linux-kernel - Re: [RFC PATCH] gpiolib: Provide and export gpiod_export

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Message-ID: <20140829173746.GA14028@roeck-us.net>
Date:	Fri, 29 Aug 2014 10:37:46 -0700
From:	Guenter Roeck <linux@...ck-us.net>
To:	Alexandre Courbot <gnurou@...il.com>
Cc:	Linus Walleij <linus.walleij@...aro.org>,
	"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>,
	Randy Dunlap <rdunlap@...radead.org>
Subject: Re: [RFC PATCH] gpiolib: Provide and export gpiod_export_name

On Fri, Aug 29, 2014 at 10:00:47AM -0700, Alexandre Courbot wrote:
> On Thu, Aug 28, 2014 at 10:54 PM, Guenter Roeck <linux@...ck-us.net> wrote:
> > On 08/28/2014 10:44 PM, Linus Walleij wrote:
> >>
> >> On Mon, Aug 11, 2014 at 5:20 PM, Alexandre Courbot <gnurou@...il.com>
> >> wrote:
> >>>
> >>> On Tue, Aug 12, 2014 at 12:15 AM, Guenter Roeck <linux@...ck-us.net>
> >>> wrote:
> >>
> >>
> >>>> This is just one of many patches which would make it possible to submit
> >>>> the rest, which would make use of it. What you are saying is that it
> >>>> won't
> >>>> make sense to submit that series into the kernel, because one of the
> >>>> very
> >>>> first patches needed to enable that won't be accepted. Kind of a
> >>>> circular
> >>>> argument, but I guess I'll have to live with it.
> >>>
> >>>
> >>> Well I have not seen the other patches you mention and cannot guess
> >>> their existence. If you send the full series it will of course be
> >>> considered as such, but right now this lone patch does not hint any
> >>> upstream user for this interface.
> >>>
> >>> Note that this doesn't change anything to the core of the argument ;
> >>> we have not heard what Linus thinks about named GPIOs in
> >>> /sys/class/gpio yet, maybe he will have a different opinion...
> >>
> >>
> >> The sysfs is sort of broken by design because of things like this and
> >> some other stuff.
> >>
> >> I think the sysfs is scary, for example since it's not hierarchical
> >> but flat and build on the assumption that there is one single
> >> GPIO numberspace. As pointed out in some other message
> >> in the thread it would be nicer to have:
> >>
> >> /dev/gpiochip0/gpio0
> >> /dev/gpiochip0/gpio1
> >> ....
> >>
> >> instead of the horrid sysfs ABI that will have to maintain forever.
> >>
> > Blocking any attempts to make it more useful doesn't help much, though.
> 
> This patch is not making it more useful. It just introduces an
> inferior way to do something that is already possible.
> 
> I have stated it countless times already, but again:
> "/sys/bus/.../device/gpio_function" is better than
> "/sys/class/gpio/device_function_foo_whatnot" (and actually,
> "/sys/bus/.../device/gpios/function" would be even better).
> 
> You can already do the former. I just don't see the need to introduce
> an API to do the latter.
> 
> Why is the former better? Because it uses the sysfs hierarchy to make
> the GPIO visible under their consumer's node. That's how sysfs is
> intended to be used.
> 
> Just explain me why you cannot live with this or why your proposal is
> better, and my concerns about this patch will be lifted.
> 

"device" is a platform driver and thus platform specific, which defeats
the purpose of having a well defined path and makes it even more difficult
to find a pin across multiple platform variants (which will have different
platform drivers). It means user space will still need platform specific
configuration data to find the pin. If user space needs such configuration
data anyway, it may as well be a "<gpio chip, pin>" tuple instead of
"<platform driver path, gpio pin name>". Even worse, in one of my use
cases some of the platform drivers may have multiple instances with dynamic
instance numbers, meaning I don't even have a well defined path to start with.

On the other side, as it turns out, most of the gpio chips I deal with are
company specific, and you can not prevent me from populating the 'names'
array for those. It is a bit on the clumsy side, as I would prefer to have
the consumer select the name instead of the provider, but it works. For the
other drivers (so far only one) I only need an out-of-tree patch with a
couple of lines of code to be able to populate the 'names' field. So unless
you take the 'names' field away, which would break the existing ABI and is
thus at least somewhat unlikely, I can still do what I need to do.

Guenter
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