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maigret
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PostPosted: Wed Nov 16, 2005 9:11 am    Post subject: To the mods: FAQ need a little update Reply with quote

The FAQ forum topic: KC2: devfs is no more up to date, because devfs is not in the kernel anymore. Would maybe be useful to explain about udev ?

Or should I first open a bug for this? ;)
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Adrien
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PostPosted: Wed Nov 16, 2005 9:13 am    Post subject: Re: To the mods: FAQ need a little update Reply with quote

maigret wrote:
Or should I first open a bug for this? ;)

:lol:
Maybe you could update it yourself! :wink:
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maigret
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PostPosted: Wed Nov 16, 2005 9:24 am    Post subject: Reply with quote

I though to that first... But only the moderators can update the FAQ section of the forum.
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kallamej
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PostPosted: Wed Nov 16, 2005 10:45 am    Post subject: Reply with quote

Feel free to post an updated text here and we will move/copy it to the FAQ forum. You'll get a little thank you note in the FAQ. :)
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maigret
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PostPosted: Wed Nov 16, 2005 1:21 pm    Post subject: Reply with quote

Thank you for your answer! I'll try to post you a draft by the beginning of next week (very busy these times).
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maigret
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PostPosted: Sat Nov 19, 2005 10:13 am    Post subject: Reply with quote

I've began to search some informations. Now what I would like to know: Would this text replace the original on devfs, be a new thread in the FAQ or be a following post to the devfs thread? Because it would influence the way I will write the article. Thanks.
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Earthwings
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PostPosted: Sat Nov 19, 2005 11:10 am    Post subject: Reply with quote

IMHO this should be its own udev FAQ entry (new thread). We can then edit the devfs one and mention that devfs is only available in older kernels and people should use udev.
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amne
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PostPosted: Sat Nov 19, 2005 11:23 am    Post subject: Reply with quote

Sounds reasonable to me, too.
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maigret
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PostPosted: Sun Nov 20, 2005 9:45 pm    Post subject: Reply with quote

Here is a first draft. Not a lot of information, since it's already so well documented in the Gentoo udev guide. My opinion on this is that we have to make the links as visible as possible so the reader can jump on them without losing time. I'm sure my English is not very good so I will not be angry against anyone who would correct this document. Tell me what you think of the structure, everyone is free to use it and change it. Just a base for a discussion. I'm very busy these times and may be slow to answer, sorry.
My draft wrote:

Question: What is udev? What does it have to do with devfs?

udev and devfs are softwares that manage the device files in your /dev
directory in a dynamic way; it means you don't have to create all the
device files yourself, which is quite important. Actually neither
devfs or udev are required, but without it would be much more
difficult.

Since the kernel 2.6.13, devfs is not anymore in the Linux
kernel. That means that if you are using now devfs, you will have to
switch to udev when upgrading to a kernel newer newer than 2.6.13.
If you want to know more, there are some simple explanations about
this switch in this FAQ:
http://www.kernel.org/pub/linux/utils/kernel/hotplug/udev-FAQ

For a quick documentation about udev that will teach you how to deploy
udev, there is a special gentoo guide located under:
http://www.gentoo.org/doc/en/udev-guide.xml


For who wants to go further, two links:
- udev official site:
http://www.kernel.org/pub/linux/utils/kernel/hotplug/udev.html

- a guide about how writing udev rules:
http://www.reactivated.net/writing_udev_rules.html


EDIT: forgot to add a link to the old devfs FAQ thread; but I'm sure the admin will do that when including this thread in the FAQ
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maigret
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PostPosted: Sat Nov 26, 2005 11:42 pm    Post subject: Reply with quote

... still no answer. Any critics, ideas about this text? Please say something, at least...
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Earthwings
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PostPosted: Sun Nov 27, 2005 12:02 am    Post subject: Reply with quote

Sorry, forgot to answer. It looks good so far, here's an edited version with some minor changes:
Quote:

Question: What is udev? What does it have to do with devfs?

udev and devfs manage the device files in your /dev directory in a dynamic way. This means udev and devfs take care to create device files for you, which is as important as convenient. Actually neither devfs or udev are required, but without one of them the setup will be much more difficult.

devfs has been removed from the Linux kernel in version 2.6.13 and later. If you are using devfs now and plan to upgrade, you will have to switch to udev. There is a FAQ about the switch at kernel.org

The Gentoo udev Guide teaches you how to deploy udev. Further resources are the udev site at kernel.org and a guide about writing udev rules


Please reply once again with some final version that we can move to the FAQ forum. We'll take care of the correct title and links in the devfs faq etc. Thanks for your help.
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Earthwings
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PostPosted: Sun Nov 27, 2005 10:21 am    Post subject: Reply with quote

Here you are: KC16: What is udev? What does it have to do with devfs?
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PostPosted: Tue Dec 20, 2005 6:17 pm    Post subject: Reply with quote

Here's how I would say it... first, describe the problem:

Did you ever notice that the "/dev" directory on your Linux system might contain literally thousands of files? What are those files, and why do I have so many? The answer is that the entries in "/dev" actually correspond to physical devices, and historically, Linux has been supplied with an entry for every device that "could possibly" exist .. not those that actually "do" exist on your machine. What we would really prefer to have, of course, is one entry in "/dev" for each device that does exist. And, since we're wishing, we'd really like for it all to be smart enough to understand removable devices ... the printers, cameras, disks and so-forth that we can plug in and remove. Your wish is granted.

