Re: [PATCH v2 2/2] gpio: 74x164: support lines-initial-states for boot-time output state

정찬홍
2026-04-30T22:14:15Z

Thanks Linus, appreciate the review.

2026년 4월 30일 (목) 오후 9:09, Linus Walleij <linusw@kernel.org>님이 작성:
>
> On Wed, Apr 29, 2026 at 5:51 AM Chanhong Jung <happycpu@gmail.com> wrote:
>
> > 74HC595 and 74LVC594 chains retain their output state from the first
> > serial write onwards. Today the driver always kicks that first write
> > from a zero-initialised buffer, so every output comes up low until user
> > space issues a write. Boards that rely on the chain to drive signals
> > whose power-on state matters (active-low indicators, reset lines, etc.)
> > have no way to express the desired initial pattern via DT.
> >
> > Read the optional lines-initial-states bitmask, recently documented for
> > this binding, into chip->buffer before the first
> > __gen_74x164_write_config() so the chain comes up in a known state on
> > the very first SPI transaction. Bit N maps to GPIO line N (matching the
> > nxp,pcf8575 convention); on this output-only device, bit=0 drives the
> > line low and bit=1 drives it high. Property absence keeps the existing
> > zeroing behaviour intact.
> >
> > Suggested-by: Linus Walleij <linus.walleij@linaro.org>
> > Signed-off-by: Chanhong Jung <happycpu@gmail.com>
>
> Reviewed-by: Linus Walleij <linusw@kernel.org>
>
> If more users of this appears we can start thinking about brining
> the support code into the core gpiolib but for now this works
> fine I think.
>
> Yours,
> Linus Walleij