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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>
This commit is contained in:
Julius Werner 2020-08-05 15:27:11 -07:00
parent 0e0273a015
commit 7450790558
1 changed files with 6 additions and 4 deletions
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10
src/lib/gpio.c
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@ -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;
}