linux-kernel - Re: [PATCH] gpio: sysfs: Try numbered exports if symbolic names fail

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Message-ID: <X9crbWwMiCC9ePLW@localhost>
Date:   Mon, 14 Dec 2020 10:07:57 +0100
From:   Johan Hovold <johan@...nel.org>
To:     Linus Walleij <linus.walleij@...aro.org>
Cc:     Johan Hovold <johan@...nel.org>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>,
        Manivannan Sadhasivam <mani@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] gpio: sysfs: Try numbered exports if symbolic names fail

On Sat, Dec 12, 2020 at 12:41:50AM +0100, Linus Walleij wrote:
> On Thu, Dec 10, 2020 at 9:33 AM Johan Hovold <johan@...nel.org> wrote:
> 
> > I suggested having the driver set a flag which determines whether to use
> > the line names in sysfs or not.
> 
> Aha I get it.
> 
> I need to think about if I can fix that in some good way.
> 
> > The above will trigger a bunch of nasty warnings and backtraces in the
> > sysfs code (for every gpio line!), which is not something we want for
> > normal operation.
> 
> At this point I feel any use of sysfs kind of deserves that but OK
> it's a bit nasty.

It would be a bug in gpiolib as the warnings are due to collisions when
trying to register two sysfs files using the same path.

> > Having the sysfs interface for the same USB device
> > depend on probe order is not very nice either.
> 
> The sysfs for a USB device is already very dependent on probe order.
> Since all dynamic gpio_chips pass -1 as base they will be allocated
> some global GPIO numbers at random (well, semi-random)
> depending on probe order.
> 
> The user will not have any idea whatsoever what to echo into the sysfs
> export file without inspecting other things such as debugfs.
> That's how unstable this interface is, and one of the reasons we
> are trying to get rid of the global GPIO numberspace to begin with...

That's not true. The gpiochip has a "base" sysfs attribute from which
any user can construct the gpio number by simply adding an offset.

Having the lines sometimes show up using their names and sometimes
using this number, is what would be a problem.

> Maybe that is actually an argument for any multi-instance GPIO
> devices to
> depends on !GPIO_SYSFS

No, not at all. Just suppress the rename of the sysfs directories based
on line names, which was a bad idea from the start (the names should
have been exported through a separate name attribute).

Johan