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The Linux SCSI Generic (sg) HOWTO
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Appendix D. Debugging

There are various ways to debug what is happening with the sg driver. The information provided in the /proc/scsi/sg directory can be useful, especially the debug pseudo file. It outputs the state of the sg driver when it is called. Invoking it at the right time can be a challenge. One approach (used in SANE) is to invoke the system() system call like this: 
    system("cat /proc/scsi/sg/debug");
at appropriate times within an application that is using the sg driver.

Another debugging technique is to trace all system calls a program makes with the strace command (see its "man" page). This command can also be used to obtain timing information (with the "-r" and "t" options).

To debug the sg driver itself then the kernel needs to be built with CONFIG_SCSI_LOGGING selected. Then copious output will be sent by the sg driver whenever it is invoked to the log (normally /var/log/messages) and/or the console. This debug output is turned on by: 
 $ echo "scsi log timeout 7" > /proc/scsi/scsi
As the number (i.e. 7) is reduced, less output is generated. To turn off this type of debugging use: 
 $ echo "scsi log timeout 0" > /proc/scsi/scsi

If you want the system to log SCSI (CHECK_CONDITION related) errors that sg detects rather than process them within the application using sg then set ioctl(SG_SET_DEBUG) to a value greater than zero. Processing SCSI errors within the application using sg is my preference.

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