Re: [PATCH v1 1/2] dt-bindings: gpio: fairchild,74hc595: add registers-default property

[Linus Walleij]

On Wed, Apr 22, 2026 at 6:05 PM Chanhong Jung <happycpu@xxxxxxxxx> wrote:

> 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.
>
> Describe a new optional 'registers-default' property that carries a u8
> array - one byte per cascaded register, in the same order used by the
> driver's internal buffer (first byte targets the last register in the
> chain). The Linux driver change that consumes this property follows.
>
> This property is already recognised by the corresponding U-Boot driver
> (drivers/gpio/74x164_gpio.c), so documenting it here brings the two
> bindings back in sync and allows boards to initialise the chain once
> from the bootloader DT and keep the same value after the kernel takes
> over.
>
> Signed-off-by: Chanhong Jung <happycpu@xxxxxxxxx>

See
Documentation/devicetree/bindings/gpio/nxp,pcf8575.yaml

  lines-initial-states:
    $ref: /schemas/types.yaml#/definitions/uint32
    description:
      Bitmask that specifies the initial state of each line.
      When a bit is set to zero, the corresponding line will be initialized to
      the input (pulled-up) state.
      When the  bit is set to one, the line will be initialized to the
      low-level output state.
      If the property is not specified all lines will be initialized to the
      input state.

If you want to set up initial states, use this property.

This also makes it possible for us to centralize the handling later on.

Yours,
Linus Walleij