[PATCH v2] gpiolib: handle gpio-hogs only once

[Daniel Drake <dan@reactivated.net>]

Commit d1d564ec49929 ("gpio: move hogs into GPIO core") introduced a
behaviour change that breaks boot on Raspberry Pi 5 when using the
firmware-supplied device tree:

  gpiochip_add_data_with_key: GPIOs 544..575
    (/soc@107c000000/gpio@7d517c00) failed to register, -22
  brcmstb-gpio 107d517c00.gpio: Could not add gpiochip for bank 1
  brcmstb-gpio 107d517c00.gpio: probe with driver brcmstb-gpio failed
    with error -22

gpio-brcmstb registers two gpio_chips against the device tree
node gpio@7d517c00, one for each bank. The firmware-supplied DT includes
a gpio-hog on RP1 RUN, and this gpio-hog is attempted to be applied to
*both* gpio_chips. This succeeds against bank 0 (which hosts the GPIO)
and fails for bank 1 (which does not).

In the previous implementation, failures to apply gpio-hogs were
quietly ignored. In the new code, the error code propagates and causes
probe to fail.

Closely approximate the previous behaviour by using the OF_POPULATED flag
to ensure that each gpio-hog is processed only once. The flag was
previously being set before the gpio-hogs were processed, so as part
of this change, the flag now gets set only after the gpio-hog is actioned.
The handling of gpio-hogs on a DT node with multiple gpio_chips remains a
bit incomplete/unclear, but this at least retains the ability to apply
hogs to the first gpio_chip per node.

Signed-off-by: Daniel Drake <dan@reactivated.net>
---
 drivers/gpio/gpiolib-of.c | 5 -----
 drivers/gpio/gpiolib.c    | 8 ++++++++
 2 files changed, 8 insertions(+), 5 deletions(-)

v2: move OF_POPULATED flag setting to happen after the gpio-hog has
been applied (otherwise all hogs were considered already-applied
and never applied at all, oops!)

This bug is only exposed by the RPi5 firmware-provided DT that has the
gpio-hog. The DT shipped in the mainline kernel does not have the hog
here. I'm not sure to what extent Linux cares about supporting the
downstream firmware DT.

I'm also happy to consider other approaches. This multi-gpiochip setup is
a bit weird and gpio-brcmstb could perhaps be converted to register only a
single gpio_chip covering all banks. I verified that the other drivers
that obviously follow this same multiple-gpiochip pattern
(pinctrl-amlogic-a4, pinctrl-st and pinctrl-stm32) do not seem to be used by
any board DTs that include gpio-hogs.

diff --git a/drivers/gpio/gpiolib-of.c b/drivers/gpio/gpiolib-of.c
index 2c923d17541f..813dbcb91f6f 100644
--- a/drivers/gpio/gpiolib-of.c
+++ b/drivers/gpio/gpiolib-of.c
@@ -1066,11 +1066,6 @@ int of_gpiochip_add(struct gpio_chip *chip)
 
 	of_node_get(np);
 
-	for_each_available_child_of_node_scoped(np, child) {
-		if (of_property_read_bool(child, "gpio-hog"))
-			of_node_set_flag(child, OF_POPULATED);
-	}
-
 	return ret;
 }
 
diff --git a/drivers/gpio/gpiolib.c b/drivers/gpio/gpiolib.c
index 1e6dce430dca..b02d711289d0 100644
--- a/drivers/gpio/gpiolib.c
+++ b/drivers/gpio/gpiolib.c
@@ -1031,9 +1031,17 @@ static int gpiochip_hog_lines(struct gpio_chip *gc)
 		if (!fwnode_property_present(fwnode, "gpio-hog"))
 			continue;
 
+		/* The hog may have been handled by another gpio_chip on the same fwnode */
+		if (is_of_node(fwnode) &&
+		    of_node_check_flag(to_of_node(fwnode), OF_POPULATED))
+			continue;
+
 		ret = gpiochip_add_hog(gc, fwnode);
 		if (ret)
 			return ret;
+
+		if (is_of_node(fwnode))
+			of_node_set_flag(to_of_node(fwnode), OF_POPULATED);
 	}
 
 	return 0;
-- 
2.54.0