From: Chanhong Jung Date: Wed, 29 Apr 2026 03:51:33 +0000 (+0900) Subject: dt-bindings: gpio: fairchild,74hc595: add lines-initial-states property X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=d6e1a94888f5a4306c9998944a0f29f7bcd49411;p=thirdparty%2Fkernel%2Flinux.git dt-bindings: gpio: fairchild,74hc595: add lines-initial-states property The 74HC595 and 74LVC594 shift registers latch their outputs until the first serial write, so boards that depend on a specific power-on pattern (for example active-low indicators, reset lines, or other signals that must come up non-zero) have no way to express that today: the Linux driver always writes zeros from its zero-initialised buffer during probe. Document support for the existing lines-initial-states bitmask, already defined for nxp,pcf8575, so the same convention covers this output-only device. Bit N corresponds to GPIO line N. Because the 74HC595/74LVC594 family is push-pull output only (no input mode, no high-impedance state under software control), bit=0 drives the line low and bit=1 drives it high; this differs from nxp,pcf8575, where the 0/1 polarity reflects the quasi-bidirectional nature of that part. The bitmask covers up to 32 lines, which fits the typical 1-4 chip cascades that appear in tree. Should longer chains require seeding in the future, the property can be extended to a uint32-array without breaking the bit-N-equals-line-N convention. Suggested-by: Linus Walleij Signed-off-by: Chanhong Jung Reviewed-by: Rob Herring (Arm) Reviewed-by: Linus Walleij Link: https://patch.msgid.link/20260429035134.1023330-2-happycpu@gmail.com Signed-off-by: Bartosz Golaszewski --- diff --git a/Documentation/devicetree/bindings/gpio/fairchild,74hc595.yaml b/Documentation/devicetree/bindings/gpio/fairchild,74hc595.yaml index 23410aeca3000..451538df63f72 100644 --- a/Documentation/devicetree/bindings/gpio/fairchild,74hc595.yaml +++ b/Documentation/devicetree/bindings/gpio/fairchild,74hc595.yaml @@ -45,6 +45,18 @@ properties: $ref: /schemas/types.yaml#/definitions/uint32 description: Number of daisy-chained shift registers + lines-initial-states: + $ref: /schemas/types.yaml#/definitions/uint32 + description: + Bitmask that specifies the initial state of each output line, written + by the driver before the gpiochip is registered. Bit N corresponds to + GPIO line N, following the convention already documented for + nxp,pcf8575. Because the 74HC595/74LVC594 family is push-pull output + only, a bit set to zero drives the line low and a bit set to one + drives it high. The bitmask covers up to 32 lines (four cascaded + registers); outputs beyond that come up zeroed. When the property is + absent all outputs come up low, preserving the previous behaviour. + enable-gpios: description: GPIO connected to the OE (Output Enable) pin. maxItems: 1 @@ -79,6 +91,7 @@ examples: gpio-controller; #gpio-cells = <2>; registers-number = <4>; + lines-initial-states = <0xffff0000>; spi-max-frequency = <100000>; }; };