drm/virtgpu: Use vblank timer

Use a vblank timer to simulate the vblank interrupt. The DRM vblank
helpers provide an implementation on top of Linux' hrtimer. Virtgpu
enables and disables the timer as part of the CRTC. The atomic_flush
callback sets up the event. Like vblank interrupts, the vblank timer
fires at the rate of the display refresh.

Most userspace limits its page flip rate according to the DRM vblank
event. Virtgpu's virtual hardware does not provide vblank interrupts, so
DRM sends each event ASAP. With the fast access times of virtual display
memory, the event rate is much higher than the display mode's refresh
rate; creating the next page flip almost immediately. This leads to
excessive CPU overhead from even small display updates, such as moving
the mouse pointer.

This problem affects virtgpu and all other virtual displays. See [1] for
a discussion in the context of hypervdrm.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://lore.kernel.org/dri-devel/SN6PR02MB415702B00D6D52B0EE962C98D46CA@SN6PR02MB4157.namprd02.prod.outlook.com/
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Tested-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Link: https://lore.kernel.org/r/20251008130701.246988-1-tzimmermann@suse.de

diff --git a/drivers/gpu/drm/virtio/virtgpu_display.c b/drivers/gpu/drm/virtio/virtgpu_display.c
index 4b1252c04b7ade6d1f185d230484bfefa72e2d1d..e972d9b015a9e633f1513d58d6390250feb96963 100644 (file)
--- a/drivers/gpu/drm/virtio/virtgpu_display.c
+++ b/drivers/gpu/drm/virtio/virtgpu_display.c
@@ -32,6 +32,8 @@
 #include <drm/drm_gem_framebuffer_helper.h>
 #include <drm/drm_probe_helper.h>
 #include <drm/drm_simple_kms_helper.h>
+#include <drm/drm_vblank.h>
+#include <drm/drm_vblank_helper.h>
 
 #include "virtgpu_drv.h"
 
@@ -55,6 +57,7 @@ static const struct drm_crtc_funcs virtio_gpu_crtc_funcs = {
        .reset                  = drm_atomic_helper_crtc_reset,
        .atomic_duplicate_state = drm_atomic_helper_crtc_duplicate_state,
        .atomic_destroy_state   = drm_atomic_helper_crtc_destroy_state,
+       DRM_CRTC_VBLANK_TIMER_FUNCS,
 };
 
 static const struct drm_framebuffer_funcs virtio_gpu_fb_funcs = {
@@ -99,6 +102,7 @@ static void virtio_gpu_crtc_mode_set_nofb(struct drm_crtc *crtc)
 static void virtio_gpu_crtc_atomic_enable(struct drm_crtc *crtc,
                                          struct drm_atomic_state *state)
 {
+       drm_crtc_vblank_on(crtc);
 }
 
 static void virtio_gpu_crtc_atomic_disable(struct drm_crtc *crtc,
@@ -108,6 +112,8 @@ static void virtio_gpu_crtc_atomic_disable(struct drm_crtc *crtc,
        struct virtio_gpu_device *vgdev = dev->dev_private;
        struct virtio_gpu_output *output = drm_crtc_to_virtio_gpu_output(crtc);
 
+       drm_crtc_vblank_off(crtc);
+
        virtio_gpu_cmd_set_scanout(vgdev, output->index, 0, 0, 0, 0, 0);
        virtio_gpu_notify(vgdev);
 }
@@ -121,9 +127,10 @@ static int virtio_gpu_crtc_atomic_check(struct drm_crtc *crtc,
 static void virtio_gpu_crtc_atomic_flush(struct drm_crtc *crtc,
                                         struct drm_atomic_state *state)
 {
-       struct drm_crtc_state *crtc_state = drm_atomic_get_new_crtc_state(state,
-                                                                         crtc);
+       struct drm_device *dev = crtc->dev;
+       struct drm_crtc_state *crtc_state = drm_atomic_get_new_crtc_state(state, crtc);
        struct virtio_gpu_output *output = drm_crtc_to_virtio_gpu_output(crtc);
+       struct drm_pending_vblank_event *event;
 
        /*
         * virtio-gpu can't do modeset and plane update operations
@@ -133,6 +140,20 @@ static void virtio_gpu_crtc_atomic_flush(struct drm_crtc *crtc,
         */
        if (drm_atomic_crtc_needs_modeset(crtc_state))
                output->needs_modeset = true;
+
+       spin_lock_irq(&dev->event_lock);
+
+       event = crtc_state->event;
+       crtc_state->event = NULL;
+
+       if (event) {
+               if (drm_crtc_vblank_get(crtc) == 0)
+                       drm_crtc_arm_vblank_event(crtc, event);
+               else
+                       drm_crtc_send_vblank_event(crtc, event);
+       }
+
+       spin_unlock_irq(&dev->event_lock);
 }
 
 static const struct drm_crtc_helper_funcs virtio_gpu_crtc_helper_funcs = {
@@ -359,6 +380,10 @@ int virtio_gpu_modeset_init(struct virtio_gpu_device *vgdev)
        for (i = 0 ; i < vgdev->num_scanouts; ++i)
                vgdev_output_init(vgdev, i);
 
+       ret = drm_vblank_init(vgdev->ddev, vgdev->num_scanouts);
+       if (ret)
+               return ret;
+
        drm_mode_config_reset(vgdev->ddev);
        return 0;
 }