[PATCH v4 2/2] cpufreq: acpi-cpufreq: fix P-state index mismatch in get_cur_freq_on_cpu()

[lirongqing <lirongqing@baidu.com>]

From: Li RongQing <lirongqing@baidu.com>

get_cur_freq_on_cpu() uses perf->state, which is an index into
perf->states[], to index policy->freq_table[]. However, freq_table[]
is built by filtering _PSS entries that are not lower in frequency
than the previous one, so its index space no longer matches
perf->states[]. The original P-state index for each remaining
freq_table entry is stored in driver_data.

Once an entry has been skipped, using perf->state as an index into
freq_table[] can therefore select the frequency of a different
P-state.

The reported current frequency itself remains correct because it is
obtained from extract_freq(). The mismatch only affects the cached
frequency used by get_cur_freq_on_cpu() to detect a firmware frequency
change behind our back.

If the wrong table entry contains a frequency different from the one
the CPU is actually running at, the check falsely detects a frequency
change and sets data->resume. The next ->target() call then performs a
redundant control-register write even if the requested P-state is
already the current P-state.

Conversely, if the wrong table entry happens to contain the frequency
to which firmware has changed the CPU, the frequency change is missed
and data->resume remains clear. A subsequent ->target() call for the
P-state that the cpufreq core believes to be current can then
short-circuit without rewriting the control register, leaving the CPU
at the firmware-selected frequency until a different P-state is
requested.

Fix this by taking the cached frequency directly from
perf->states[perf->state].core_frequency. perf->state and
perf->states[] use the same P-state index space, and converting
core_frequency to kHz yields the same value stored in the corresponding
freq_table entry during initialization.

Fixes: e56a727b023d ("[CPUFREQ] Make acpi-cpufreq more robust against BIOS freq changes behind our back.")
Reported-by: Zhongqiu Han <zhongqiu.han@oss.qualcomm.com>
Suggested-by: Zhongqiu Han <zhongqiu.han@oss.qualcomm.com>
Signed-off-by: Li RongQing <lirongqing@baidu.com>
Reviewed-by: Zhongqiu Han <zhongqiu.han@oss.qualcomm.com>
---
 drivers/cpufreq/acpi-cpufreq.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/cpufreq/acpi-cpufreq.c b/drivers/cpufreq/acpi-cpufreq.c
index a797bb2..3313c74 100644
--- a/drivers/cpufreq/acpi-cpufreq.c
+++ b/drivers/cpufreq/acpi-cpufreq.c
@@ -353,6 +353,7 @@ static u32 get_cur_val(const struct cpumask *mask, struct acpi_cpufreq_data *dat
 
 static unsigned int get_cur_freq_on_cpu(unsigned int cpu)
 {
+	struct acpi_processor_performance *perf;
 	struct acpi_cpufreq_data *data;
 	struct cpufreq_policy *policy;
 	unsigned int freq;
@@ -368,7 +369,9 @@ static unsigned int get_cur_freq_on_cpu(unsigned int cpu)
 	if (unlikely(!data || !policy->freq_table))
 		return 0;
 
-	cached_freq = policy->freq_table[to_perf_data(data)->state].frequency;
+	perf = to_perf_data(data);
+	cached_freq = perf->states[perf->state].core_frequency * 1000;
+
 	freq = extract_freq(policy, get_cur_val(cpumask_of(cpu), data));
 	if (freq != cached_freq) {
 		/*
-- 
2.9.4