Hi Alexander, On Mon, Dec 12, 2022 at 11:35:22AM +0100, Alexander Stein wrote: > Hi all, > > this series is an RFC for a general approach to solve the issue at [1]. While I'm impressed by how fast you came up with a solution :-) > 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(). While I like consistency, I wonder if it wouldn't be better in this case to use a list of <gpio-number delay> cells in gpio-ramp-up-delays-us. In typical use cases, very few GPIOs will need a delay, and a GPIO controller could support a very large number of GPIOs, which would make your current proposal cumbersome. > 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. Agreed. > Laurent suggest to add a GPIO delay node in DT. IMHO this increased the DT > complexity unnecessarily. But comments are welcome. It's an alternative approach that could be considered if this one is rejected, but I have a preference for your solution. > 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? How so ? > 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 assume you're talking about the gpiod_set_array_value() API. That sounds OK as an initial implementation, a caller of that function needs to be prepared for the GPIOs being set in a random order due to hardware delays, so it shouldn't break the API contract. I would however state this explicitly in the function documentation. > 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(+) -- Regards, Laurent Pinchart
