tegra124: support tri-state Board Id

Since Nyan Big board id pins may layout in tri-state, we need code
to support this change.

We use every two-bit to encode a gpio state as below:
 b00: LOW
 b01: HIGH
 b10: TRI-STATE

BUG=none
TEST=emerge-nyan chromeos-coreboot-nyan
     coreboot log shows "Board TRISTATE ID: 0x41" for gpios 1001

Change-Id: I05d63f0bdafd533ef9d9e0da668837ecd2f4c8c0
Signed-off-by: Jimmy Zhang <jimmzhang@nvidia.com>
Reviewed-on: https://chromium-review.googlesource.com/183855
Reviewed-by: Gabe Black <gabeblack@chromium.org>
Commit-Queue: Doug Anderson <dianders@chromium.org>

src/mainboard/google/nyan_big/boardid.c

@@ -19,6 +19,7 @@
 
 #include <console/console.h>
 #include <soc/nvidia/tegra124/gpio.h>
+#include <stdlib.h>
 
 #include "boardid.h"
 
@@ -27,11 +28,17 @@ uint8_t board_id(void)
 	static int id = -1;
 
 	if (id < 0) {
-		id = gpio_get_in_value(GPIO(Q3)) << 0 |
-		     gpio_get_in_value(GPIO(T1)) << 1 |
-		     gpio_get_in_value(GPIO(X1)) << 2 |
-		     gpio_get_in_value(GPIO(X4)) << 3;
-		printk(BIOS_SPEW, "Board ID: %#x.\n", id);
+		gpio_t gpio[] = {GPIO(Q3), GPIO(T1), GPIO(X1), GPIO(X4)};
+		int value[ARRAY_SIZE(gpio)];
+
+		gpio_get_in_tristate_values(gpio, ARRAY_SIZE(gpio), value);
+
+		/* A gpio state is encoded in every two-bit */
+		id = value[0] << 0 |
+		     value[1] << 2 |
+		     value[2] << 4 |
+		     value[3] << 6;
+		printk(BIOS_SPEW, "Board TRISTATE ID: %#x.\n", id);
 	}
 
 	return id;

src/soc/nvidia/tegra/gpio.c

@@ -22,6 +22,7 @@
 #include <soc/addressmap.h>
 #include <stddef.h>
 #include <stdint.h>
+#include <delay.h>
 
 #include "gpio.h"
 #include "pinmux.h"
@@ -174,6 +175,58 @@ int gpio_get_in_value(gpio_t gpio)
 	return (port & (1 << bit)) != 0;
 }
 
+int gpio_get_in_tristate_values(gpio_t gpio[], int num_gpio, int value[])
+{
+	/*
+	 * GPIOs which are tied to stronger external pull up or pull down
+	 * will stay there regardless of the internal pull up or pull
+	 * down setting.
+	 *
+	 * GPIOs which are floating will go to whatever level they're
+	 * internally pulled to.
+	 */
+
+	int temp;
+	int index;
+
+	/* Enable internal pull up */
+	for (index = 0; index < num_gpio; ++index)
+		gpio_input_pullup(gpio[index]);
+
+	/* Wait until signals become stable */
+	udelay(10);
+
+	/* Get gpio values at internal pull up */
+	for (index = 0; index < num_gpio; ++index)
+		value[index] = gpio_get_in_value(gpio[index]);
+
+	/* Enable internal pull down */
+	for (index = 0; index < num_gpio; ++index)
+		gpio_input_pulldown(gpio[index]);
+
+	/* Wait until signals become stable */
+	udelay(10);
+
+	/*
+	 * Get gpio values at internal pull down.
+	 * Compare with gpio pull up value and then
+	 * determine a gpio final value/state:
+	 *  0: pull down
+	 *  1: pull up
+	 *  2: floating
+	 */
+	for (index = 0; index < num_gpio; ++index) {
+		temp = gpio_get_in_value(gpio[index]);
+		value[index] = ((value[index] ^ temp) << 1) | temp;
+	}
+
+	/* Disable pull up / pull down to conserve power */
+	for (index = 0; index < num_gpio; ++index)
+		gpio_input(gpio[index]);
+
+	return 0;
+}
+
 int gpio_get_int_status(gpio_t gpio)
 {
 	int bit = gpio % GPIO_GPIOS_PER_PORT;

src/soc/nvidia/tegra/gpio.h

@@ -81,6 +81,7 @@ void gpio_set_out_value(gpio_t gpio, int value);
 int gpio_get_out_value(gpio_t gpio);
 
 int gpio_get_in_value(gpio_t gpio);
+int gpio_get_in_tristate_values(gpio_t gpio[], int num_gpio, int value[]);
 
 int gpio_get_int_status(gpio_t gpio);