[PATCH v3 0/2] cpufreq/amd-pstate: Fix null pointer dereference when frequency invariance is disabled

[Wyes Karny]

Currently, amd_pstate only uses `adjust_perf` and `target` callback
functions to get the frequency/effective utilization data from scaling
governors. But the governor can switch back to fast_switch callback when
frequency invariance is disabled. In that case, if the driver didn't set
fast_switch function pointer, null pointer dereference can happen and
kernel will crash.

Currently, frequency invariance is turned off when any anomaly is
detected with aperf/mperf readings. Which triggers this problem.

Fix this by adding `fast_switch` function in the amd_pstate driver. The
same problem could arise with intel_pstate driver when the driver is in
passive mode and the governor is schedutil.

Additionally, display a warning message if the scaling driver sets
fast_swich_possible flag but not fast_switch function pointer.

Changelog:
v2 -> v3
- Add fast_switch callback in amd_pstate and warn if any driver
  sets fast_swich_possible flag but not fast_switch function pointer.

v1 -> v2
- Remove fast_switch_enabled flag dependency for adjust_perf callback

v1: https://lore.kernel.org/linux-pm/20230410095045.14872-1-wyes.karny@xxxxxxx/
v2: https://lore.kernel.org/linux-pm/20230509180503.739208-1-wyes.karny@xxxxxxx/

Gautham R. Shenoy (1):
  amd_pstate: Add ->fast_switch() callback

Wyes Karny (1):
  cpufreq: Warn if fast_switch is not set

 drivers/cpufreq/amd-pstate.c | 35 +++++++++++++++++++++++++++++------
 drivers/cpufreq/cpufreq.c    | 18 ++++++++++++++++++
 include/linux/cpufreq.h      |  5 ++++-
 3 files changed, 51 insertions(+), 7 deletions(-)

-- 
2.34.1