Re: [PATCH] gpio: 74x164: Enable output pins after registers are reset

[Arturas Moskvinas <arturas.moskvinas@gmail.com>]

Hello,

On 2/26/24 16:01, Andy Shevchenko wrote:
> On Mon, Feb 26, 2024 at 03:46:56PM +0200, Arturas Moskvinas wrote:
>> Move output enabling after chip registers are cleared.
> Does this fix anything? If so, maybe elaborate a bit the potential behavioural
> changes on the real lines.

Chip outputs are enabled[1] before actual reset is performed[2] which 
might cause pin output value to flip flop if previous pin value was set 
to 1 in chip. Change fixes that behavior by making sure chip is fully 
reset before all outputs are enabled.

Flip-flop can be noticed when module is removed and inserted again and 
one of the pins was changed to 1 before removal. 100 microsecond 
flipping is noticeable on oscilloscope (100khz SPI bus).

For a properly reset chip - output is enabled around 100 microseconds 
(on 100khz SPI bus) later during probing process hence should be 
irrelevant behavioral change.

[1] - 
https://elixir.bootlin.com/linux/v6.7.4/source/drivers/gpio/gpio-74x164.c#L130
[2] - 
https://elixir.bootlin.com/linux/v6.7.4/source/drivers/gpio/gpio-74x164.c#L150

Arturas Moskvinas