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Thomas Zimmermann
fa4b91eea4
[ Upstream commit74afeb8128] 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 commit3a0709928b("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. There's a possible deadlock between drm_crtc_handle_vblank() and hrtimer_cancel(). [1] The implementation avoids to call hrtimer_cancel() directly and instead signals to the timer function to not restart itself. v4: - fix possible race condition between timeout and atomic commit (Michael) v3: - avoid deadlock when cancelling timer (Ville, Lyude) 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> Tested-by: Michael Kelley <mhklinux@outlook.com> Link: https://lore.kernel.org/all/20250510094757.4174662-1-zengheng4@huawei.com/ # [1] Link: https://lore.kernel.org/r/20250916083816.30275-2-tzimmermann@suse.de Signed-off-by: Mingyu Wang <25181214217@stu.xidian.edu.cn> Signed-off-by: Sasha Levin <sashal@kernel.org>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.