Date:	Thu, 6 Oct 2011 09:43:02 +0200
From:	Jean Delvare <khali@...ux-fr.org>
To:	Guenter Roeck <guenter.roeck@...csson.com>
Cc:	Himanshu Chauhan <hschauhan@...ltrace.org>,
	lm-sensors@...sensors.org, linux-kernel@...r.kernel.org
Subject: Re: [lm-sensors] [PATCH] hwmon class driver registration with a 
 device number

I am purposely removing kernelnewbies from the CC list, as it seems
quite irrelevant for this discussion.

On Wed, 5 Oct 2011 22:19:55 -0700, Guenter Roeck wrote:
> On Thu, Oct 06, 2011 at 12:06:59AM -0400, Himanshu Chauhan wrote:
> > Hi,
> > 
> > > I can not comment on the merits of your patch. Unless I am missing
> > > something, which may well be since I only spent a couple of minutes on
> > > it, other device classes don't seem to provide a similar API, so I don't
> > > know if or why it would make sense for hwmon. Maybe a driver which wants
> > > to register a character device interface should do so independently of
> > > hwmon.
> > > 
> > 
> > The idea here is to sit in the same class directory as of hwmon. Devices
> > registered with this interface will have "dev" under, for example,
> > /sys/class/hwmon/hwmon0/dev. To do the same inside the driver will be
> > a bit more involved than a call.
> > 
> > In my opinion other classes should also have similar interfaces.
> > 
> I think you'll have to spend some more time and effort explaining the "what for".
> 
> Apparently no other device class needs this functionality so far, yet you
> suggest that such an interface should exist for all device classes.

Actually a lot of class devices do have a device node:
$ ls -1 /sys/class/*/*/dev | wc -l
252
This includes block, drm, dvb, input, msr, sound and tty class devices,
to name just a few. But this isn't the problem. All these are
documented in Documentation/devices.txt, and have a properly defined
API. There is no such thing for hwmon devices at this point.

> But you do so without explanation, or in other words without use case.

This is indeed the key point. Creating a device node is pointless
without a standard API to make use of it. So Himanshu will have to
document that first.

> I for my part have no idea what you would use or need this new interface for,
> and if there would be other less intrusive means to accomplish the same goal.
> And I would want to see really good reasons to make a change like this.

Another good point.

> Specifically looking at the hwmon subsystem, you are expected to use the lm-sensors
> library to access all hwmon attributes. So I would expect your explanation to include
> exactly what you want to accomplish and why, details why you believe that you can not
> use the lm-sensors library, why you believe that the current infrastructure
> does not provide the means you need to accomplish your goals, and why you
> think that the existing infrastructure can not be modified to let you accomplish
> what you want to do without such a - from a conceptual perspective - substantial change.

I again agree. Which means that Himanshu is still 3 steps away from
getting his patch accepted:
* Explaining why the current sysfs interface is insufficient and can't
  be fixed.
* Getting official device node numbers registered for hwmon use.
* Defining an API for these device nodes.

Before then, there's no point in resending this patch, it will be
rejected.

-- 
Jean Delvare
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