linux-kernel - Re: [lm-sensors] [PATCH] hwmon class driver registration with a device number

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Date:	Thu, 6 Oct 2011 16:46:00 +0100
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Himanshu Chauhan <hschauhan@...ltrace.org>
Cc:	Jean Delvare <khali@...ux-fr.org>,
	Guenter Roeck <guenter.roeck@...csson.com>,
	lm-sensors@...sensors.org, linux-kernel@...r.kernel.org
Subject: Re: [lm-sensors] [PATCH] hwmon class driver registration with a
 device number

> AFAIK, the current sysfs interface is to read standard attributes from
> the device. My reasoning is:

and any other attributes.

> 1. This will give hwmon devices to have their "dev" in their sysfs folder
>    as like other classes and in turn be visible in /dev by mdev or udev.

Well there is in theory no reason a device shouldn't do that, but most
sysfs devices don't need to so making it generic seems odd.

> 2. Implementing IOCTL if required. I know that kernel hackers believe
>    they are "backdoors" and "can't be done right" but sometimes there
>    isn't any get away with them. I don't know when their support will
>    be completely removed. Rather, I see that instead of deprecation,
>    new IOCTL vectors like compat_ioctl have been introduced.

Certain things need ioctl, in fact some problems are *very* hard to
solve any other way: transactional updates (changing a set of fields at
once all or none), and query/response data being two.
 
> 2. During probing, for each device, call "hwmon_device_register_numbered"
>    and let mdev create a /dev node for it. I don't say that this interface
>    be imposed on drivers. If they want they can still call "hwmon_device_register"
>    if they don't want to implement standard ioctl, read, write calls.

I guess I don't see why a device that is more than just a monitoring
interface can't allocate a misc device or similar if it needs one.

For the more complex cases take a look at drivers/staging/iio also.
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