On Mon, Oct 24, 2011 at 03:26, Linus Walleij wrote: > On Sat, Oct 22, 2011 at 7:44 PM, Mike Frysinger wrote: >> On Tue, Oct 4, 2011 at 16:35, Grant Likely wrote: >>> On Sat, Oct 01, 2011 at 12:39:21PM +0200, Linus Walleij wrote: >>>> 2011/9/30 Grant Likely: >>>> > I'm not convinced that the sysfs approach is >>>> > actually the right interface here (I'm certainly not a fan of the gpio >>>> > sysfs i/f), and I'd rather not be putting in unneeded stuff until the >>>> > userspace i/f is hammered out. >>>> >>>> Actually, thinking about it I cannot see what would be wrong >>>> with /dev/gpio0 & friends in the first place. >>>> >>>> Using sysfs as swiss army knife for custom I/O does not >>>> seem like it would be long-term viable so thanks for this >>>> observation, and I think we need /dev/gpio* put on some >>>> mental roadmap somewhere. >>> >>> Agreed. I don't want to be in the situation we are now with GPIO, >>> where every time I look at the sysfs interface I shudder. >> >> the problem with that is it doesn't scale. if i have a device with >> over 150 GPIOs on the SoC itself (obviously GPIO expanders can make >> that much bigger), i don't want to see 150+ device nodes in /dev/. >> that's a pretty big waste. sysfs only allocates/frees resources when >> userspace actually wants to utilize a GPIO. > > I was more thinking along the lines of one device per GPIO controller, > then you ioctl() to ask /dev/gpio0 how many pins it has or so.
that brings its own set of trade offs. this might be OK from a debugging point of view, but it means security wise we have to grant access on a per-gpiochip basis instead of a per-gpio basis. i think the sysfs interface has this granularity of support already as the root user can chmod/chown the files after exporting them. Grant suggested we extend UIO to export GPIOs. this would be a good trade off i think -- sysfs is a good on-the-fly debugging/scripting interface, but UIO gets us the performance. sysfs overhead can be mitigated by using pwrite/pread, but without pwritev/preadv, we're stuck with 1-transition-per-syscall. -mike _______________________________________________ linaro-dev mailing list linaro-dev@lists.linaro.org http://lists.linaro.org/mailman/listinfo/linaro-dev
