linux-kernel - Re: [PATCH v2 4/4] docs: counter: Document character device interface

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Date:   Fri, 29 May 2020 15:26:04 +0200
From:   Pavel Machek <pavel@....cz>
To:     William Breathitt Gray <vilhelm.gray@...il.com>
Cc:     jic23@...nel.org, kamel.bouhara@...tlin.com, gwendal@...omium.org,
        alexandre.belloni@...tlin.com, david@...hnology.com,
        linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-stm32@...md-mailman.stormreply.com,
        linux-arm-kernel@...ts.infradead.org, syednwaris@...il.com,
        patrick.havelange@...ensium.com, fabrice.gasnier@...com,
        mcoquelin.stm32@...il.com, alexandre.torgue@...com
Subject: Re: [PATCH v2 4/4] docs: counter: Document character device interface

On Sat 2020-05-16 15:20:02, William Breathitt Gray wrote:
> This patch adds high-level documentation about the Counter subsystem
> character device interface.
> 
> Signed-off-by: William Breathitt Gray <vilhelm.gray@...il.com>
> ---
>  Documentation/driver-api/generic-counter.rst | 112 +++++++++++++------
>  1 file changed, 76 insertions(+), 36 deletions(-)
> 
> diff --git a/Documentation/driver-api/generic-counter.rst b/Documentation/driver-api/generic-counter.rst
> index 8f85c30dea0b..58045b33b576 100644
> --- a/Documentation/driver-api/generic-counter.rst
> +++ b/Documentation/driver-api/generic-counter.rst

> +
> +Counter chrdev
> +--------------
> +Translates counter data to the standard Counter character device; data
> +is transferred via standard character device read/write calls.
> +
> +Sysfs Interface
> +===============
> +
> +Several sysfs attributes are generated by the Generic Counter interface,
> +and reside under the `/sys/bus/counter/devices/counterX` directory,
> +where `X` is to the respective counter device id. Please see
> +Documentation/ABI/testing/sysfs-bus-counter 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.
> +
> +Counter Character Device
> +========================
> +
> +Counter character device nodes are created under the `/dev` directory as
> +`counterX`, where `X` is the respective counter device id. Defines for
> +the standard Counter data types are exposed via the userspace
> +`include/uapi/linux/counter-types.h` file.
> +
> +The first 196095 bytes of the character device serve as a control
> +selection area where control exposure of desired Counter components and
> +extensions may be selected. Each byte serves as a boolean selection
> +indicator for a respective Counter component or extension. The format of
> +this area is as follows:
> +
> +* For each device extension, a byte is required.
> +* For each Signal, a byte is reserved for the Signal component, and a
> +  byte is reserved for each Signal extension.
> +* For each Count, a byte is reserved for the Count component, a byte is
> +  reserved for the count function, a byte is reserved for each Synapse
> +  action, and byte is reserved for each Count extension.
> +
> +The selected Counter components and extensions may then be interfaced
> +after the first 196095 bytes via standard character device read/write
> +operations. The number of bytes available for each component or
> +extension is dependent on their respective data type: u8 will have 1
> +byte available, u64 will have 8 bytes available, strings will have 64
> +bytes available, etc.

This looks like very, very strange interface, and not described in detail
required to understand it.

Could you take a look at input subsystem, /dev/input/event0? Perhaps it is 
directly usable, and if not something similar should probably be acceptable.

Best regards,
									Pavel