On Sat, Dec 12, 2020 at 01:03:32AM +0100, Linus Walleij wrote: > On Thu, Dec 10, 2020 at 9:53 AM Johan Hovold <johan@xxxxxxxxxx> wrote: > > On Wed, Dec 09, 2020 at 05:25:32PM +0100, Linus Walleij wrote: > > > I just replied to that thread, but to summarize, you can't rely on > > having the sysfs code detect collisions since that will trigger a bunch > > of nasty warnings and backtraces. We also don't want the sysfs interface > > for a specific USB device to depend on probe order (only the first one > > plugged in gets to use the line names). And adding line names now could > > in fact be what breaks currently working scripts. > > Yes the sysfs ABI is very volatile and easy to break. > > As pointed out in the other reply, sysfs base GPIO number is all > wibbly-wobbly on anything hot-pluggable so in a way I feel it > is the right thing to disallow sysfs altogether on hotpluggable > devices. No, the gpio numbers will of course vary, but since gpiolib exports the base number for the chip, a scripts can easily determine the right gpio number as base + offset. Having probe order break that by sometimes exporting the line using it's name is what would be a problem. > > Just did a super quick check and it seems libgpiod still assumes a flat > > name space. For example, gpiod_chip_find_line() returns only the first > > line found that matches a name. Shouldn't be impossible to extend, but > > just want to make sure this flat namespace assumption hasn't been to > > heavily relied upon. > > The unique way to identify a GPIO is gpiochip instance (with > topology from sysfs) and then a line number on that chip. > This is done e.g. in the example tool > tools/gpio/gpio-hammer.c > > As you can see the tool doesn't use these line names. > > The line names are really like symbolic links or something. > But they are indeed in a flat namespace so we should try to > at least make them unique if it turns out people love to use > these. Not necessarily, they could be unique per chip as we're discussing here with respect to hotpluggable controllers. We just can't have it both ways. > As it is now system designers mostly use device tree to assign > line names and they try to make these unique because they don't > like the nasty warnings from gpiolib. > > If I google for the phrase "Detected name collision for GPIO name" > I just find the code, our discussions and some USB serial devices > warning about this so far. > > Maybe we should just make a patch to disallow it? That would make it impossible to provide name lines on hotpluggable controllers, which would be nice to support. Johan
