• Suzuki K Poulose's avatar
    arm-cci: CCI-500: Work around PMU counter writes · a445fcc9
    Suzuki K Poulose authored
    The CCI PMU driver sets the event counter to the half of the maximum
    value(2^31) it can count before we start the counters via
    pmu_event_set_period(). This is done to give us the best chance to
    handle the overflow interrupt, taking care of extreme interrupt latencies.
    
    However, CCI-500 comes with advanced power saving schemes, which
    disables the clock to the event counters unless the counters are enabled to
    count (PMCR.CEN). This prevents the driver from writing the period to the
    counters before starting them.  Also, there is no way we can reset the
    individual event counter to 0 (PMCR.RST resets all the counters, losing
    their current readings). However the value of the counter is preserved and
    could be read back, when the counters are not enabled.
    
    So we cannot reliably use the counters and compute the number of events
    generated during the sampling period since we don't have the value of the
    counter at start.
    
    This patch works around this issue by changing writes to the counter
    with the following steps.
    
     1) Disable all the counters (remembering any counters which were enabled)
     2) Enable the PMU, now that all the counters are disabled.
    
     For each counter to be programmed, repeat steps 3-7
     3) Save the current event and program the target counter to count an
        invalid event, which by spec is guaranteed to not-generate any events.
     4) Enable the target counter.
     5) Write to the target counter.
     6) Disable the target counter
     7) Restore the event back on the target counter.
    
     8) Disable the PMU
     9) Restore the status of the all the counters
    
    Cc: Punit Agrawal <punit.agrawal@arm.com>
    Acked-by: default avatarOlof Johansson <olof@lixom.net>
    Acked-by: default avatarMark Rutland <mark.rutland@arm.com>
    Signed-off-by: default avatarSuzuki K Poulose <suzuki.poulose@arm.com>
    Signed-off-by: default avatarWill Deacon <will.deacon@arm.com>
    a445fcc9
arm-cci.c 61.4 KB