Message-ID: <53E32C09.5000704@roeck-us.net>
Date:	Thu, 07 Aug 2014 00:34:33 -0700
From:	Guenter Roeck <linux@...ck-us.net>
To:	Alexandre Courbot <gnurou@...il.com>
CC:	"linux-gpio@...r.kernel.org" <linux-gpio@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	Linus Walleij <linus.walleij@...aro.org>,
	Randy Dunlap <rdunlap@...radead.org>
Subject: Re: [RFC PATCH] gpiolib: Provide and export gpiod_export_name

On 08/06/2014 11:25 PM, Alexandre Courbot wrote:
> Sorry for the delayed reply...
>
> On Thu, Jul 24, 2014 at 3:33 PM, Guenter Roeck <linux@...ck-us.net> wrote:
>>> As I explained on the other thread, I still encourage you to have
>>> these GPIOs under device nodes that give a hint of who is provided the
>>> GPIO (effectively exporting the (dev, function) tuple to user-space)
>>> instead of having them popping out under /sys/class/gpio where nobody
>>> knows where they come from and name collisions are much more likely.
>>>
>>> Your message sounds like you have no choice but have the named GPIO
>>> link under the gpiochip's device node, but this is not the case -
>>> gpio_export_link() let's you specify the device under which the link
>>> should appear. Make that device be your "scu" device and you can have
>>> a consistent sysfs path to access your GPIOs.
>>>
>> Yes, I understand, but that is not acceptable for the users - see below.
>>
>>
>>> Allowing GPIOs to pop up in the same directory with an arbitrary name
>>> is just a recipe for a mess. But that's a recipe that is already
>>> allowed thanks to that 'names' array. So I'm really confused about
>>> what to do with this patch. If you can do with gpio_export_link() (and
>>> I have not seen evidence of the contrary), please go that way instead.
>>>
>>
>> I personally don't think it is that much of a mess. Sure, once has to be
>> careful when selecting names, but I don't see a problem with that.
>>
>> I have two users for this. Interestingly the problem is pretty
>> much the same, though the applications are completely different.
>>
>> One is the company using the scu.c file. They are currently using the
>> pca953x driver approach (using the names array), but they also have
>> a new version of their product which also uses gpio pins from gpio-ich.
>> For consistency, they want all gpio pins in the same directory, meaning
>> /sys/class/gpio.
>>
>> The currently implemented solution is to have a weak pointer to the names
>> array in gpio-ch.c and override it with a pointer from scu.c.
>>
>> /* SCU specific gpio pin names. Only works if module is built into kernel.
>> */
>> static const char * const scu_ichx_gpiolib_names[128] = {
>>          [0] = "switch_interrupt",       /* GPI0 */
>>          [3] = "ac_loss_detect",         /* GPI3 */
>>          [16] = "debug_out",             /* GPO0 */
>>          [20] = "switch_reset",          /* GPO3 */
>> };
>>
>> [ switch_interrupt and switch_reset will ultimately make it into the kernel,
>>    to be used by a dsa driver, but that is besides the point. ]
>>
>> const char * const (* const ichx_gpiolib_names)[] = &scu_ichx_gpiolib_names;
>>
>> and ichx_gpiolib_names is declared as
>>
>> /* Allow overriding default gpio pin names */
>> const char * const (* const __weak ichx_gpiolib_names)[];
>>
>> in gpio-ich.c. Pretty hackish, but at least it works. Slim chance though
>> to get this accepted upstream. Since the ultimate idea is to submit all
>> this code upstream, I would prefer to find a solution which is
>> acceptable for both the user and for upstream integration.
>>
>> The second user is my employer. Same thing here, though even more complex
>> as there are several different platforms to support with different platform
>> drivers. Each platform exports a number of gpio pins to user space, often
>> with
>> the same functionality across platforms. The request here is to have all
>> those pins in the same directory, for all platforms, which again suggests
>> using /sys/class/gpio.
>>
>> Current approach is to use the "export" file to request pin exports,
>> which has its own challenges such as having to search for the pin numbers.
>> Well defined names and pins exported from the kernel would be much cleaner
>> and be easier to handle.
>>
>> Of course, I could try to come up with a new class named "gpios" or similar,
>> put everything there, and start selling that idea. Somehow that doesn't
>> sound like a good idea to me.
>
> Or you could have a platform device which sole purpose is to request
> the GPIOs to export and export them using gpio_export() and
> gpio_export_link() using itself as the device parameters. All your
> links would then appear under the same sysfs directory, and that
> directory name would be consistent across platforms. What is wrong
> with this approach?
>
The link does not appear in /sys/class/gpio. I understand you don't like the
idea of having named gpio pins in that directory, but that is supported today,
it works, and others do like it.

> As for the patch itself, as I said before I am not a huge fan of
> putting randomly-named exports under /sys/class/gpio, but since there
> is a precedent with the driver GPIO names array I don't think this
> makes the situation much worse. Still I'd like you to explain me what
> I missed and why you cannot use the technique described above to
> achieve your goal with the currently existing interfaces.
>

I thought I explained it before; my users (ie the people writing user space
applications accessing the pins) expect to see the exported pins in /sys/class/gpio
and not in a more or less arbitrary directory. They are used to this
approach, and they are less than enthusiastic to change it. The code needed
to generate the exported pin names is a bit messy, it being tied to the driver,
but it does both exist and work. This patch was an attempt to provide a
cleaner API to accomplish the same without having to touch various gpio
drivers which don't provide the ability to configure the names array.

Note that I don't consider the names "random". They are much less random
than gpioX, where X can change each time the system boots or, for OIR
capable systems, each time a gpio driver is instantiated or removed.
In today's system, without well defined names, one never knows if gpioX points
to the pin the user is looking for. If the pin is named "this-is-your-pin"
it is much easier to write user space code using it.

Oddly enough, if I would use the platform driver approach you suggest,
I would still need a well defined directory, say, /sys/class/named_gpios,
and I would still have to handle the potential situation that some genius
tries to export two different pins with the same name, and handle
the resulting failure. I would still need an API to tell that driver to export
a GPIO pin under a specific name. I don't really see how that would be
different to providing that API in the gpio subsystem, and to using
/sys/class/gpio to start with and just fail the export if a duplicate
name is specified ... which happens to be exactly what is done today
already. On top of that, this would result in two different means
to export gpio pins to user space, one triggered through the gpio driver
and one triggered from outside through a new driver and API.
I can not imagine that anyone would be happy about that.
So, quite frankly, I don't entirely understand your objection.

Of course, thinking about it, maybe I should just export the gpio_class
variable in my kernel tree. Looks like I'll have to maintain local kernel
changes anyway, and it seems to be easier to export gpio_class and use a
symlink to gpio_class than to use driver based names, per driver hacks,
or a new class / driver. Hmmm ... I'll start looking into that.
A two-line change is much easier to maintain for me than all those
other options. I should have had that idea before. And I guess people
can live with symlinks from /sys/class/gpio/this-is-your-pin to
/sys/class/gpio/gpioX.

Guenter

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