linux-kernel - Re: [PATCH 0/4] USB: ftdio

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Date:   Mon, 07 Dec 2020 14:41:03 +0000
From:   Marc Zyngier <maz@...nel.org>
To:     Johan Hovold <johan@...nel.org>
Cc:     linux-usb@...r.kernel.org, linux-gpio@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Linus Walleij <linus.walleij@...aro.org>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        kernel-team@...roid.com
Subject: Re: [PATCH 0/4] USB: ftdio_sio: GPIO validity fixes

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...