Re: [PATCH v2 1/2] dt-bindings: gpio: fairchild,74hc595: add lines-initial-states property

["Rob Herring (Arm)" <robh@kernel.org>]

On Wed, 29 Apr 2026 12:51:33 +0900, Chanhong Jung 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.
> 
> 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 <linus.walleij@linaro.org>
> Signed-off-by: Chanhong Jung <happycpu@gmail.com>
> ---
>  .../devicetree/bindings/gpio/fairchild,74hc595.yaml | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 

Reviewed-by: Rob Herring (Arm) <robh@kernel.org>