linux-kernel - Re: [v7,03/10] docs: Add Generic Counter interface documentation

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Date:   Wed, 4 Jul 2018 19:23:26 +0200
From:   Linus Walleij <linus.walleij@...aro.org>
To:     William Breathitt Gray <vilhelm.gray@...il.com>
Cc:     David Lechner <david@...hnology.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Jonathan Cameron <jic23@...nel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-iio@...r.kernel.org,
        Fabrice Gasnier <fabrice.gasnier@...com>,
        Benjamin Gaignard <benjamin.gaignard@...com>,
        Rob Herring <robh+dt@...nel.org>,
        Hartmut Knaack <knaack.h@....de>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Peter Meerwald <pmeerw@...erw.net>,
        Mark Rutland <mark.rutland@....com>
Subject: Re: [v7,03/10] docs: Add Generic Counter interface documentation

On Tue, Jul 3, 2018 at 4:16 PM William Breathitt Gray
<vilhelm.gray@...il.com> wrote:
> On Mon, Jul 02, 2018 at 02:37:53PM -0500, David Lechner wrote:
> >On 06/21/2018 04:07 PM, William Breathitt Gray wrote:
> >> +Userspace Interface
> >> +===================
> >> +
> >> +Several sysfs attributes are generated by the Generic Counter interface,
> >> +and reside under the /sys/bus/counter/devices/counterX directory, where
> >> +counterX refers to the respective counter device. Please see
> >> +Documentation/ABI/testing/sys-bus-counter-generic-sysfs for detailed
> >> +information on each Generic Counter interface sysfs attribute.
> >> +
> >> +Through these sysfs attributes, programs and scripts may interact with
> >> +the Generic Counter paradigm Counts, Signals, and Synapses of respective
> >> +counter devices.
> >> +
> >
> >Have you considered a character device in addition to the sysfs interface?
> >
> >I basically have many of the same concerns that resulted in a char dev for
> >gpio[1].
> >
> >- With sysfs, you *can* technically poll for events, but then you have to
> >   seek and read or re-open the file.
> >- File permissions are annoying if you want a non root user to be able to
> >   use the device.
> >- A single program can't claim exclusive access to a device.
> >- There is no automatic cleanup if a userspace program accessing the device
> >    crashes.
> >
> >[1]: https://www.elinux.org/images/7/74/Elce2017_new_GPIO_interface.pdf
>
> Those look like good technical reasons for implementing a character
> device for the Generic Counter interface. I chose to implement the sysfs
> interface because I was using the IIO code as a reference, but I
> personally don't have much opposition to a character device in addition.

IIO is also using a character device for outputting events and sensor
data. In IIO sysfs is only used for configuring what events and
data should come out of the character device.

> I'd like to get Jonathan's opinion on this as well if possible -- I
> vaguely recall us considering this option briefly last year when the
> Generic Counter interface was still in its beginnings. I've CC'd Linus
> Walleij as well for input as the GPIO maintainer.

The GPIO character device was based on the IIO character device.

We have one TODO item: to merge the timestamping format
between GPIO and IIO and use the same sysfs file for configuring
the time base.

Yours,
Linus Walleij