On 10/11/2014 10:54 AM, Anthony Wright wrote: > On 10/10/2014 18:00, Bruce Dubbs wrote: >> Anthony Wright wrote: >>> I'm running a modified version of LFS 7.5 where I have an initramfs that >>> populates /dev/ with busybox's mdev before handing over to the udevd >>> that's part of LFS 7.5 >>> >>> Everything seemed to be fine until a partitioned a disk and discovered >>> that the partition device nodes weren't created by udevd. When I do >>> 'udevadm info -e' it lists the /dev/sda1, /dev/sda2 devices but they >>> don't exists in /dev. >>> >>> I deleted all of /dev, re-created /dev/null and then tried to get udevd >>> to re-populate it using the standard LFS commands (run udevd, then call >>> udevadm trigger). However, while udevadm info listed all the devices, it >>> only created a few symbolic links and did no mknod()s at all. >>> >>> I tried to track down the issue in the udev code with a view to putting >>> in some debugging patches. However, while I can find a few mknod calls >>> inside systemd, I can't find any that look like they are used by udev to >>> create the device nodes in /dev/. >>> >>> Could anybody explain how the actual mknod() calls are made but udev. >>> Are the calls made by udev itself or does it call out to something else >>> to do this final step. Does it use mknod() to create the /dev entries or >>> is there another mechanism that I'm unaware of. >> >> udev does not create the devices. The kernel does. What udev does is >> create symlinks, change permissions, ownership, etc. Look at the udev >> rules to see. >> >> -- Bruce >> > I spent the whole day looking at this, and finally discovered the > devtmpfs filesystem. Version 176 of udev switched from using a tmpfs for > /dev and doing the mknod()s itself to using a devtmpfs for /dev and > letting the kernel do the mknod()s for it. This was particularly > confusing for me since everybody says the purpose of udev is to populate > /dev, and clearly that's not it's purpose any more. > > Given that udev's role has clearly changed, I'd like to understand what > it's role is now. It seems to be involved in doing the modprobes, but > does it do more than that now? >
It does more and Bruce already said what. Hotplug handing, device renaming/symlinking and managing ownership and permissions of the device nodes. You could call it a "Device manager" nowadays, as it simply manages device nodes. -- Note: My last name is not Krejzi.
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