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authorJulius Werner <jwerner@chromium.org>2020-08-05 15:27:11 -0700
committerJulius Werner <jwerner@chromium.org>2020-08-06 23:54:41 +0000
commit7450790558f3b1f85e9a11df252dd27f1047bfeb (patch)
tree4534b2e8d65d250686cd2268d5d9bd60fbd21078
parent0e0273a015f57e244015c80f0fa37192f1e569f3 (diff)
downloadcoreboot-7450790558f3b1f85e9a11df252dd27f1047bfeb.tar.xz
gpio: Pull down HiZ pins after reading tristate GPIO strapping
People who know a lot more about electrons and stuff than I do tell me that leaving a HiZ pin floating without a pull resistor may waste power. So if we find a pin to be HiZ when reading tristate strapping GPIOs, we should make sure the internal pull-down is enabled when we're done with it. (For pins that are externally pulled high or low, we should continue to leave the internal pull disabled instead.) Signed-off-by: Julius Werner <jwerner@chromium.org> Change-Id: I1669823c8a7faab536e0441cb4c6cfeb9f696189 Reviewed-on: https://review.coreboot.org/c/coreboot/+/44253 Reviewed-by: Douglas Anderson <dianders@chromium.org> Reviewed-by: Alexandru Stan <amstan@google.com> Reviewed-by: Furquan Shaikh <furquan@google.com> Reviewed-by: Aaron Durbin <adurbin@chromium.org> Reviewed-by: Angel Pons <th3fanbus@gmail.com> Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
-rw-r--r--src/lib/gpio.c10
1 files changed, 6 insertions, 4 deletions
diff --git a/src/lib/gpio.c b/src/lib/gpio.c
index 3f3ae60d91..801a3b3b9c 100644
--- a/src/lib/gpio.c
+++ b/src/lib/gpio.c
@@ -114,6 +114,12 @@ uint32_t _gpio_base3_value(const gpio_t gpio[], int num_gpio, int binary_first)
printk(BIOS_DEBUG, "%c ", tristate_char[temp]);
result = (result * 3) + temp;
+ /* Disable pull to avoid wasting power. For HiZ we leave the
+ pull-down enabled, since letting them float freely back and
+ forth may waste power in the SoC's GPIO input logic. */
+ if (temp != Z)
+ gpio_input(gpio[index]);
+
/*
* For binary_first we keep track of the normal ternary result
* and whether we found any pin that was a Z. We also determine
@@ -159,10 +165,6 @@ uint32_t _gpio_base3_value(const gpio_t gpio[], int num_gpio, int binary_first)
printk(BIOS_DEBUG, "= %d (%s base3 number system)\n", result,
binary_first ? "binary_first" : "standard");
- /* Disable pull up / pull down to conserve power */
- for (index = 0; index < num_gpio; ++index)
- gpio_input(gpio[index]);
-
return result;
}