archive mirror
 help / color / mirror / Atom feed
From: Marc Zyngier <>
To: Johan Hovold <>
	Linus Walleij <>,
	Bartosz Golaszewski <>,
	Greg Kroah-Hartman <>,
Subject: Re: [PATCH 0/4] USB: ftdio_sio: GPIO validity fixes
Date: Mon, 07 Dec 2020 14:41:03 +0000	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <X841xwCChUEqi5Ad@localhost>

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, 
> 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.

Jazz is not dead. It just smells funny...

  reply	other threads:[~2020-12-07 14:42 UTC|newest]

Thread overview: 23+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-12-04 16:47 Marc Zyngier
2020-12-04 16:47 ` [PATCH 1/4] gpiolib: cdev: Flag invalid GPIOs as used Marc Zyngier
2020-12-07 14:16   ` Johan Hovold
2020-12-07 14:59     ` Marc Zyngier
2020-12-09  9:25   ` Linus Walleij
2020-12-04 16:47 ` [PATCH 2/4] USB: serial: ftdi_sio: Report the valid GPIO lines to gpiolib Marc Zyngier
2020-12-09  9:28   ` Linus Walleij
2020-12-04 16:47 ` [PATCH 3/4] USB: serial: ftdi_sio: Log the CBUS GPIO validity Marc Zyngier
2020-12-07 14:29   ` Johan Hovold
2020-12-07 15:00     ` Marc Zyngier
2020-12-07 15:19       ` Johan Hovold
2020-12-09  9:35         ` Linus Walleij
2020-12-09 17:05           ` Johan Hovold
2020-12-09 17:39             ` Johan Hovold
2020-12-04 16:47 ` [PATCH 4/4] USB: serial: ftdi_sio: Drop GPIO line checking dead code Marc Zyngier
2020-12-07  9:55 ` [PATCH 0/4] USB: ftdio_sio: GPIO validity fixes Andy Shevchenko
2020-12-07 14:01 ` Johan Hovold
2020-12-07 14:41   ` Marc Zyngier [this message]
2020-12-07 15:08     ` Johan Hovold
2020-12-07 15:34       ` Marc Zyngier
2020-12-07 15:49         ` Johan Hovold
2020-12-09  9:20           ` Linus Walleij
2020-12-09 15:42             ` Johan Hovold

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \ \ \ \ \ \ \ \ \ \ \
    --subject='Re: [PATCH 0/4] USB: ftdio_sio: GPIO validity fixes' \

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).