On Tue, Jul 5, 2016 at 8:04 PM, Uwe Kleine-König <u.kleine-koenig@xxxxxxxxxxxxxx> wrote: > [Rob]: >> Like SPI and I2C, I'm against further abuse of aliases for this purpose >> [1]. So what about the usecase for serial ports, where we use this to make sure the console come out where we want it? Is that also considered abuse or legitimate use? Note: I'm not trying to be snarky, I'm trying to understand what is the right and wrong use of alias. I'm confused about it right now :( >> Why is deterministic numbering needed? This is the big question, we all agree on that. > in my case (with a pre 4.8 kernel) it's to control GPIO48 with > /sys/class/gpio/gpio48. That is understandable. However as many people have pointed out, if this is inherently broken since the introduction of off-chip devices and even more by deferred probing. For some odd silicon state making one GPIO's clock or something randomly defer the probe, this would happen to become unpredictable also on SoC-embedded GPIO chips. It's just that we don't see it very much because it is deterministic in practice. We need to face it: relying on fixed GPIO numbering is just a big fragile mess that we're trying to duct-tape. > But also when using the gpio chardev device > (that will hit 4.8-rc1 AFAIK) there is one device file per gpio chip. Yeah and the argument is that the numbering also of the chip instance should be made more deterministic, so gpiochip0, gpiochip1 ... gpiochipN are intuitive to a human user. > root@hostname:/sys/bus/gpio/devices ls -l > lrwxrwxrwx 1 root root 0 Jul 5 20:52 gpiochip0 -> ../../../devices/platform/soc/53f00000.aips/53f9c000.gpio/gpiochip0 > lrwxrwxrwx 1 root root 0 Jul 5 20:52 gpiochip1 -> ../../../devices/platform/soc/53f00000.aips/53fa4000.gpio/gpiochip1 > lrwxrwxrwx 1 root root 0 Jul 5 20:52 gpiochip2 -> ../../../devices/platform/soc/53f00000.aips/53fcc000.gpio/gpiochip2 > lrwxrwxrwx 1 root root 0 Jul 5 20:52 gpiochip3 -> ../../../devices/platform/soc/53f00000.aips/53fd0000.gpio/gpiochip3 > > That is we have: > > Hardware name | software gpiochip > GPIO4 | gpiochip0 > GPIO3 | gpiochip1 > GPIO1 | gpiochip2 > GPIO2 | gpiochip3 > > I bet that's the probe order because when sorted by address (and so > by order in the device tree) we have exactly this ordering. (Compare > with $(grep gpio@ arch/arm/boot/dts/imx25.dtsi).) That is super-unintuitive for a human user, I agree 100%. > For a new interface this is OK, still I predict users will complain if > the numbers used don't match naturally the hardware names. And IMHO they > are right. I agree, and I think that if aliases can alleviate the situation we should allow them and encourage them. They are the best duct-tape we can find for the DT systems. Unfortunately it is not my call, because DT bindings and alias use is not under my jurisdiction. I *THINK* the view of the device core maintainers is that udev and sysfs hierarchies should be used to uniquely identify a certain device, and that relying on device numbering is too fragile. The network and disks people faced this problem before us, and their solution was things like using sysfs/udev to make sure the device get presented the right way to userspace and handled the way it should. Yours, Linus Walleij -- 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
