On Sat, Dec 13, 2014 at 12:28 AM, Arnd Bergmann <arnd@xxxxxxxx> wrote: > On Friday 12 December 2014 22:05:37 Alexandre Courbot wrote: >> On Fri, Dec 12, 2014 at 9:08 PM, Arnd Bergmann <arnd@xxxxxxxx> wrote: >> > On Thursday 11 December 2014 16:05:04 Ray Jui wrote: >> >> + >> >> +- linux,gpio-base: >> >> + Base GPIO number of this controller >> >> + >> >> >> > >> > We've NAK'ed properties like this multiple times before, and it >> > doesn't get any better this time. What are you trying to achieve >> > here? >> >> I am to blame for suggesting using this property to Ray, and I am >> fully aware that this has been rejected before, but look at what >> people came with recently to palliate the lack of control over the >> GPIO number space for DT platforms: >> >> http://www.spinics.net/lists/arm-kernel/msg384847.html >> https://lkml.org/lkml/2014/12/10/133 >> >> Right now GPIO numbering for platforms using DT is a very inconsistent >> process, subject to change by the simple action of adjusting the value >> of ARCH_NR_GPIOS (which we did recently, btw), adding a new GPIO >> controller, or changing the probe order of devices. For users of the >> integer or sysfs interfaces, this results in GPIO numbers that change, >> and drivers and/or user-space programs that behave incorrectly. >> Ironically, the only way to have consistent numbers is to use the old >> platform files, where you can specify the base number of a gpio_chip. >> >> DT is actually probably not such a bad place to provide consistency in >> GPIO numbering. It has a global vision of the system layout, including >> all GPIO controllers and the number of GPIOs they include, and thus >> can make informed decisions. It provides a consistent result >> regardless of probe order. And allowing it to assign GPIO bases to >> controllers will free us from the nonsensical dependency of some >> arbitrary upper-bound for GPIO numbers that ARCH_NR_GPIOS imposes on >> us. Also about ARCH_NR_GPIOS, the plan is to eventually remove it >> since we don't need it anymore after the removal of the global >> gpio_descs array. This will again interfere with the numbering of GPIO >> chips that do not have a base number provided. >> >> Note that I don't really like this, either - but the problem is the >> GPIO integer interface. Until everyone has upgraded to gpiod and we >> have a replacement for the current sysfs interface (this will take a >> while) we have to cope with this. This issue has been bothering users >> for years, so this time I'd like to try and solve it the less ugly >> way. If there is a better solution, of course I'm all for it. > > I think the scheme will fail if you ever get gpio controllers that are > not part of the DT: We have hotpluggable devices (PCI, USB, ...) that > are not represented in DT and that may also provide GPIOs for internal > uses. > > The current state of affairs is definitely problematic, but defining > the GPIO numbers in DT properties would only be a relative improvement, > not a solution, and I fear it would make it harder to change the kernel > to remove the gpio numbers eventually. You are absolutely right that this would be only a partial solution. However this is a situation where there is no absolute fix (besides dropping the GPIO numbers completely) and the relief this property would brings makes it up for its shortcomings IMHO. > I wonder if we could instead come up with an approach that completely > randomizes the gpio numbers (as a compile-time option) to find any > places that still rely on specific numbers. A.k.a. Linus and Alex' hate mail generator. :P Actually we are not that far from being able to do completely without any GPIO number, and maybe that's what we should aim for. I think the only remaining offender is the sysfs interface. If we could reach GPIO controllers through a fixed path and just export their GPIOs there, I believe we would have fixed the whole issue. -- To unsubscribe from this list: send the line "unsubscribe devicetree" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
