.. SPDX-License-Identifier: GPL-2.0
===============
NETIF Msg Level
===============
The design of the network interface message level setting.
History
-------
The design of the debugging message interface was guided and
constrained by backwards compatibility previous practice. It is useful
to understand the history and evolution in order to understand current
practice and relate it to older driver source code.
From the beginning of Linux, each network device driver has had a local
integer variable that controls the debug message level. The message
level ranged from 0 to 7, and monotonically increased in verbosity.
The message level was not precisely defined past level 3, but were
always implemented within +-1 of the specified level. Drivers tended
to shed the more verbose level messages as they matured.
- 0 Minimal messages, only essential information on fatal errors.
- 1 Standard messages, initialization status. No run-time messages
- 2 Special media selection messages, generally timer-driver.
- 3 Interface starts and stops, including normal status messages
- 4 Tx and Rx frame error messages, and abnormal driver operation
- 5 Tx packet queue information, interrupt events.
- 6 Status on each completed Tx packet and received Rx packets
- 7 Initial contents of Tx and Rx packets
Initially this message level variable was uniquely named in each driver
e.g. "lance_debug", so that a kernel symbolic debugger could locate and
modify the setting. When kernel modules became common, the variables
were consistently renamed to "debug" and allowed to be set as a module
parameter.
This approach worked well. However there is always a demand for
additional features. Over the years the following emerged as
reasonable and easily implemented enhancements
- Using an ioctl() call to modify the level.
- Per-interface rather than per-driver message level setting.
- More selective control over the type of messages emitted.
The netif_msg recommendation adds these features with only a minor
complexity and code size increase.
The recommendation is the following points
- Retaining the per-driver integer variable "debug" as a module
parameter with a default level of '1'.
- Adding a per-interface private variable named "msg_enable". The
variable is a bit map rather than a level, and is initialized as::
1 << debug
Or more precisely::
debug < 0 ? 0 : 1 << min(sizeof(int)-1, debug)
Messages should changes from::
if (debug > 1)
printk(MSG_DEBUG "%s: ...
to::
if (np->msg_enable & NETIF_MSG_LINK)
printk(MSG_DEBUG "%s: ...
The set of message levels is named
========= =================== ============
Old level Name Bit position
========= =================== ============
0 NETIF_MSG_DRV 0x0001
1 NETIF_MSG_PROBE 0x0002
2 NETIF_MSG_LINK 0x0004
2 NETIF_MSG_TIMER 0x0004
3 NETIF_MSG_IFDOWN 0x0008
3 NETIF_MSG_IFUP 0x0008
4 NETIF_MSG_RX_ERR 0x0010
4 NETIF_MSG_TX_ERR 0x0010
5 NETIF_MSG_TX_QUEUED 0x0020
5 NETIF_MSG_INTR 0x0020
6 NETIF_MSG_TX_DONE 0x0040
6 NETIF_MSG_RX_STATUS 0x0040
7 NETIF_MSG_PKTDATA 0x0080
========= =================== ============