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drm_vblank_restore() exists because certain power saving states
can clobber the hardware frame counter. The way it does this is
by guesstimating how many frames were missed purely based on
the difference between the last stored timestamp vs. a newly
sampled timestamp.

If we should call this function before a full frame has
elapsed since we sampled the last timestamp we would end up
with a possibly slightly different timestamp value for the
same frame. Currently we will happily overwrite the already
stored timestamp for the frame with the new value. This
could cause userspace to observe two different timestamps
for the same frame (and the timestamp could even go
backwards depending on how much error we introduce when
correcting the timestamp based on the scanout position).

To avoid that let's not update the stored timestamp at all,
and instead we just fix up the last recorded hw vblank counter
value such that the already stored timestamp/seq number will
match. Thus the next time a vblank irq happens it will calculate
the correct diff between the current and stored hw vblank counter
values.

Sidenote: Another possible idea that came to mind would be to
do this correction only if the power really was removed since
the last time we sampled the hw frame counter. But to do that
we would need a robust way to detect when it has occurred. Some
possibilities could involve some kind of hardare power well
transition counter, or potentially we could store a magic value
in a scratch register that lives in the same power well. But
I'm not sure either of those exist, so would need an actual
investigation to find out. All of that is very hardware specific
of course, so would have to be done in the driver code.

Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210218160305.16711-1-ville.syrjala@linux.intel.com

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>