Messages in this thread | | | Date | Mon, 07 Dec 2020 14:41:03 +0000 | From | Marc Zyngier <> | Subject | Re: [PATCH 0/4] USB: ftdio_sio: GPIO validity fixes |
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On 2020-12-07 14:01, Johan Hovold wrote: > 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.
That's not what I suggest. If you want fixed GPIO offsets, fine by me. But telling the user "these are GPIOs you can use", and then "on second though, you can't" is not exactly consistent.
>> - 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.
-ENODEV really means "no GPIOchip" in this context. The fact that other drivers return -ENODEV for reserved pins looks like a bug to me.
> -EBUSY could also be an alternative, but that's used to indicate that a > line is already in use as a gpio.
Or something else. Which is exactly the case, as it's been allocated to another function.
>> 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. ;)
I sure will, right after I have fixed the rest of the kernel bugs I have introduced. With a bit of luck, that's right after I finally kick the bucket.
M. -- Jazz is not dead. It just smells funny...
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