ppc64_cpu: Clean up sysfs smt/control error handling

When the kernel does not support the sysfs interface do not report an
error, fall back to the old method silently.

Suggested-by: Nathan Lynch<nathanl@linux.ibm.com>
Signed-off-by: Michal Suchanek <msuchanek@suse.de>
Signed-off-by: Tyrel Datwyler <tyreld@linux.ibm.com>

src/ppc64_cpu.c

@@ -364,14 +364,30 @@ static int is_dscr_capable(void)
 
 /*
  * Depends on kernel's CONFIG_HOTPLUG_CPU
+ * Return -1 for fatal error, -2 to retry.
  */
 static int set_smt_control(int smt_state)
 {
 	if (set_attribute(SYS_SMT_CONTROL, "%d", smt_state)) {
-		/* Silently ignore kernel not supporting this feature */
-		if (errno != ENODEV)
-			perror(SYS_SMT_CONTROL);
-		return -1;
+		switch (errno) {
+			case ENOENT:
+			/*
+			 * The kernel does not have the interface.
+			 * Try the old method.
+			 */
+				return -2;
+			case ENODEV:
+			/*
+			 * Setting SMT state not supported by this interface.
+			 * On older kernels (before Linux 6.6) the generic interface
+			 * may exist but is not hooked on powerpc resulting in ENODEV
+			 * on kernels that can set SMT using the old interface.
+			 */
+				return -2;
+			default:
+				perror(SYS_SMT_CONTROL);
+				return -1;
+		}
 	}
 	return 0;
 }
@@ -405,7 +421,7 @@ static int do_smt(char *state, bool numeric)
 		}
 
 		/* Try using smt/control if failing, fall back to the legacy way */
-		if (set_smt_control(smt_state))
+		if ((rc = set_smt_control(smt_state)) == -2)
 			rc = set_smt_state(smt_state);
 	}