[coreboot-gerrit] Change in coreboot[master]: gpio: Pull down HiZ pins after reading tristate GPIO strapping

[Julius Werner (Code Review)]

Julius Werner has submitted this change. ( https://review.coreboot.org/c/coreboot/+/44253 )

Change subject: gpio: Pull down HiZ pins after reading tristate GPIO strapping
......................................................................

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 
Change-Id: I1669823c8a7faab536e0441cb4c6cfeb9f696189
Reviewed-on: https://review.coreboot.org/c/coreboot/+/44253
Reviewed-by: Douglas Anderson 
Reviewed-by: Alexandru Stan 
Reviewed-by: Furquan Shaikh 
Reviewed-by: Aaron Durbin 
Reviewed-by: Angel Pons 
Tested-by: build bot (Jenkins) 
---
M src/lib/gpio.c
1 file changed, 6 insertions(+), 4 deletions(-)

Approvals:
  build bot (Jenkins): Verified
  Aaron Durbin: Looks good to me, approved
  Furquan Shaikh: Looks good to me, approved
  Douglas Anderson: Looks good to me, but someone else must approve
  Angel Pons: Looks good to me, approved
  Alexandru Stan: Looks good to me, but someone else must approve

diff --git a/src/lib/gpio.c b/src/lib/gpio.c
index 3f3ae60..801a3b3 100644
--- a/src/lib/gpio.c
+++ b/src/lib/gpio.c
@@ -114,6 +114,12 @@
    	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 @@
    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;
 }