linux-kernel - Re: Creating dev nodes from driver or via uevent?

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Message-ID: <ac3eb2510810150145q4ee66c17xde59d966053d6e7f@mail.gmail.com>
Date:	Wed, 15 Oct 2008 10:45:10 +0200
From:	"Kay Sievers" <kay.sievers@...y.org>
To:	linux-kernel@...r.kernel.org
Subject: Re: Creating dev nodes from driver or via uevent?

On Wed, Oct 15, 2008 at 8:39 AM, Fabiano Sidler <lists@...scode.ch> wrote:
> Probably a very basic question, but is it better to directly mknod from a
> module's init funtion or to use the uevent/udev mechanism?

You better do not touch any userspace managed filesystem from inside
the kernel. No driver is doing that.

> If the latter,
> are there good overviews, explanations and comprehensive code samples
> especially for using udev, as well as how to package the userspace program
> creating the nodes along with a driver?

You do not package anything along with the driver, unless you have
_very_ special requirements for userspace device setup. Then you could
ship a udev rules file.

There is nothing special to do, just run udev, and register a "struct
device" inside the kernel's driver core, and you will see a device
node in /dev/ with the name of your kernel device, and the major/minor
you assigned to the struct device before registration.

Almost every Linux distro, and every driver in the kernel works that
way. I suggest looking at the current kernel code and running a Linux
distro and loading/unloading an existing module, to see it working.

You can also run "/sbin/udevadm monitor --env --kernel" to see the
"raw" kernel events "struct device" registration causes, and which
lets userspace create the device nodes.

Kay
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