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Subject: Re: [v7,03/10] docs: Add Generic Counter interface documentation
To:     William Breathitt Gray <vilhelm.gray@gmail.com>,
        gregkh@linuxfoundation.org
Cc:     jic23@kernel.org, linux-arm-kernel@lists.infradead.org,
        devicetree@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-iio@vger.kernel.org, fabrice.gasnier@st.com,
        benjamin.gaignard@st.com, robh+dt@kernel.org, knaack.h@gmx.de,
        lars@metafoo.de, pmeerw@pmeerw.net, mark.rutland@arm.com
References: <7606bdc53c26c332b2bbff0f865380fb0e874b56.1529607879.git.vilhelm.gray@gmail.com>
From:   David Lechner <david@lechnology.com>
Message-ID: <b7c690eb-274e-3377-cf8a-2f9ab836677e@lechnology.com>
Date:   Mon, 2 Jul 2018 14:37:53 -0500
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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