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

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Date:   Sun, 31 May 2020 09:31:47 -0400
From:   William Breathitt Gray <vilhelm.gray@...il.com>
To:     Pavel Machek <pavel@....cz>
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 Fri, May 29, 2020 at 03:26:04PM +0200, Pavel Machek wrote:
> 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

Yes, I don't think this is a good interface afterall. I'm implementing a
different design for v3 that should be more intuitive. The input
subsystem could be useful for streams of events such as timestamps, so
I'll take a look at it as well in case something similar to it could be
used.

William Breathitt Gray

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