor damage invoked by mouse movement is unconstrained by
screen refresh rate, and therefore consumes most of Xorg's update
opportunities, starving normal graphical updates, resulting in
plummeting frame rate.

Hyper-V virtual display framebuffer is able to accept multiple writes in
one update window, so the commit broke this expected behavior.

The regression is also reported by other users:

Link: https://www.mail-archive.com/ubuntu-bugs@xxxxxxxxxxxxxxxx/msg6302857.html

Add a read-only module parameter hyperv_drm.vblank_timer, which is
enabled by default. Disabling it prevents the timer's enable callback
from starting the timer. The vblank framework and the existing atomic
callbacks remain registered. When disabled, the first
drm_crtc_vblank_get() fails through the callback, so the existing helper
sends the atomic event immediately and skips the vblank wait, matching
the behavior before the timer was introduced.

This provides an opt-out for affected virtual display users.

Benchmark results:

Workload: Inject mouse movement at 125 Hz against a flashing
animation running at 60 Hz, matching the screen refresh rate, for 4
seconds.

Tested versions: 6.18 (the last major version before the offending
commit), version 6.19 (the first major version containing that commit),
version 7.1.8 (on which the patch is based), and its patched version.

Version  Anim. Hz            Mouse Hz         Max client-request gap
====================================================================
6.18     59.66-60.00        122.00-123.33     17.0 ms
6.19     0-0.162            59.73-59.86       7.53 s
7.1.8    0.133-0.162        59.83-59.86       6.16 s
Patched  59.75-60.00        122.75-124.50     14.0 ms

The patch restores both animation and mouse update rates, and reduces
the maximum client-request gap from seconds to milliseconds.

Fixes: 52e6b1988334 ("drm/hypervdrm: Use vblank timer")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Lanfeng Wei <xlfylawarthur@xxxxxxxxx>
---
The parameter defaults to enabled to preserve the current kernel
behavior. I do not know the full set of users who rely on the vblank
timer, so changing the default could introduce a different regression.
Affected users can set hyperv_drm.vblank_timer=0.

If the maintainers prefer the parameter to default to disabled, or if
there is a better way to detect the affected configuration
automatically, I'm happy to send a v2.
 drivers/gpu/drm/hyperv/hyperv_drm_modeset.c | 23 ++++++++++++++++++++-
 1 file changed, 22 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/hyperv/hyperv_drm_modeset.c b/drivers/gpu/drm/hyperv/hyperv_drm_modeset.c
index d48ca6c23..527385748 100644
--- a/drivers/gpu/drm/hyperv/hyperv_drm_modeset.c
+++ b/drivers/gpu/drm/hyperv/hyperv_drm_modeset.c
@@ -4,6 +4,7 @@
  */
 
 #include <linux/hyperv.h>
+#include <linux/moduleparam.h>
 
 #include <drm/drm_atomic.h>
 #include <drm/drm_crtc_helper.h>
@@ -25,6 +26,19 @@
 
 #include "hyperv_drm.h"
 
+static bool vblank_timer = true;
+module_param(vblank_timer, bool, 0444);
+MODULE_PARM_DESC(vblank_timer,
+		 "Enable the Hyper-V DRM vblank timer (default: true)");
+
+static int hyperv_crtc_enable_vblank(struct drm_crtc *crtc)
+{
+	if (!vblank_timer)
+		return -EOPNOTSUPP;
+
+	return drm_crtc_vblank_helper_enable_vblank_timer(crtc);
+}
+
 static int hyperv_blit_to_vram_rect(struct drm_framebuffer *fb,
 				    const struct iosys_map *vmap,
 				    struct drm_rect *rect)
@@ -132,7 +146,14 @@ static const struct drm_crtc_funcs hyperv_crtc_funcs = {
 	.page_flip = drm_atomic_helper_page_flip,
 	.atomic_duplicate_state = drm_atomic_helper_crtc_duplicate_state,
 	.atomic_destroy_state = drm_atomic_helper_crtc_destroy_state,
-	DRM_CRTC_VBLANK_TIMER_FUNCS,
+	/*
+	 * expanded DRM_CRTC_VBLANK_TIMER_FUNCS to override
+	 * enable_vblank with hyperv_crtc_enable_vblank
+	 */
+	.enable_vblank = hyperv_crtc_enable_vblank,
+	.disable_vblank = drm_crtc_vblank_helper_disable_vblank_timer,
+	.get_vblank_timestamp =
+		drm_crtc_vblank_helper_get_vblank_timestamp_from_timer,
 };
 
 static int hyperv_plane_atomic_check(struct drm_plane *plane,
-- 
2.55.0