Hi Alexander, On Tue, Dec 13, 2022 at 08:49:06AM +0100, Alexander Stein wrote: > Am Montag, 12. Dezember 2022, 13:40:50 CET schrieb Laurent Pinchart: > > 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. > > That's a good idea. I would even go a step further to specify both ramp-up and > ramp-down in one cell, e.g. <gpio-number ramp-up ramp-down>. This way a second > property is not needed. > > > > 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 ? > > Given the name ramp-up (& ramp-down) I would assume they affect the voltage > low -> high change (resp. high -> low), not just gpiod_set_value(..., 1). Good point. They should indeed. > > > 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. > > Okay, that seems sensible. Will do it. > > > > 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
