gpio: add a function to read GPIO array as base-2 value

This adds gpio_base2_value() which reads an array of 2-state
GPIOs and returns a base-2 value, where gpio[0] represents the
least significant bit.

BUG=none
BRANCH=none
TEST=tested with follow-up patches for pinky

Change-Id: Ia7ffc16eb60e93413c0812573b9cf0999b92828e
Signed-off-by: David Hendricks <dhendrix@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/228323
Reviewed-by: Julius Werner <jwerner@chromium.org>

src/include/gpio.h

@@ -34,6 +34,15 @@ void gpio_input_pullup(gpio_t gpio);
 void gpio_input(gpio_t gpio);
 void gpio_output(gpio_t gpio, int value);
 
+/*
+ * Read the value presented by the set of GPIOs, when each pin is interpreted
+ * as a base-2 digit (LOW = 0, HIGH = 1).
+ *
+ * gpio[]: pin positions to read. gpio[0] is less significant than gpio[1].
+ * num_gpio: number of pins to read.
+ */
+int gpio_base2_value(gpio_t gpio[], int num_gpio);
+
 /*
  * Read the value presented by the set of GPIOs, when each pin is interpreted
  * as a base-3 digit (LOW = 0, HIGH = 1, Z/floating = 2).

src/lib/gpio.c

@@ -22,6 +22,18 @@
 #include <delay.h>
 #include <gpio.h>
 
+int gpio_base2_value(gpio_t gpio[], int num_gpio)
+{
+	int i, result = 0;
+
+	for (i = 0; i < num_gpio; i++) {
+		gpio_input(gpio[i]);
+		result |= gpio_get(gpio[i]) << i;
+	}
+
+	return result;
+}
+
 int gpio_base3_value(gpio_t gpio[], int num_gpio)
 {
 	/*