• Matt Roper's avatar
    drm/i915/gt: Manage uncore->lock while waiting on MCR register · 41f425ad
    Matt Roper authored
    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: 3068bec8 ("drm/i915/gt: Add intel_gt_mcr_wait_for_reg_fw()")
    Cc: Lucas De Marchi <lucas.demarchi@intel.com>
    Signed-off-by: default avatarMatt Roper <matthew.d.roper@intel.com>
    Reviewed-by: default avatarLucas De Marchi <lucas.demarchi@intel.com>
    Link: https://patchwork.freedesktop.org/patch/msgid/20221117173358.1980230-1-matthew.d.roper@intel.com
    (cherry picked from commit 192bb40f)
    Signed-off-by: default avatarRodrigo Vivi <rodrigo.vivi@intel.com>
    41f425ad
intel_gt_mcr.h 2.37 KB