Use a vblank timer to simulate the vblank interrupt. The DRM vblank helpers provide an implementation on top of Linux' hrtimer. Cirrus-qemu 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. Cirrus-qemu' 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 cirrus-qemu and all other virtual displays. See [1] for a discussion in the context of hypervdrm. Signed-off-by: Thomas Zimmermann <[email protected]> Link: https://lore.kernel.org/dri-devel/sn6pr02mb415702b00d6d52b0ee962c98d4...@sn6pr02mb4157.namprd02.prod.outlook.com/ # [1] --- drivers/gpu/drm/tiny/cirrus-qemu.c | 11 +++++++++++ 1 file changed, 11 insertions(+) diff --git a/drivers/gpu/drm/tiny/cirrus-qemu.c b/drivers/gpu/drm/tiny/cirrus-qemu.c index 97a93adc5669..f728fa48ac88 100644 --- a/drivers/gpu/drm/tiny/cirrus-qemu.c +++ b/drivers/gpu/drm/tiny/cirrus-qemu.c @@ -45,6 +45,8 @@ #include <drm/drm_modeset_helper_vtables.h> #include <drm/drm_module.h> #include <drm/drm_probe_helper.h> +#include <drm/drm_vblank.h> +#include <drm/drm_vblank_helper.h> #define DRIVER_NAME "cirrus-qemu" #define DRIVER_DESC "qemu cirrus vga" @@ -404,11 +406,15 @@ static void cirrus_crtc_helper_atomic_enable(struct drm_crtc *crtc, #endif drm_dev_exit(idx); + + drm_crtc_vblank_on(crtc); } static const struct drm_crtc_helper_funcs cirrus_crtc_helper_funcs = { .atomic_check = cirrus_crtc_helper_atomic_check, + .atomic_flush = drm_crtc_vblank_atomic_flush, .atomic_enable = cirrus_crtc_helper_atomic_enable, + .atomic_disable = drm_crtc_vblank_atomic_disable, }; static const struct drm_crtc_funcs cirrus_crtc_funcs = { @@ -418,6 +424,7 @@ static const struct drm_crtc_funcs cirrus_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, }; static const struct drm_encoder_funcs cirrus_encoder_funcs = { @@ -493,6 +500,10 @@ static int cirrus_pipe_init(struct cirrus_device *cirrus) if (ret) return ret; + ret = drm_vblank_init(dev, 1); + if (ret) + return ret; + return 0; } -- 2.51.0