Now what the feature, old and new, is supposed to do:

The first attempt to solve that problem was called "devfs," and while it worked okay, some other folks came up with what turned out to be a better way, which they called "udev." We don't need to explain too much about how it works; only what it does. It creates an entry in "/dev" for each device that actually exists. And, when you plug-in and unplug removable devices, it keeps up with you .. adding and removing entries on-the-fly. It's pretty smart, too: the device names that it assigns are actually meaningful, and when you unplug a device from one slot and plug it back in to another slot, it keeps its name. The name, in other words, describes what the device is, not just where it is. Cool, huh? udev has a file of "naming rules" that teaches it just how you want to do that. It's a lot more flexible than devfs was... it replaces devfs altogether, of course.

And the caveat:

One thing that Gentoo users need to bear in mind is that there's a "feature" that can interfere with what udev is doing. Not "interfere," really, so much as to just keep it from being able to remove all those "/dev" files. Gentoo will make a backup of all the device-names (all so-many thousand of them...) when you shut down, and then restore all those names when you boot up again. Unless you tell it not to, which you probably should. Here's how: in the file "/etc/conf/rc" set "RC_DEVICE_TARBALL=No".
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PostPosted: Tue Dec 29, 2009 5:49 am    Post subject: Reply with quote

Earthwings wrote:
Here you are: KC16: What is udev? What does it have to do with devfs?


hiya

what's the low down on devtmpfs in the newer (.32) kernels?

I can't tell if it's supposed to be a replacement for udev, or complementary to it.

I have it enabled - don't know if there's any real configuration to it, if this means I can nuke udev from sysinit, or what have you.
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PostPosted: Tue Dec 29, 2009 12:18 pm    Post subject: Reply with quote

it is sth. distros have been pushing for, but it is not a replacement for udev. If i understand it correctly it allows the kernel to create several devices without the need of udev, which should speed up things a bit until udev is called later by init and does the rest of the job.

there has been a lot of dicussion about this before inclussion in mainline, see http://thread.gmane.org/gmane.linux.kernel/830032 f.ex.

cheers
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PostPosted: Tue Dec 29, 2009 8:18 pm    Post subject: Reply with quote

gringo wrote:
it is sth. distros have been pushing for, but it is not a replacement for udev. If i understand it correctly it allows the kernel to create several devices without the need of udev, which should speed up things a bit until udev is called later by init and does the rest of the job.

there has been a lot of dicussion about this before inclussion in mainline, see http://thread.gmane.org/gmane.linux.kernel/830032 f.ex.

cheers


yip, read that one, still not finding anywhere where it's particularly dumbed down enough for me to understand *what* devices it's dumping in tmpfs

and I've noticed zero improvement in boot speed whatsoever since its inclusion in my kernel (and yes, I did include it)
far as I can tell (and admittedly I'm not savvy enough to know for certain) there's no real speed benefit at boot, since udev still ends up going back over everything, doing double duty.

Maybe the benefit isn't to be seen until one goes through and prunes out items from udev that are now handled by devtmpfs?

Dunno. As much as they've hyped it am surprised I can't find any decent power-user-friendly doc explaining what it all means for the actual user.
(and now I'm infinitely curious what all has been done for Android to remove the need for udev)
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gringo
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PostPosted: Wed Dec 30, 2009 9:14 am    Post subject: Reply with quote

Quote:
and I've noticed zero improvement in boot speed whatsoever since its inclusion in my kernel


i think the speedup is only noticeable ( if at all) by those using initrds or such things because udev doesn´t have to be in there anymore.
Or that´s what i think is the purpose of all this stuff : giving a minimal environment for the initrd to function without udev.
It´s a long time since i used initrds so i really can´t say and i might be wrong of course ...

Quote:
(and now I'm infinitely curious what all has been done for Android to remove the need for udev)


i´m curious too, but android is a different playground IMO and i can understand they don´t want udev at all, for such a device a static /dev is probably the best choice.

cheers
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PostPosted: Wed Dec 30, 2009 9:57 am    Post subject: Reply with quote

gringo wrote:

i´m curious too, but android is a different playground IMO and i can understand they don´t want udev at all, for such a device a static /dev is probably the best choice.


and that's another place I end up lost.
my hardware isn't going to change from one boot to another

so why the need to probe every single time rather than using what was discovered previously?

*shrug*

Things like this I get to the point where ricing my boot time becomes less and less important, and I just blindly trust people far more clever than myself know what's going on.
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