[PATCH v2 1/2] dt-bindings: gpio: nxp,pcf8575: add reset GPIO

[Quentin Schulz <foss+kernel@0leil.net>]

From: Quentin Schulz <quentin.schulz@cherry.de>

A few of the I2C GPIO expander chips supported by this binding have a
RESETN pin to be able to reset the chip. The chip is held in reset while
the pin is low, therefore the polarity of reset-gpios is expected to
reflect that, i.e. a GPIO_ACTIVE_HIGH means the GPIO will be driven high
for reset and then driven low, GPIO_ACTIVE_LOW means the GPIO will be
driven low for reset and then driven high. If a GPIO is directly routed
to RESETN pin on the IC without any inverter, GPIO_ACTIVE_LOW is thus
expected.

Out of the supported chips, only PCA9670, PCA9671, PCA9672 and PCA9673
show a RESETN pin in their datasheets. They all share the same reset
timings, that is 4+us reset pulse[0] and 100+us reset time[0].

When performing a reset, "The PCA9670 registers and I2C-bus state
machine will be held in their default state until the RESET input is
once again HIGH."[1] meaning we now know the state of each line
controlled by the GPIO expander. Therefore, setting lines-initial-states
and reset-gpios both does not make sense and their presence is XOR'ed.

[0] https://www.nxp.com/docs/en/data-sheet/PCA9670.pdf Fig 22.
[1] https://www.nxp.com/docs/en/data-sheet/PCA9670.pdf 8.5

Tested-by: Heiko Stuebner <heiko@sntech.de> # exclusion logic
Signed-off-by: Quentin Schulz <quentin.schulz@cherry.de>
---
 .../devicetree/bindings/gpio/nxp,pcf8575.yaml      | 38 ++++++++++++++++++++++
 1 file changed, 38 insertions(+)

diff --git a/Documentation/devicetree/bindings/gpio/nxp,pcf8575.yaml b/Documentation/devicetree/bindings/gpio/nxp,pcf8575.yaml
index 3718103e966a13e1d77f73335ff73c18a3199469..633ac5cfa04a10bcbb748b6580938cddae9e5596 100644
--- a/Documentation/devicetree/bindings/gpio/nxp,pcf8575.yaml
+++ b/Documentation/devicetree/bindings/gpio/nxp,pcf8575.yaml
@@ -73,6 +73,44 @@ properties:
 
   wakeup-source: true
 
+  reset-gpios:
+    maxItems: 1
+    description:
+      GPIO controlling the (reset active LOW) RESET# pin.
+
+      The active polarity of the GPIO must translate to the low state
+      of the RESET# pin on the IC, i.e. if a GPIO is directly routed
+      to the RESET# pin without any inverter, GPIO_ACTIVE_LOW is
+      expected.
+
+      Performing a reset makes all lines initialized to their input (pulled-up)
+      state.
+
+allOf:
+  - if:
+      properties:
+        compatible:
+          not:
+            contains:
+              enum:
+                - nxp,pca9670
+                - nxp,pca9671
+                - nxp,pca9672
+                - nxp,pca9673
+    then:
+      properties:
+        reset-gpios: false
+
+  # lines-initial-states XOR reset-gpios
+  # Performing a reset reinitializes all lines to a known state which
+  # may not match passed lines-initial-states
+  - if:
+      required:
+        - lines-initial-states
+    then:
+      properties:
+        reset-gpios: false
+
 patternProperties:
   "^(.+-hog(-[0-9]+)?)$":
     type: object

-- 
2.48.1