Re: [gentoo-user] serial in /sys

[Jorge Almeida]

On Thu, Nov 29, 2012 at 10:35 PM, Walter Dnes <waltdnes@××××××××.org> wrote:
> On Thu, Nov 29, 2012 at 07:31:10PM +0000, Jorge Almeida wrote
>
>
>   I have a couple of scripts that automount USB devices under mdev.  See
> https://wiki.gentoo.org/wiki/Mdev for the general setup, and
> https://wiki.gentoo.org/wiki/Mdev/Automount_USB and
> https://wiki.gentoo.org/wiki/Mdev/Automount_USB/automount
>

I don't care for automounting, I just want customized symlinks. Using fstab is
not a second choice, it is how I want it.

>
>   The output below shows that not every USB device has a serial number,
> and the "Patriot Memory" key has a serial number but no manufacturer...
>
> [d531][waltdnes][~] find /sys/devices/pci0* -name serial | xargs cat
>

OK. I found that my pens have serial numbers. I setup /proc/sys/kernel/hotplug
with a script that prints the environment. When a pen is inserted, this is
what is set:

ACTION=add
DEVPATH=/devices/pci0000:00/0000:00:04.1/usb1/1-4/1-4:1.0/host12/target12:0:0/12:0:0:0/block/sdd
SUBSYSTEM=block
MAJOR=8
MINOR=48
DEVNAME=sdd
DEVTYPE=disk
SEQNUM=1750

Following your suggestion:
# find /sys/devices/pci0* -name serial
/sys/devices/pci0000:00/0000:00:04.0/usb5/serial
/sys/devices/pci0000:00/0000:00:04.1/usb1/1-4/serial
/sys/devices/pci0000:00/0000:00:04.1/usb1/serial
/sys/devices/pci0000:00/0000:00:06.0/usb6/serial
/sys/devices/pci0000:00/0000:00:06.1/usb3/3-4/serial
/sys/devices/pci0000:00/0000:00:06.1/usb3/serial
/sys/devices/pci0000:00/0000:00:0c.0/0000:02:00.0/0000:03:01.0/0000:04:00.0/usb2/serial
/sys/devices/pci0000:00/0000:00:0c.0/0000:02:00.0/0000:03:01.0/0000:04:00.0/usb4/serial

# cat /sys/devices/pci0000:00/0000:00:04.1/usb1/1-4/serial
AA04012700011287

So, the serial number is here. To retrieve it out of $DEVPATH, I can do (in
this case, at least!):
# cat /sys/devices/pci0000:00/0000:00:04.1/usb1/1-4/1-4:1.0/host12/target12:0:0/12:0:0:0/block/sdd/../../../../../../serial
AA04012700011287

I'm assuming that this scenario is consistent for usb pen drives. I'll think
of external hard disks next...

BTW, the idea behind this is:
-- have s6-devd listen to the netlink interface
(http://www.skarnet.org/software/s6-linux-utils/s6-devd.html)

-- when a device is inserted, s6-devd launches a program that tries to obtain
the serial number, uses it as key to seek a string some_name in a constant
database, and creates the symlink /dev/some_name --> $DEVNAME. On failure,
exec mdev with the environment passed by the kernel.

I'm supposing that the environment on netlink is the same as with the hotplug
mechanism. Would this be true?

I would be interested in hearing any comments...

Thanks

Joreg Almeida

>
> --
> Walter Dnes <waltdnes@××××××××.org>
> We are apparently better off trying to avoid udev like the plague.

Precisely.

> Linus Torvalds; 2012/10/03 https://lkml.org/lkml/2012/10/3/349
>