Re: replace dev.d/ with a rule based program execution

[Kay Sievers <kay.sievers@vrfy.org>]

On Fri, 2005-02-25 at 15:26 -0800, Greg KH wrote:
>On Mon, Feb 21, 2005 at 07:12:42PM +0100, Kay Sievers wrote:
>> Here is an experimental patch to replace the brute-force dev.d/ script
>> execution by a rule based model, with the same logic we currently use to
>> name a device. While searching for a rule to apply, we collect programs
>> to execute after node creation/removal.
>> 
>> This makes it possible to gain complete control of the execution of programs
>> for a specific device instead of letting the programs exit if they don't
>> want to handle the device.
>> 
>> We apply the rule on device remove events too. A ACTION="<value>" match can
>> be used to write rules that are only applied on a specific action.
>> 
>> I've replaced the following:
>>   [kay@pim ~]$ tree /etc/dev.d/
>>   /etc/dev.d/
>>   |-- default
>>   |   |-- 00-log.dev
>>   |   |-- 05-pam_console.dev -> ../../udev/scripts/pam_console.dev
>>   |   `-- 10-hal.dev -> /usr/libexec/hal.dev
>>   |-- fd0
>>   |   `-- 10-MAKEDEV.dev -> ../../udev/scripts/MAKEDEV.dev
>>   |-- fd1
>>   |   `-- 10-MAKEDEV.dev -> ../../udev/scripts/MAKEDEV.dev
>>   |-- fd2
>>   |   `-- 10-MAKEDEV.dev -> ../../udev/scripts/MAKEDEV.dev
>>   |-- fd3
>>   |   `-- 10-MAKEDEV.dev -> ../../udev/scripts/MAKEDEV.dev
>>   `-- sound
>>       `-- alsa.dev
>> 
>> With these rules:
>>   KERNEL="ttyUSB*", HOTPLUG="<usb-serial-device program>"
>>   SUBSYSTEM="tty", OPTIONS="no_hotplug"
>>   SUBSYSTEM="vc", OPTIONS="no_hotplug"
>> 
>>   ACTION="add", SUBSYSTEM="sound", HOTPLUG="/etc/dev.d/sound/alsa.dev"
>>   ACTION="add", KERNEL="fd*", HOTPLUG="/etc/udev/scripts/MAKEDEV.dev"
>>   ACTION="add", HOTPLUG="/etc/udev/scripts/pam_console.dev"
>> 
>>   HOTPLUG="/usr/libexec/hal.dev"
>>   HOTPLUG="/etc/dev.d/default/00-log.dev"
>> 
>> On my box, udevstart takes 2.1 seconds instead of 8.6 seconds to run with
>> my default setup. Mainly because it will not run anything for all the virtual
>> tty devices. Any tty device which should be catched, needs a rule before the
>> "no_hotplug" option.
>> 
>> It is also possible to pass arguments to the scripts, which sometimes
>> makes it no longer necessary to wrap a program with a shell script.
>
>I like the general idea, but we can't break the /etc/dev.d/ functionalty
>so much.
>
>If we just add a udev option (like you show above) for "no_dev.d", we
>could still have the massive udevstart speedups, right?  So this is ok
>with me.

We should plug that in with a external program called by a rule.

>Remember, it's easier for a package to drop a symlink into the
>/etc/dev.d/ directory tree, than it is for it to modify / add a udev
>rule.

Packages should drop a single rules file instead.

>> We could also remove all the hardcoded knowledge about sysfs in the
>> wait_for_sysfs logic and replace it by a few simple ignore_device rules.
>
>How would that happen?

We match against the SUBSYSTEM and KERNEL and request a specific sysfs
value, which will wait until it appears. That is generic, nice,
configurable, elegant and users can tweak it for their needs.

>> The same model could easily replace the whole hotplug.d/ multiplexing and
>> give use an efficient rule based event management with a single source of
>> policy.
>
>See my previous response as to why we can't do this.

Just let the users decide. We would ship a backwards compatible default
setup, but the user can remove the multiplex emulation rule and has a
nice, clean rule-based solution without all that crap compiled in.

Thanks,
Kay



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