gpio: Add support for binary_first base3 number system

This patch adds support for an alternative ternary number system in
which group of GPIOs can be interpreted. In this system, the digit
combinations that would form a binary number (i.e. that contain no 'Z'
state) are used to represent the lower values in the way they're used in
the normal binary system, and all the combinations that do contain a 'Z'
are used to represent values above those. We can use this for boards
that originally get strapped with binary board IDs but eventually
require more revisions than that representation allows. We can switch
their code to binary_first base3 and all old revisions with already
produced boards will still get read as the correct numbers.

Credit for the algorithm idea goes to Haran Talmon.

BRANCH=None
BUG=None
TEST=Stubbed out the actual GPIO reading and simulated all combinations
of 4 ternary digits for both number systems.

Change-Id: Ib5127656455f97f890ce2999ba5ac5f58a20cf93
Signed-off-by: Julius Werner <jwerner@chromium.org>
Reviewed-on: https://review.coreboot.org/14116
Tested-by: build bot (Jenkins)
Reviewed-by: David Hendricks <dhendrix@chromium.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

1 files changed, 35 insertions, 1 deletions

diff --git a/src/include/gpio.h b/src/include/gpio.h
index ea03a23231..4627e4412c 100644
--- a/src/include/gpio.h
+++ b/src/include/gpio.h
@@ -29,6 +29,7 @@ void gpio_input_pulldown(gpio_t gpio);
 void gpio_input_pullup(gpio_t gpio);
 void gpio_input(gpio_t gpio);
 void gpio_output(gpio_t gpio, int value);
+int _gpio_base3_value(gpio_t gpio[], int num_gpio, int binary_first);
 
 /*
  * Read the value presented by the set of GPIOs, when each pin is interpreted
@@ -48,6 +49,39 @@ int gpio_base2_value(gpio_t gpio[], int num_gpio);
  * gpio[]: pin positions to read. gpio[0] is less significant than gpio[1].
  * num_gpio: number of pins to read.
  */
-int gpio_base3_value(gpio_t gpio[], int num_gpio);
+static inline int gpio_base3_value(gpio_t gpio[], int num_gpio)
+{
+	return _gpio_base3_value(gpio, num_gpio, 0);
+}
+
+/*
+ * 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) in a non-standard
+ * ternary number system where the first 2^n natural numbers are represented
+ * as they would be in a binary system (without any Z digits), and the following
+ * 3^n-2^n numbers use the remaining ternary representations in the normal
+ * ternary system order (skipping the values that were already used up).
+ * This is useful for boards which initially used a binary board ID and later
+ * decided to switch to tri-state after some revisions have already been built.
+ * Example: For num_gpio = 2 we get the following representation:
+ *
+ *   Number      X1     X0
+ *     0          0      0
+ *     1          0      1
+ *     2          1      0
+ *     3          1      1	// Start counting ternaries back at 0 after this
+ *     4          0      2	// Skipping 00 and 01 which are already used up
+ *     5          1      2	// Skipping 10 and 11 which are already used up
+ *     6          2      0
+ *     7          2      1
+ *     8          2      2
+ *
+ * gpio[]: pin positions to read. gpio[0] is less significant than gpio[1].
+ * num_gpio: number of pins to read.
+ */
+static inline int gpio_binary_first_base3_value(gpio_t gpio[], int num_gpio)
+{
+	return _gpio_base3_value(gpio, num_gpio, 1);
+}
 
 #endif /* __SRC_INCLUDE_GPIO_H__ */