[PATCH v3] cpufreq/amd-pstate: Skip auto_sel write when it already matches the mode

[Wentao Guan]

Since commit 9dfd13f80c85 ("cpufreq/amd-pstate: Toggle auto_sel in active
mode on shared memory systems"), shmem_init_perf() programs
AUTONOMOUS_SELECTION_ENABLE unconditionally, including in active mode.

Platforms that exclusively support autonomous selection report this field
as an Integer of 1, as required by the ACPI specification, instead of as a
register descriptor.  cppc_get_reg_val() reads such a field back
successfully, but cppc_set_reg_val() rejects the write with -EOPNOTSUPP
because the entry is not an ACPI_TYPE_BUFFER.

shmem_init_perf() propagates that error, so every amd_pstate_epp_cpu_init()
fails, cpufreq_register_driver() ends up with an empty policy list and
returns -ENODEV, and amd-pstate refuses to load at all:

  amd_pstate: failed to set auto_sel, ret: -95
  amd_pstate: Failed to initialize CPU 0: -95
  ...
  amd_pstate: failed to register with return -19

Skip the write when the register already holds the value the requested mode
needs.  Active and guided mode are fixed on these platforms, which report
auto_sel as 1, while passive mode still fails to load on a platform that
cannot disable autonomous selection, exactly as it did before the
offending commit.

Fixes: 9dfd13f80c85 ("cpufreq/amd-pstate: Toggle auto_sel in active mode on shared memory systems")
Cc: stable@xxxxxxxxxxxxxxx
Reviewed-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>
Tested-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>
Tested-by: LFRon <ronforever@xxxxxxxxxxx>
Signed-off-by: Wentao Guan <guanwentao@xxxxxxxxxxxxx>
---
changelog v3:
Include the K Prateek Nayak reviews to remove that ternary op base on current 
condition now have.  

v2 link: 
https://lore.kernel.org/stable/c3eb86bd-70a6-4bb2-8eff-b2a3c0817514@xxxxxxx/#R
changelog v2:
Accecpt and tested the K Prateek Nayak reviews.
v1 link:
https://lore.kernel.org/stable/20260918054808.48999-1-guanwentao@xxxxxxxxxxxxx/#R
---
---
 drivers/cpufreq/amd-pstate.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/cpufreq/amd-pstate.c b/drivers/cpufreq/amd-pstate.c
index 80d99ba1902c9..2d1f5292d1738 100644
--- a/drivers/cpufreq/amd-pstate.c
+++ b/drivers/cpufreq/amd-pstate.c
@@ -532,8 +532,10 @@ static int shmem_init_perf(struct amd_cpudata *cpudata)
 		return 0;
 	}
 
-	ret = cppc_set_auto_sel(cpudata->cpu,
-			(cppc_state == AMD_PSTATE_PASSIVE) ? 0 : 1);
+	if ((cppc_state != AMD_PSTATE_PASSIVE) == auto_sel)
+		return 0;
+
+	ret = cppc_set_auto_sel(cpudata->cpu, !auto_sel);
 
 	if (ret)
 		pr_warn("failed to set auto_sel, ret: %d\n", ret);
-- 
2.30.2