Providing physical IDs that are, in common usage, stable: "minor"As well as updating some internal docs so they're current, this patch adds the physical path to the data available through hotplugging. This feature was requested at the LinuxKongress hotplug BOF, and was also on my 2.5 list.And is already present with the DEVPATH environment variable, right?No, DEVPATH isn't as stable as the physical path. ....Ok, I'm probably confused here, what exactly is the problem that this is trying to solve?
config changes don't break the IDs ... like adding and removing other
USB controllers, enabling or disabling their drivers, reboooting,
and so on.
That lets people label USB ports like they label TTY ports ... so
application level configuration data can be saved. That way the
whole system (kernel + apps + users + sysadmins) can keep running
without needing someone ("sysadmin") to re-configure applications.
Reconfiguration is error prone, and costs time better spent on other
things ... and it's something you never want to need, as a surprise
at 2am after a system rebooted for some reason. So is the initial
config, which is a reason to want the shorter of these two IDs since
it's easier to copy (read over phone, etc) without errors:
USB_PATH=usb-02:00.0-1.4
DEVPATH=root/pci0/00:03.0/02:00.0/usb1/1-1/1-1.4/1-1.4:0
Such physical IDs are the only solution for distinguishing N (>1)
devices that are otherwise identical (no serial numbers etc).
It's how people use almost every kind of I/O device, except
that they can't (yet) set up "hands-off admin" with USB.
The basic problem with $DEVPATH is its use of the USB "bus number".
It changes easily in the face of those minor config changes, and
doesn't correspond directly to any physical reality that a sysadmin
controls. The PCI slot_name doesn't have those disadvantages(*),
so for PCI based controllers that's a much more useful bus ID.
- Dave
(*) Except for rare configurations with hardware that
most people don't have, PCI "bus numbers" won't change
except for major config changes. Like enabling ACPI or
switching to an OS that assigns them differently.
People already expect such changes to break a lot!
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