[Intel-gfx] [Resend for flip-flops] drm/i915: Always wake the device to flush the GTT

[Chris Wilson]

Quoting Chris Wilson (2017-08-29 20:25:46)
> Since we hold the device wakeref when writing through the GTT (otherwise
> the writes would fail), we presumed that before the device sleeps those
> writes would naturally be flushed and that we wouldn't need our mmio
> read trick. However, that presumption seems false and a sleepy bxt seems
> to require us to always manually flush the GTT writes prior to direct
> access.
> 
> Fixes: e2a2aa36a509 ("drm/i915: Check we have an wake device before flushing GTT writes")
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>
> Reviewed-by: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>

On the basis of two samples, this seems to fix the pwrite flip-flops,
so pushed for a wider sampling.
-Chris