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Old 10-24-2011, 11:19 AM
Ivan Shmakov
 
Default udev: what does it used for in Debian?

I've found that a few packages, contrary to my expectations,
have Depends: on udev. I'm primarily concerned with alsa-base
and initramfs-tools, but also wonder about libcomedi0, dkopp,
python-expeyes, libnjb5, media-player-info, pulseaudio, ukopp,
xserver-xorg-core, and midisport-firmware.

The backstory is that I'm about to install Debian (either stable
or testing) on a tiny Architecture: i386 system, and consider
excluding udev from the installation, as the hardware in
question has virtually no support for any pluggable devices
whatsoever.

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Old 10-24-2011, 12:11 PM
Neil Williams
 
Default udev: what does it used for in Debian?

On Mon, 24 Oct 2011 18:19:08 +0700
Ivan Shmakov <ivan@gray.siamics.net> wrote:

> I've found that a few packages, contrary to my expectations,
> have Depends: on udev. I'm primarily concerned with alsa-base
> and initramfs-tools, but also wonder about libcomedi0, dkopp,
> python-expeyes, libnjb5, media-player-info, pulseaudio, ukopp,
> xserver-xorg-core, and midisport-firmware.
>
> The backstory is that I'm about to install Debian (either stable
> or testing) on a tiny Architecture: i386 system, and consider
> excluding udev from the installation, as the hardware in
> question has virtually no support for any pluggable devices
> whatsoever.

udev isn't just for pluggable devices, packages can provide udev rules
to ensure that devices appear with a consistent name,
e.g. /dev/input/event[0-9] does not include only pluggable or external
devices, it can contain several internal input devices as HIDs but the
actual number is not predictable. To make sure the package reads from
the correct device, it is wiser to provide a udev rule which gives a
particularly identified input device (by classification / type /
interface or even vendor) a known /dev location as a name or symlink.

--


Neil Williams
=============
http://www.linux.codehelp.co.uk/
 
Old 10-24-2011, 02:20 PM
Ivan Shmakov
 
Default udev: what does it used for in Debian?

>>>>> Neil Williams <codehelp@debian.org> writes:
>>>>> On Mon, 24 Oct 2011 18:19:08 +0700 Ivan Shmakov wrote:

>> I've found that a few packages, contrary to my expectations, have
>> Depends: on udev. I'm primarily concerned with alsa-base and
>> initramfs-tools, but also wonder about libcomedi0, dkopp,
>> python-expeyes, libnjb5, media-player-info, pulseaudio, ukopp,
>> xserver-xorg-core, and midisport-firmware.

>> The backstory is that I'm about to install Debian (either stable or
>> testing) on a tiny Architecture: i386 system, and consider excluding
>> udev from the installation, as the hardware in question has
>> virtually no support for any pluggable devices whatsoever.

> udev isn't just for pluggable devices, packages can provide udev
> rules to ensure that devices appear with a consistent name,

It doesn't seem like a good reason for the aforementioned
dependency, does it?

And what the initramfs-tools package has to do with consistent
devices' filenames?

> e.g. /dev/input/event[0-9] does not include only pluggable or
> external devices, it can contain several internal input devices as
> HIDs but the actual number is not predictable. To make sure the
> package reads from the correct device, it is wiser to provide a udev
> rule which gives a particularly identified input device (by
> classification / type / interface or even vendor) a known /dev
> location as a name or symlink.

Indeed, thanks.

Somehow, I assume that given a relatively small number of
devices per bus, this wasn't a problem, say, a decade or so ago.
(Think of, say, 2 IDE or floppy drives per IDE or FDD
controller, one keyboard, one PS/2 mouse, two RS-232 ports per
UART, etc.) Or was it rather that there was much less variation
between different instances Intel-based computers' hardware?

However, I wonder, how often these numbers will change given
that the system's hardware configuration will essentially be
fixed for all the foreseeable future? I guess it won't be
something like “every (other) kernel's release”, right?

TIA.

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Old 10-24-2011, 02:29 PM
 
Default udev: what does it used for in Debian?

On Oct 24, Ivan Shmakov <ivan@gray.siamics.net> wrote:

> It doesn't seem like a good reason for the aforementioned
> dependency, does it?
There are many other reasons, you can find them in /lib/udev/.

> And what the initramfs-tools package has to do with consistent
> devices' filenames?
udev is needed to automatically load all the modules you need, for a
start.

> However, I wonder, how often these numbers will change given
> that the system's hardware configuration will essentially be
> fixed for all the foreseeable future? I guess it won't be
> something like “every (other) kernel's release”, right?
Names could change even at every boot.

Anyway, you have equiv. Try to use it and look what happens.

--
ciao,
Marco
 
Old 10-24-2011, 02:32 PM
Timo Juhani Lindfors
 
Default udev: what does it used for in Debian?

Ivan Shmakov <ivan@gray.siamics.net> writes:
> And what the initramfs-tools package has to do with consistent
> devices' filenames?

Initramfs runs udev. This allows you to use e.g.

root=/dev/disk/by-id/ata-WDC_WD3200AAJS-65B4A0_WD-WMAT13954017-part1

instead of

root=/dev/sda1

to specify where your root filesystem is.


