Hi Rob, On Tue, Dec 13, 2022 at 08:20:57AM -0600, Rob Herring wrote: > On Mon, Dec 12, 2022 at 4:35 AM Alexander Stein wrote: > > > > 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. > > At least for DT, I think this belongs (if at all) in the consumers, > rather than a producer property. The options there are > 'foo-gpios-ramp-us' for 'foo-gpios' or add some delay bits to GPIO > flags. My main requirement is to handle these properties in a central place, without having to patch individual drivers, so this would work for me. > We already have some of the former for various 'generic' power > sequencing related delays. Of course, there's no real pattern to them > as they all get added as we go without much foresight. In this case > even, there are 4 possible delays: pre and post ramp up and down. > > > 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. > > Many/most GPIO controllers can read the actual state of an output > (IIRC, i.MX ctrlr can). Perhaps that capability could be used to delay > until the state of the signal matches the set state. And you'd > probably want to measure how long that took and then add some more > time based on it. This of course gets into the electricals of at what > levels a low or high state will register. If you can't read the state, > then you would be stuck with some maximum timeout. -- Regards, Laurent Pinchart
