• Chris Wilson's avatar
    drm/i915/ringbuffer: Fix context restore upon reset · b3ee09a4
    Chris Wilson authored
    The discovery with trying to enable full-ppgtt was that we were
    completely failing to the load both the mm and context following the
    reset. Although we were performing mmio to set the PP_DIR (per-process
    GTT) and CCID (context), these were taking no effect (the assumption was
    that this would trigger reload of the context and restore the page
    tables). It was not until we performed the LRI + MI_SET_CONTEXT in a
    following context switch would anything occur.
    
    Since we are then required to reset the context image and PP_DIR using
    CS commands, we place those commands into every batch. The hardware
    should recognise the no-ops and eliminate the expensive context loads,
    but we still have to pay the cost of using cross-powerwell register
    writes. In practice, this has no effect on actual context switch times,
    and only adds a few hundred nanoseconds to no-op switches. We can improve
    the latter by eliminating the w/a around known no-op switches, but there
    is an ulterior motive to keeping them.
    
    Always emitting the context switch at the beginning of the request (and
    relying on HW to skip unneeded switches) does have one key advantage.
    Should we implement request reordering on Haswell, we will not know in
    advance what the previous executing context was on the GPU and so we
    would not be able to elide the MI_SET_CONTEXT commands ourselves and
    always have to emit them. Having our hand forced now actually prepares
    us for later.
    
    Now since that context and mm follow the request, we no longer (and not
    for a long time since requests took over!) require a trace point to tell
    when we write the switch into the ring, since it is always. (This is
    even more important when you remember that simply writing into the ring
    bears no relation to the current mm.)
    
    v2: Sandybridge has to agree to use LRI as well.
    
    Testcase: igt/drv_selftests/live_hangcheck
    Signed-off-by: default avatarChris Wilson <chris@chris-wilson.co.uk>
    Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
    Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
    Cc: Matthew Auld <matthew.william.auld@gmail.com>
    Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
    Reviewed-by: default avatarJoonas Lahtinen <joonas.lahtinen@linux.intel.com>
    Link: https://patchwork.freedesktop.org/patch/msgid/20180611110845.31890-1-chris@chris-wilson.co.uk
    b3ee09a4
i915_trace.h 27.6 KB