Generalize revision number calculation function

Some platforms use tertiary interpretation of GPIO input state to
increase number of distinct values represented by a limited number of
GPIOs. The three states are

- external pull down (interpreted as 0)
- external pull up (1)
- not connected (2)

This has been required by Nvidia devices so far, but Exynos and
Ipq8086 platforms need this too.

This patch moves the function reading the tertiary state into the
library and exposes the necessary GPIO API functions in a new include
file. The functions are still supposed to be provided by platform
specific modules.

The function interpreting the GPIO states has been modified to allow
to interpret the state either as a true tertiary number or as a set
two bit fields.

Since linker garbage collection is not happening when building x86
targets, a new configuration option is being added to include the new
module only when needed.

BUG=chrome-os-partner:30489
TEST=verified that nyan_big still reports proper revision ID.

Change-Id: I243c9f43c82bd4a41de2154bbdbd07df0a241046
Signed-off-by: Vadim Bendebury <vbendeb@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/209673
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

src/include/gpiolib.h

@@ -0,0 +1,47 @@
+/*
+ * This file is part of the coreboot project.
+ *
+ * Copyright 2014 Google Inc.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; version 2 of the License.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+#ifndef __SRC_INCLUDE_GPIOLIB_H__
+#define __SRC_INCLUDE_GPIOLIB_H__
+
+/* A generic type, use accessor macros to actually access the hardware. */
+typedef unsigned gpio_t;
+
+/*
+ * Read the value presented by the set of GPIOs, when each pin is interpreted
+ * as a number in 0..2 range depending on the external pullup situation.
+ *
+ * Depending on the third parameter, the return value is either a set of two
+ * bit fields, each represeting one GPIO value, or a number where each GPIO is
+ * included multiplied by 3^gpio_num, resulting in a true tertiary value.
+ *
+ */
+int gpio_board_id(gpio_t gpio[], int num_gpio, int tertiary);
+
+/*
+ * The following functions are not provided by the common library, but must be
+ * implemented by the appropriate SOC/board instead.
+ */
+int gpio_get_in_value(gpio_t gpio);
+void gpio_set_out_value(gpio_t gpio, int value);
+void gpio_input_pulldown(gpio_t gpio);
+void gpio_input_pullup(gpio_t gpio);
+void gpio_input(gpio_t gpio);
+
+#endif