Re: [PATCH v2 1/4] drm/vblank: Add vblank timer

[Javier Martinez Canillas <javierm@redhat.com>]

Thomas Zimmermann <tzimmermann@suse.de> writes:

> The vblank timer simulates a vblank interrupt for hardware without
> support. Rate-limits the display update frequency.
>
> DRM drivers for hardware without vblank support apply display updates
> ASAP. A vblank event informs DRM clients of the completed update.
>
> Userspace compositors immediately schedule the next update, which
> creates significant load on virtualization outputs. Display updates
> are usually fast on virtualization outputs, as their framebuffers are
> in regular system memory and there's no hardware vblank interrupt to
> throttle the update rate.
>
> The vblank timer is a HR timer that signals the vblank in software.
> It limits the update frequency of a DRM driver similar to a hardware
> vblank interrupt. The timer is not synchronized to the actual vblank
> interval of the display.
>
> The code has been adopted from vkms, which added the funtionality
> in commit 3a0709928b17 ("drm/vkms: Add vblank events simulated by
> hrtimers").
>
> The new implementation is part of the existing vblank support,
> which sets up the timer automatically. Drivers only have to start
> and cancel the vblank timer as part of enabling and disabling the
> CRTC. The new vblank helper library provides callbacks for struct
> drm_crtc_funcs.
>
> The standard way for handling vblank is to call drm_crtc_handle_vblank().
> Drivers that require additional processing, such as vkms, can init
> handle_vblank_timeout in struct drm_crtc_helper_funcs to refer to
> their timeout handler.
>
> v2:
> - implement vblank timer entirely in vblank helpers
> - downgrade overrun warning to debug
> - fix docs
>
> Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
> Tested-by: Louis Chauvet <louis.chauvet@bootlin.com>
> Reviewed-by: Louis Chauvet <louis.chauvet@bootlin.com>
> ---

Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>

-- 
Best regards,

Javier Martinez Canillas
Core Platforms
Red Hat