pinctrl: aspeed: Allow disabling Port D and Port E loopback mode

Port D and port E GPIO loopback modes are commonly enabled via hardware
straps for use with front-panel buttons.  When the BMC is powered
off or fails to boot, the front-panel buttons are directly connected to
the host chipset via the loopback to allow direct power-on and reset
control. Once the BMC has booted, the loopback mode must be disabled for
the BMC to take over control of host power-on and reset.

Disabling these loopback modes requires writing to the hardware strap
register which violates the current design of assuming the system
designer chose the strap settings for a specific reason and they should
be treated as read-only. Only the two bits of the strap register related
to these loopback modes are allowed to be written and comments have been
added to explain why.

Signed-off-by: Rick Altherr <raltherr@google.com>
Acked-by: Joel Stanley <joel@jms.id.au>
Reviewed-by: Andrew Jeffery <andrew@aj.id.au>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/pinctrl/aspeed/pinctrl-aspeed.c

@@ -198,9 +198,19 @@ static int aspeed_sig_expr_set(const struct aspeed_sig_expr *expr,
 		 * them. This may mean that certain functions cannot be
 		 * deconfigured and is the reason we re-evaluate after writing
 		 * all descriptor bits.
+		 *
+		 * Port D and port E GPIO loopback modes are the only exception
+		 * as those are commonly used with front-panel buttons to allow
+		 * normal operation of the host when the BMC is powered off or
+		 * fails to boot. Once the BMC has booted, the loopback mode
+		 * must be disabled for the BMC to control host power-on and
+		 * reset.
 		 */
-		if ((desc->reg == HW_STRAP1 || desc->reg == HW_STRAP2) &&
-				desc->ip == ASPEED_IP_SCU)
+		if (desc->ip == ASPEED_IP_SCU && desc->reg == HW_STRAP1 &&
+		    !(desc->mask & (BIT(21) | BIT(22))))
+			continue;
+
+		if (desc->ip == ASPEED_IP_SCU && desc->reg == HW_STRAP2)
 			continue;
 
 		ret = regmap_update_bits(maps[desc->ip], desc->reg,