On Thu, Nov 17, 2022 at 09:33:58AM -0800, Matt Roper wrote:
The GT MCR code currently relies on uncore->lock to avoid race
conditions on the steering control register during MCR operations. The
*_fw() versions of MCR operations expect the caller to already hold
uncore->lock, while the non-fw variants manage the lock internally.
However the sole callsite of intel_gt_mcr_wait_for_reg_fw() does not
currently obtain the forcewake lock, allowing a potential race condition
(and triggering an assertion on lockdep builds). Furthermore, since
'wait for register value' requests may not return immediately, it is
undesirable to hold a fundamental lock like uncore->lock for the entire
wait and block all other MMIO for the duration; rather the lock is only
needed around the MCR read operations and can be released during the
delays.
Convert intel_gt_mcr_wait_for_reg_fw() to a non-fw variant that will
manage uncore->lock internally. This does have the side effect of
causing an unnecessary lookup in the forcewake table on each read
operation, but since the caller is still holding the relevant forcewake
domain, this will ultimately just incremenent the reference count and
won't actually cause any additional MMIO traffic.
In the future we plan to switch to a dedicated MCR lock to protect the
steering critical section rather than using the overloaded and
high-traffic uncore->lock; on MTL and beyond the new lock can be
implemented on top of the hardware-provided synchonization mechanism for
steering.
Fixes: 3068bec83eea ("drm/i915/gt: Add intel_gt_mcr_wait_for_reg_fw()")
Cc: Lucas De Marchi <lucas.demarchi@xxxxxxxxx>
Signed-off-by: Matt Roper <matthew.d.roper@xxxxxxxxx>
Reviewed-by: Lucas De Marchi <lucas.demarchi@xxxxxxxxx>
thanks
Lucas De Marchi