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Old 10-24-2011, 03:06 PM
Ivan Shmakov
 
Default udev: what does it used for in Debian?

>>>>> Marco d'Itri <md@Linux.IT> writes:
>>>>> On Oct 24, Ivan Shmakov <ivan@gray.siamics.net> wrote:

>> It doesn't seem like a good reason for the aforementioned
>> dependency, does it?

> There are many other reasons, you can find them in /lib/udev/.

ACK, thanks.

>> And what the initramfs-tools package has to do with consistent
>> devices' filenames?

> udev is needed to automatically load all the modules you need, for a
> start.

So, the kernel won't try to autoload a module when the
corresponding device gets accessed?

It was my understanding that, e. g., should a 10, 1 character
device (/dev/psaux) be accessed, the kernel will spawn
modprobe(8) against char-major-10-1, which is then resolved to
‘psmouse’ thanks to modprobe.conf(5). At the least, it was used
to do just that something like a decade ago (IOW, as of 2.4.)

$ /sbin/modprobe -c | grep -F -- 'char-major-10-1 '
alias char-major-10-1 psmouse
$

>> However, I wonder, how often these numbers will change given that
>> the system's hardware configuration will essentially be fixed for
>> all the foreseeable future? I guess it won't be something like
>> “every (other) kernel's release”, right?

> Names could change even at every boot.

Any reasons for that other than the order in which the modules
get loaded?

> Anyway, you have equiv. Try to use it and look what happens.

I've never heard of that (and Googling for “linux equiv” doesn't
seem to return anything relevant to the problem.) What's it?

TIA.

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Old 10-24-2011, 03:12 PM
Ivan Shmakov
 
Default udev: what does it used for in Debian?

>>>>> Timo Juhani Lindfors <timo.lindfors@iki.fi> writes:
>>>>> Ivan Shmakov <ivan@gray.siamics.net> writes:

>> And what the initramfs-tools package has to do with consistent
>> devices' filenames?

> Initramfs runs udev. This allows you to use e.g.

> root=/dev/disk/by-id/ata-WDC_WD3200AAJS-65B4A0_WD-WMAT13954017-part1

> instead of

> root=/dev/sda1

> to specify where your root filesystem is.

Indeed, it's a good option to have. But isn't it something that
can be, in certain circumstances, however marginal, considered
an extra?

BTW, does the root=UUID= variant require udev as well?

TIA.

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Old 10-24-2011, 03:18 PM
 
Default udev: what does it used for in Debian?

On Oct 24, Ivan Shmakov <ivan@gray.siamics.net> wrote:

> So, the kernel won't try to autoload a module when the
> corresponding device gets accessed?
Not for *hardware* drivers, since it cannot know which driver is needed.

> I've never heard of that (and Googling for “linux equiv” doesn't
> seem to return anything relevant to the problem.) What's it?
I meant equivs.

Anyway, I think that if you want to learn how udev works then you
should send your questions to an users support mailing
list/newsgroup/forum/etc.

--
ciao,
Marco
 
Old 10-24-2011, 04:08 PM
Ivan Shmakov
 
Default udev: what does it used for in Debian?

>>>>> Marco d'Itri <md@Linux.IT> writes:
>>>>> On Oct 24, Ivan Shmakov <ivan@gray.siamics.net> wrote:

>> So, the kernel won't try to autoload a module when the corresponding
>> device gets accessed?

> Not for *hardware* drivers, since it cannot know which driver is
> needed.

Not even via modprobe.conf(5)?

>> I've never heard of that (and Googling for “linux equiv” doesn't
>> seem to return anything relevant to the problem.) What's it?

> I meant equivs.

Somehow, I've totally missed this one. Which, indeed, I've a
plenty uses for. Thanks!

> Anyway, I think that if you want to learn how udev works then you
> should send your questions to an users support mailing
> list/newsgroup/forum/etc.

Well, seems like there're little specific to Debian in how udev
works. Thanks.

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Old 10-24-2011, 04:10 PM
Neil Williams
 
Default udev: what does it used for in Debian?

On Mon, 24 Oct 2011 21:20:01 +0700
Ivan Shmakov <ivan@gray.siamics.net> wrote:

> >>>>> Neil Williams <codehelp@debian.org> writes:
> >>>>> On Mon, 24 Oct 2011 18:19:08 +0700 Ivan Shmakov wrote:
>
> > udev isn't just for pluggable devices, packages can provide udev
> > rules to ensure that devices appear with a consistent name,
>
> It doesn't seem like a good reason for the aforementioned
> dependency, does it?

Yes it is - otherwise devices simply stop communicating with the
package every couple of boot cycles. The package could also start
reading the wrong /dev node and get thoroughly confused.

If the device concerned is the main input HID for the machine, that is
really very, very bad and a working udev rule file and a dependency on
udev is mandatory. (My most recent udev rule encounter was precisely
this situation with an embedded device trying to read the hardware
button interface, one boot it was /dev/input/event0, next time it
was /dev/input/event1, the package failed to start and the device
could not be operated or shutdown or rebooted without connecting the
serial port.)

--


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