pwm: cros-ec: Accept more error codes from cros_ec_cmd_xfer_status

Since commit c5cd2b47 ("platform/chrome: cros_ec_proto: Report command
not supported") we can no longer assume that cros_ec_cmd_xfer_status()
reports -EPROTO for all errors returned by the EC itself. A follow-up
patch will change cros_ec_cmd_xfer_status() to report additional errors
reported by the EC as distinguished Linux error codes.

Handle this change by no longer assuming that only -EPROTO is used
to report all errors returned by the EC itself. Instead, support both
the old and the new error codes.

Add a comment describing cros_ec_num_pwms() to explain its functionality.

Cc: Gwendal Grignou <gwendal@chromium.org>
Cc: Yu-Hsuan Hsu <yuhsuan@chromium.org>
Cc: Prashant Malani <pmalani@chromium.org>
Cc: Brian Norris <briannorris@chromium.org>
Acked-by: Thierry Reding <thierry.reding@gmail.com>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>

drivers/pwm/pwm-cros-ec.c

@@ -204,6 +204,11 @@ static const struct pwm_ops cros_ec_pwm_ops = {
 	.owner		= THIS_MODULE,
 };
 
+/*
+ * Determine the number of supported PWMs. The EC does not return the number
+ * of PWMs it supports directly, so we have to read the pwm duty cycle for
+ * subsequent channels until we get an error.
+ */
 static int cros_ec_num_pwms(struct cros_ec_device *ec)
 {
 	int i, ret;
@@ -213,20 +218,30 @@ static int cros_ec_num_pwms(struct cros_ec_device *ec)
 		u32 result = 0;
 
 		ret = __cros_ec_pwm_get_duty(ec, i, &result);
-		/* We want to parse EC protocol errors */
-		if (ret < 0 && !(ret == -EPROTO && result))
-			return ret;
-
 		/*
 		 * We look for SUCCESS, INVALID_COMMAND, or INVALID_PARAM
 		 * responses; everything else is treated as an error.
+		 * The EC error codes either map to -EOPNOTSUPP / -EINVAL,
+		 * or -EPROTO is returned and the EC error is in the result
+		 * field. Check for both.
 		 */
-		if (result == EC_RES_INVALID_COMMAND)
+		switch (ret) {
+		case -EOPNOTSUPP:	/* invalid command */
 			return -ENODEV;
-		else if (result == EC_RES_INVALID_PARAM)
+		case -EINVAL:		/* invalid parameter */
 			return i;
-		else if (result)
+		case -EPROTO:
+			/* Old or new error return code: Handle both */
+			if (result == EC_RES_INVALID_COMMAND)
+				return -ENODEV;
+			else if (result == EC_RES_INVALID_PARAM)
+				return i;
 			return -EPROTO;
+		default:
+			if (ret < 0)
+				return ret;
+			break;
+		}
 	}
 
 	return U8_MAX;