1. 20 Jun, 2018 1 commit
    • Daniel Vetter's avatar
      drm/gem-fb-helper: Always do implicit sync · 9d54fcd5
      Daniel Vetter authored
      I've done a lot of history digging. The first signs of this
      optimization was introduced in i915:
      
      commit 25067bfc
      Author: 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
      
      commit ea2c67bb
      Author: 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
      
      commit cf3a7e4c
      Author: 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: default avatarEric Anholt <eric@anholt.net>
      Signed-off-by: default avatarDaniel Vetter <daniel.vetter@intel.com>
      Link: https://patchwork.freedesktop.org/patch/msgid/20180405154449.23038-8-daniel.vetter@ffwll.ch
      9d54fcd5
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