[Intel-gfx] [PATCH v2 0/3] drm/i915: Bug fixes to ring 'head' updating

[Dave Gordon]

Fixes to both the LRC and the legacy ringbuffer code to correctly
calculate and update the available space in a ring.

The logical ring code was updating the software ring 'head' value
by reading the hardware 'HEAD' register. In LRC mode, this is not
valid as the hardware is not necessarily executing the same context
that is being processed by the software. Thus reading the h/w HEAD
could put an unrelated (undefined, effectively random) value into
the s/w 'head' -- A Bad Thing for the free space calculations.

In addition, the old code could update a ringbuffer's 'head' value
from the 'last_retired_head' even when the latter hadn't been recently
updated and therefore had a value of -1; this would also confuse the
freespace calculations. Now, we consume 'last_retired_head' in just
one place, ensuring that this confusion does not arise.