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This register is sticky and remains set until reset or >> OS clears it by writing 0. >> >> The interface is exposed as: >>   /sys/devices/system/cpu/cpuX/cpufreq/perf_limited >> >> Signed-off-by: Sumit Gupta >> --- >>   drivers/acpi/cppc_acpi.c       | 26 ++++++++++++++++++++++++++ >>   drivers/cpufreq/cppc_cpufreq.c | 12 ++++++++++++ >>   include/acpi/cppc_acpi.h       | 10 ++++++++++ >>   3 files changed, 48 insertions(+) >> >> diff --git a/drivers/acpi/cppc_acpi.c b/drivers/acpi/cppc_acpi.c >> index ef53eb8a1feb..9b8da3ef06db 100644 >> --- a/drivers/acpi/cppc_acpi.c >> +++ b/drivers/acpi/cppc_acpi.c >> @@ -1810,6 +1810,32 @@ int cppc_set_max_perf(int cpu, u64 max_perf) >>   } >>   EXPORT_SYMBOL_GPL(cppc_set_max_perf); >> >> +/** >> + * cppc_get_perf_limited - Get the Performance Limited register value. >> + * @cpu: CPU from which to get Performance Limited register. >> + * @perf_limited: Pointer to store the Performance Limited value. >> + * >> + * Return: 0 for success, -EIO on register access failure, >> -EOPNOTSUPP if not supported. >> + */ >> +int cppc_get_perf_limited(int cpu, u64 *perf_limited) >> +{ >> +     return cppc_get_reg_val(cpu, PERF_LIMITED, perf_limited); >> +} >> +EXPORT_SYMBOL_GPL(cppc_get_perf_limited); >> + >> +/** >> + * cppc_set_perf_limited() - Write the Performance Limited register. >> + * @cpu: CPU on which to write register. >> + * @perf_limited: Value to write to the perf_limited register. >> + * >> + * Return: 0 for success, -EIO on register access failure, >> -EOPNOTSUPP if not supported. >> + */ >> +int cppc_set_perf_limited(int cpu, u64 perf_limited) >> +{ >> +     return cppc_set_reg_val(cpu, PERF_LIMITED, perf_limited); >> +} > > There are currently only 2 bits used: > - 0 Desired_Excursion > - 1 Minimum_Excursion > It might be worth defining these bits and mask the values when trying to > set the register. > Will do the change to allow only the clearing of these two bits and not setting. > ------ > > Also NIT: > > The spec. says: > " All accesses to the Performance Limited Register must be made using > interlocked operations, by both accessing entities." > > I am not sure I understand which synchronization issues are faced. > It's just to report the comment from the spec. > cpc_write() already has locking. So, two writes will complete without interference. As you suggested above, only the required register bit will be cleared by an operation. Thank you, Sumit Gupta ....