power: supply: cros_charge-control: hide start threshold on v2 cmd

ECs implementing the v2 command will not stop charging when the end
threshold is reached. Instead they will begin discharging until the
start threshold is reached, leading to permanent charge and discharge
cycles. This defeats the point of the charge control mechanism.

Avoid the issue by hiding the start threshold on v2 systems.
Instead on those systems program the EC with start == end which forces
the EC to reach and stay at that level.

v1 does not support thresholds and v3 works correctly,
at least judging from the code.

Reported-by: Thomas Koch <[e-mail]>
Fixes: c6ed48ef5259 ("power: supply: add ChromeOS EC based charge control driver")
Cc: [e-mail]
Signed-off-by: Thomas Weißschuh <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Sebastian Reichel <[e-mail]>

drivers/power/supply/cros_charge-control.c

···
 		return -EINVAL;
 
 	if (is_end_threshold) {
+		/* Start threshold is not exposed, use fixed value */
+		if (priv->cmd_version == 2)
+			priv->current_start_threshold = val == 100 ? 0 : val;
+
 		if (val < priv->current_start_threshold)
 			return -EINVAL;
 		priv->current_end_threshold = val;
···
 {
 	struct cros_chctl_priv *priv = cros_chctl_attr_to_priv(attr, n);
 
-	if (priv->cmd_version < 2) {
-		if (n == CROS_CHCTL_ATTR_START_THRESHOLD)
-			return 0;
-		if (n == CROS_CHCTL_ATTR_END_THRESHOLD)
-			return 0;
-	}
+	if (n == CROS_CHCTL_ATTR_START_THRESHOLD && priv->cmd_version < 3)
+		return 0;
+	else if (n == CROS_CHCTL_ATTR_END_THRESHOLD && priv->cmd_version < 2)
+		return 0;
 
 	return attr->mode;
 }