hwmon: (dell-smm) Warn if SMM call took a very long time to execute

If a particular SMM call takes a very long time to execute,
the user might experience audio problems. Print a warning
if a particular SMM call took over 0.250 seconds to execute,
so the user can check whether or not possible audio problems
are caused by this driver.
Signed-off-by: Armin Wolf <W_Armin@gmx.de>
Link: https://lore.kernel.org/r/20220426213154.724708-4-W_Armin@gmx.deSigned-off-by: Guenter Roeck <linux@roeck-us.net>

drivers/hwmon/dell-smm-hwmon.c

@@ -49,6 +49,9 @@
 #define I8K_SMM_GET_DELL_SIG1	0xfea3
 #define I8K_SMM_GET_DELL_SIG2	0xffa3
 
+/* in usecs */
+#define DELL_SMM_MAX_DURATION  250000
+
 #define I8K_FAN_MULT		30
 #define I8K_FAN_RPM_THRESHOLD	1000
 #define I8K_MAX_TEMP		127
@@ -239,6 +242,9 @@ static int i8k_smm_func(void *par)
 	pr_debug("smm(0x%.4x 0x%.4x) = 0x%.4x  (took %7lld usecs)\n", eax, ebx,
 		 (rc ? 0xffff : regs->eax & 0xffff), duration);
 
+	if (duration > DELL_SMM_MAX_DURATION)
+		pr_warn_once("SMM call took %lld usecs!\n", duration);
+
 	return rc;
 }