| 1 |
On 07/06/2013 06:12, Chris Stankevitz wrote: |
| 2 |
> Hello, |
| 3 |
> |
| 4 |
> A USB serial device is identified by the characters "/dev/ttyUSB0". |
| 5 |
> One might call this string a "device on your filesystem" and it can be |
| 6 |
> opened/closed just like any other entry in the filesystem. |
| 7 |
> |
| 8 |
> An ethernet device is sometimes represented by the string "eth0". |
| 9 |
> Regarding this string "eth0": |
| 10 |
> |
| 11 |
> 1. What does this string represent? Is it a file on a filesystem? |
| 12 |
> (no!) Is it okay for me to call it an "ethernet *device*" |
| 13 |
|
| 14 |
It's just a name. Your name is Chris, mine is Alan and it's is eth0. |
| 15 |
|
| 16 |
We usually just call it an interface |
| 17 |
> |
| 18 |
> 2. Assuming udev is not running, who/what comes up with the name |
| 19 |
> "eth0"? How does that person/thing know how many ethernet devices |
| 20 |
> there are and in what order to enumerate them? What happens if |
| 21 |
> ethernet devices are dynamically added (e.g. a USB ethernet device or |
| 22 |
> a driver being loaded/unloaded)? |
| 23 |
|
| 24 |
The kernel driver gives it that name based on what it finds when it |
| 25 |
probes your hardware. Kernel drivers strives for some form of |
| 26 |
consistency in picking names, but the order they are found in is |
| 27 |
somewhat random-ish. This means you cannot guarantee that the number on |
| 28 |
the end is always going to be the same every boot - hardware just does |
| 29 |
not work that way |
| 30 |
|
| 31 |
When you hotplug a device, the driver does what it's coded to do - |
| 32 |
usually assign the next available number. There is no guarantee all |
| 33 |
drivers will always do this always - it's by convention |
| 34 |
> |
| 35 |
> 3. How does (2) change when udev is running? |
| 36 |
|
| 37 |
Depends. |
| 38 |
|
| 39 |
If you like the kernel naming scheme and want to keep it, just tell udev |
| 40 |
to not fiddle with names and keep them as they are. |
| 41 |
|
| 42 |
If you like the way udev does things and want to go with it's scheme, |
| 43 |
follow recent recommendations on this list. udev will rename the |
| 44 |
interfaces to it's own scheme based on the rules you set up. |
| 45 |
|
| 46 |
What you CANNOT do with udev is eg switch the names eth0 and eth1 around |
| 47 |
after the kernel has named them. That was tried for years, it doesn't |
| 48 |
work. So now udev never interferes with kernel namespace, it create it's |
| 49 |
own namespace |
| 50 |
|
| 51 |
For more info, research this list going back about 4 months. The whole |
| 52 |
topic was discussed at length. Search for "udev persistent names". A |
| 53 |
word of warning - it wasn't pretty at the time and nothing changed just |
| 54 |
becuase those mails are now archived :-) |
| 55 |
|
| 56 |
|
| 57 |
|
| 58 |
-- |
| 59 |
Alan McKinnon |
| 60 |
alan.mckinnon@×××××.com |
Archives