On Wed, Aug 14, 2019 at 2:34 PM Emil Lenngren <emil.lenngren@gmail.com> wrote:
> > Isn't this why we have udev, to create fancy by-id/by-path/... naming conventions?
> >
> > Thanks,
> > //richard
>
> I actually implemented the same change some time ago, for the exact
> same reason (swapping two volumes and then reboot). Referring to an
> ubi volume by name is more convenient than volume numbers, since names
> can be changed and numbers can't.
> Is it maybe possible to define both /dev/ubiblock%d_%d and
> /dev/ubiblock%d_%s at the same time?
> How would this be done with udev instead to get "fancy by-id"?

Thanks for bringing this topic up again.
udev allows rules and even helper (scripts) to gain device
attributes. These can then be used to name/symlink devices as you want.

For example, if you want to have device symlinks by name for UBI
volumes, you can create a rule like that:
KERNEL=="ubi?_?", IMPORT{program}="/etc/udev/rules.d/ubi_probe
$devnode", SYMLINK="ubi%E{MTD_NUM}_%E{VOL_NAME}"

With ubi_probe being:
#!/bin/sh
VOL_NAME="$(cat /sys/${DEVPATH}/name)"
MTD_NUM="$(cat /sys/${DEVPATH}/../mtd_num)"
echo "VOL_NAME=${VOL_NAME}"
echo "MTD_NUM=${MTD_NUM}"

I took the chance and reviewed UBI's sysfs interface and found a few
things which need
improvement. With these issues addressed you can also work with
ubiblock by name:
1. UBI does not export the UBI device number via sysfs. In 99% of all
cases mtd_num will be correct,
but you can select the UBI number upon attach time.
2. UBI does not emit a change kevent when an volume is renamed. So
udev does not see the rename command.
3. ubiblock does not set the UBI volume as parent device.

So we can then have a rule like:
KERNEL=="ubiblock?_?",
IMPORT{program}="/etc/udev/rules.d/ubiblock_probe $devnode"
SYMLINK="ubiblock$number_%E{VOL_NAME}"

With ubiblock_probe being:
#!/bin/sh
VOL_NAME="$(cat /sys/${DEVPATH}/device/name)"
echo "VOL_NAME=${VOL_NAME}"

# ls -ltr /dev/ubiblock*
brw-rw---- 1 root disk 251, 0 18. Aug 22:41 /dev/ubiblock7_0
lrwxrwxrwx 1 root root     11 18. Aug 22:41 /dev/ubiblock0_test -> ubiblock7_0
# ubirename /dev/ubi7 test lala
# ls -ltr /dev/ubiblock*
brw-rw---- 1 root disk 251, 0 18. Aug 22:42 /dev/ubiblock7_0
lrwxrwxrwx 1 root root     11 18. Aug 22:42 /dev/ubiblock0_lala -> ubiblock7_0

Does this help? If you need other/more sysfs changes, please tell. :-)
Attached you can find my WIP patch for these changes. I need to double check
a few things first before I will send a formal patch.

I think it would even make sense to integrate a more powerful
ubi_probe (as C program) into systemd-udev
like we already have for mtd itself.

-- 
Thanks,
//richard