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If we miss the current vblank because the gpu was busy, that may cause a jitter as the frame rate temporarily drops. We try to limit the impact of this by then boosting the GPU clock to deliver the frame as quickly as possible. Originally done in commit6ad790c0f5("drm/i915: Boost GPU frequency if we detect outstanding pageflips") but was never forward ported to atomic and finally dropped in commitfd3a40242e("drm/i915: Rip out legacy page_flip completion/irq handling"). One of the most typical use-cases for this is a mostly idle desktop. Rendering one frame of the desktop's frontbuffer can easily be accomplished by the GPU running at low frequency, but often exceeds the time budget of the desktop compositor. The result is that animations such as opening the menu, doing a fullscreen switch, or even just trying to move a window around are slow and jerky. We need to respond within a frame to give the best impression of a smooth UX, as a compromise we instead respond if that first frame misses its goal. The result should be a near-imperceivable initial delay and a smooth animation even starting from idle. The cost, as ever, is that we spend more power than is strictly necessary as we overestimate the required GPU frequency and then try to ramp down. This of course is reactionary, too little, too late; nevertheless it is surprisingly effective. Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=102199 Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk> Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com> Cc: Ville Syrjälä <ville.syrjala@linux.intel.com> Cc: Daniel Vetter <daniel.vetter@intel.com> Link: https://patchwork.freedesktop.org/patch/msgid/20170817123706.6777-1-chris@chris-wilson.co.uk Tested-by: Lyude Paul <lyude@redhat.com> Reviewed-by: Radoslaw Szwichtenberg <radoslaw.szwichtenberg@intel.com>
Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. 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.