Hi Stephen, On Fri, Oct 04, 2019 at 01:07:23PM -0600, Stephen Warren wrote: [..] > I think the DT should represent the device that's attached to the GPIOs. > That way, there's already a clear way to represent the GPIO polarity, as > described in the document linked by Eugenui in [2] below. > > If for some reason that's not possible, then I think keeping track of the > GPIO polarity in user-space is entirely reasonable, and is the correct > approach. To claim that tracking GPIO polarity in user-space is too much > burden, yet to also allow user-space to control GPIOs at all, and hence to > know exactly which GPIOs must be controlled, is an inconsistent assertion. > > Put another way: If a piece of user-space SW controls GPIOs, it must know > which GPIO number to use for each logical purpose. This information > presumably varies on different platforms, so the SW must have a list of GPIO > numbers and GPIO controller IDs per platform. Additionally storing a > polarity bit along with that information seems entirely trivial to me. > > Is there some other issue that I'm overlooking? Based on the discussions so far, the user who requested this feature intends to (in fact already does) "mark" the userspace-relevant gpio lines via the "gpio-line-names" [5] DT property, implemented by Linus in v4.7 commit [6]. By keeping track of "gpio line name" both in DT and in user-space, apparently the user is able to accurately map the "line name" (visible in userspace) to the corresponding gpio chip/name and gpio line offset in a "platform/board-independent" way. Do you think this is unorthodox? > If the list of GPIO IDs is retrieved from DT by the user-space SW, I could > see an argument for storing the polarity information in DT along with that > list of GPIO IDs. However, I don't believe there's any standard way of > representing "a list of GPIO IDs for user space use" in DT. > > > If we hog a GPIO pin in DTS (which allows specifying its polarity), > > userspace no longer has access to that pin. There isn't a way to define > > GPIO polarity by means of DTS without affecting userspace access > > (can anybody contradict this statement?). > > GPIO hog doesn't seem like the right approach; its intent is to actively > configure the GPIO in a fixed state, which is logically incompatible with > user-space control of the GPIO. Agreed. Thanks for strengthening the idea behind hogging the gpios. [..] > > [1] https://marc.info/?l=linux-gpio&m=139204273132477&w=4 ("Correct meaning of the GPIO active low flag") > > [2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=51e8afc1c43c75 ("gpio: document polarity flag best practices") > > [3] https://marc.info/?l=linux-gpio&m=155721267517644&w=2 ("[PATCH V1 1/2] gpio: make it possible to set active-state on GPIO lines") > > [4] https://marc.info/?l=linux-gpio&m=155713157122847&w=2 ("[PATCH V1 1/2] gpio: make it possible to set active-state on GPIO lines") [5] https://marc.info/?l=linux-gpio&m=155712945922102&w=2 [6] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=fd9c55315db9 ("gpio: of: make it possible to name GPIO lines") -- Best Regards, Eugeniu
