• Chris Wilson's avatar
    drm/i915: Use exponential backoff for wait_for() · a54b1873
    Chris Wilson authored
    Instead of sleeping for a fixed 1ms (roughly, depending on timer slack),
    start with a small sleep and exponentially increase the sleep on each
    cycle.
    
    A good example of a beneficiary is the guc mmio communication channel.
    Typically we expect (and so spin) for 10us for a quick response, but this
    doesn't cover everything and so sometimes we fallback to the millisecond+
    sleep. This incurs a significant delay in time-critical operations like
    preemption (igt/gem_exec_latency), which can be improved significantly by
    using a small sleep after the spin fails.
    
    We've made this suggestion many times, but had little experimental data
    to support adding the complexity.
    
    v2: Bump the minimum usleep to 10us on advice of
    Documentation/timers/timers-howto.txt (Tvrko)
    v3: Specify min, max range for usleep intervals -- some code may
    crucially depend upon and so want to specify the sleep pattern.
    
    References: 1758b90e ("drm/i915: Use a hybrid scheme for fast register waits")
    Signed-off-by: default avatarChris Wilson <chris@chris-wilson.co.uk>
    Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
    Cc: John Harrison <John.C.Harrison@intel.com>
    Cc: Michał Winiarski <michal.winiarski@intel.com>
    Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
    Reviewed-by: default avatarMichał Winiarski <michal.winiarski@intel.com>
    Link: https://patchwork.freedesktop.org/patch/msgid/20171124130031.20761-2-chris@chris-wilson.co.uk
    a54b1873
intel_drv.h 66.9 KB