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

Krzysztof Kozlowski
2026-04-28T08:40:55Z

On 23/04/2026 12:33, Chanhong Jung wrote:
> Hi Krzysztof,
>
> Thanks for the review.
>
>> You can fix the driver not to do that. Or fix the driver to properly
>> parse hogs. I don't understand why exactly this one driver needs
>> default registers stored in DT, but all other drivers in the kernel
>> don't.
>
> I thought about both, but I don't think either works for this chip.
>
> 74HC595/74LVC594 is pure write-only: no MISO path, no register
> read-back, and the data sheet defines the power-on state of the
> parallel outputs as whatever /SRCLR and /OE do before the SPI master
> is up. That's different from MCP23xxx, PCA953x, STMPE and the rest,
> which are either readable (so the driver can RMW) or have a defined
> reset value. None of those drivers need a seed property. This one
> does, because there is nothing to read back and no defined state to
> fall back on.
>
> Partial updates aren't possible either. set_value() has to shift the
> full chain every time - see __gen_74x164_write_config() - so whatever

This should also be explained in the commit msg.

Best regards,
Krzysztof