On Fri, Dec 04, 2020 at 04:47:35PM +0000, Marc Zyngier wrote: > Having recently tried to use the CBUS GPIOs that come thanks to the > ftdio_sio driver, it occurred to me that the driver has a couple of > usability issues: > > - it advertises potential GPIOs that are reserved to other uses (LED > control, or something else) Consider the alternative, that the gpio offsets (for CBUS0, CBUS1, CBUS2 or CBUS4) varies depending on how the pins have been muxed. Hardly very user friendly. > - it returns an odd error (-ENODEV), instead of the expected -EINVAL > when a line is unavailable, leading to a difficult diagnostic Hmm, maybe. Several gpio driver return -ENODEV when trying to request reserved pins. Even gpiolib returns -ENODEV when a pins is not yet available due to probe deferal. -EBUSY could also be an alternative, but that's used to indicate that a line is already in use as a gpio. > We address the issues in a number of ways: > > - Stop reporting invalid GPIO lines as valid to userspace. It > definitely seems odd to do so. Instead, report the line as being > used, making the userspace interface a bit more consistent. > > - Implement the init_valid_mask() callback in the ftdi_sio driver, > allowing it to report which lines are actually valid. > > - As suggested by Linus, give an indication to the user of why some of > the GPIO lines are unavailable, and point them to a useful tool > (once per boot). It is a bit sad that there next to no documentation > on how to use these CBUS pins. Don't be sad, Marc; write some documentation. ;) Johan
