Messages in this thread |  | | From | Linus Walleij <> | | Date | Thu, 30 Apr 2026 14:09:32 +0200 | | Subject | Re: [PATCH v2 2/2] gpio: 74x164: support lines-initial-states for boot-time output state |
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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
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