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>

0 files changed, 0 insertions, 0 deletions