[PATCH v2 1/3] cpufreq: amd-pstate: Restore previous mode when changing driver mode fails

["Mario Limonciello (AMD)" <superm1@kernel.org>]

From: Mario Limonciello <mario.limonciello@amd.com>

amd_pstate_change_driver_mode() unregisters the currently active driver
before registering the requested mode.  If amd_pstate_register_driver()
fails for the new mode, the function returns the error without restoring
anything, leaving the system with no cpufreq scaling driver at all until
a valid mode is manually re-selected.

Remember the mode that was active before the transition and, if
registering the requested mode fails, register the previous mode again so
the system keeps a working scaling driver.  The original error is still
returned to the caller so the sysfs write reports the failure.

Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://sashiko.dev/#/bug/linux-5b138ddb-c88b-4f82-8a2d-f39446b625d4
Fixes: 3ca7bc818d8c ("cpufreq: amd-pstate: Add guided mode control support via sysfs")
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Link: https://lore.kernel.org/r/20260921190235.3651688-1-mario.limonciello@amd.com
Signed-off-by: Mario Limonciello <superm1@kernel.org>
---
 drivers/cpufreq/amd-pstate.c | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/drivers/cpufreq/amd-pstate.c b/drivers/cpufreq/amd-pstate.c
index 743d90ed6af46..a1d58d1f5f3aa 100644
--- a/drivers/cpufreq/amd-pstate.c
+++ b/drivers/cpufreq/amd-pstate.c
@@ -1837,6 +1837,7 @@ static int amd_pstate_change_mode_without_dvr_change(int mode)
 
 static int amd_pstate_change_driver_mode(int mode)
 {
+	int old_mode = cppc_state;
 	int ret;
 
 	lockdep_assert_held(&amd_pstate_driver_lock);
@@ -1846,10 +1847,16 @@ static int amd_pstate_change_driver_mode(int mode)
 		return ret;
 
 	ret = amd_pstate_register_driver(mode);
-	if (ret)
-		return ret;
+	if (ret) {
+		pr_err("Failed to register %s mode, restoring %s mode\n",
+		       amd_pstate_get_mode_string(mode),
+		       amd_pstate_get_mode_string(old_mode));
+		if (amd_pstate_register_driver(old_mode))
+			pr_err("Failed to restore %s mode\n",
+			       amd_pstate_get_mode_string(old_mode));
+	}
 
-	return 0;
+	return ret;
 }
 
 static cppc_mode_transition_fn mode_state_machine[AMD_PSTATE_MAX][AMD_PSTATE_MAX] = {
-- 
2.53.0