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I've done a lot of history digging. The first signs of this optimization was introduced in i915: commit25067bfc06Author: Gustavo Padovan <gustavo.padovan@collabora.co.uk> Date: Wed Sep 10 12:03:17 2014 -0300 drm/i915: pin sprite fb only if it changed without much justification. Pinning already pinned stuff is real cheap (it's just obj->pin_count++ really), and the missing implicit sync was entirely forgotten about it seems. It's at least not mentioned anywhere it the commit message. It was also promptly removed shortly afterwards in commitea2c67bb4aAuthor: Matt Roper <matthew.d.roper@intel.com> Date: Tue Dec 23 10:41:52 2014 -0800 drm/i915: Move to atomic plane helpers (v9) again without really mentioning the side-effect that plane updates with the same fb now again obey implicit syncing. Note that this only ever applied to the plane_update hook, all other legacy entry points (set_base, page_flip) always obeyed implicit sync in the drm/i915 driver. The real source of this code here seems to be msm, copied to vc4, then copied to tinydrm. I've also tried to dig around in all available msm sources, but the corresponding check for fb != old_fb is present ever since the initial merge in commitcf3a7e4ce0Author: Rob Clark <robdclark@gmail.com> Date: Sat Nov 8 13:21:06 2014 -0500 drm/msm: atomic core bits The only older version I've found of msm atomic code predates the atomic helpers, and so didn't even use any of this. It also does not have a corresponding check (because it simply did no implicit sync at all). I've chatted with Rob on irc, and he didn't remember the reason for this either. Note we had epic amounts of fun with too much syncing against _vblank_, especially around cursor updates. But I don't ever discussing a need for less syncing against implicit fences. Also note that explicit fencing allows you to sidetrack all of this, at least for all the drivers correctly implemented using drm_atomic_set_fence_for_plane(). Given that it seems to be an accident of history, and that big drivers like i915 (and also nouveau it seems, I didn't follow the amdgpu/radeon sync code to figure this out properly there) never have done it, let's remove this. Cc: Rob Clark <robdclark@gmail.com> Cc: Matt Roper <matthew.d.roper@intel.com> Cc: Gustavo Padovan <gustavo.padovan@collabora.co.uk> Cc: Ville Syrjälä <ville.syrjala@linux.intel.com> Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com> Cc: Sean Paul <seanpaul@chromium.org> Cc: David Airlie <airlied@linux.ie> Cc: "Noralf Trønnes" <noralf@tronnes.org> Reviewed-by: Eric Anholt <eric@anholt.net> Signed-off-by: Daniel Vetter <daniel.vetter@intel.com> Link: https://patchwork.freedesktop.org/patch/msgid/20180405154449.23038-8-daniel.vetter@ffwll.ch
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 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.