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From: Johan Hovold <johan@kernel.org>
To: Marc Zyngier <maz@kernel.org>
Cc: linux-usb@vger.kernel.org, linux-gpio@vger.kernel.org,
	linux-kernel@vger.kernel.org,
	Linus Walleij <linus.walleij@linaro.org>,
	Bartosz Golaszewski <bgolaszewski@baylibre.com>,
	Johan Hovold <johan@kernel.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	kernel-team@android.com
Subject: Re: [PATCH 0/4] USB: ftdio_sio: GPIO validity fixes
Date: Mon, 7 Dec 2020 15:01:43 +0100	[thread overview]
Message-ID: <X841xwCChUEqi5Ad@localhost> (raw)
In-Reply-To: <20201204164739.781812-1-maz@kernel.org>

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

  parent reply	other threads:[~2020-12-07 14:02 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 [this message]
2020-12-07 14:41   ` Marc Zyngier
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

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