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