[RFC PATCH 0/3] gpiolib: ramp-up delay support

[Alexander Stein <alexander.stein@xxxxxxxxxxxxxxx>]

Hi all,

this series is an RFC for a general approach to solve the issue at [1]. While
a device specific property works as well, a more generic approach is preferred.
In short: When enabling a GPIO the actual ramp-up time might be (much) bigger
than what software usually assume, in my case >100ms. Adding a delay to each
driver is cumbersome.
Instead the (optional) ramp-up delay is added to each gpio_desc. The delays can
be specified per gpio-controller, similar to 'gpio-line-names'. Actually the
parsing code is almost a 1:1 copy of devprop_gpiochip_set_names(). Due to
(temporary) memory allocation, I opted for a separate function, there is code
duplication, but handling both properties in a single function seemed too
tedious, let alone the to be added ramp-down delays.

This feature could also be added as a callback in gpio_chip, but the callbacks
have to be added to each driver then. I would prefer a single one-fits-all
implementation and another indirection in the GPIO call chain.

Laurent suggest to add a GPIO delay node in DT. IMHO this increased the DT
complexity unnecessarily. But comments are welcome.

The following 3 patches are a proof-of-concept on my platform, consisting of:
Patch 1 is the proposed bindings and straight forward.
Patch 2 is the current implementation
Patch 3 is an actual usage example for specifying the delays

TODO:
1. Adding ramp-down delays (Just the inverse copy of ramp-up delay)
2. Should these delays take active low flags into account?
3. How to deal with setting multiple GPIOs at once?

I skipped 1. for now, because this is just a copy with ramp-up being replaced
with ramp-down.

I'm not that well versed in gpiolib code, so I'm not sure if I got all placed
where GPIOs are set. So patch 2 might be incomplete.

For now I skipped setting multiple GPIOs at once completely, so to get some
feedback on this approach. A possible solution is to check for the bigest delay
in the set and use that for all afterwards. But I'm not sure about the overhead
in this case.

I hope there is some feedback. While thinking about this issue appears to be
more widespread than I expected.

Best regards,
Alexander

[1] https://lore.kernel.org/all/20221209083339.3780776-1-alexander.stein@xxxxxxxxxxxxxxx/

Alexander Stein (3):
  dt-bindings: gpio: Add optional ramp-up delay property
  gpiolib: Add support for optional ramp-up delays
  arm64: dts: mba8mx: Add GPIO ramp-up delays

 .../devicetree/bindings/gpio/gpio.txt         | 22 +++++
 arch/arm64/boot/dts/freescale/mba8mx.dtsi     |  5 ++
 drivers/gpio/gpiolib.c                        | 80 +++++++++++++++++++
 drivers/gpio/gpiolib.h                        |  3 +
 4 files changed, 110 insertions(+)

-- 
2.34.1