Log Debug Messages without Debug Serial on Shipped Device
- by Kate Moss' Open Space
Debug message is one of the ancient but useful way for problem resolving. Message is redirected to PB if KITL is enabled otherwise it goes to default debug port, usually a serial port on most of the platform but it really depends on how OEMWriteDebugString and OEMWriteDebugByte are implemented.
For many reasons, we don't want to have a debug serial port, for example, we don't have enough spare serial ports and it can affect the performance. So some of the BSP designers decide to dump the messages into other media, could be a log file, shared memory or any solution that is suitable for the need.
In CE 5.0 and previous, OAL and Kernel are linked into one binaries; in the other word, you can use whatever function in kernel, such as SC_CreateFileW to access filesystem in OAL, even this is strongly not recommended. But since the OAL is being a standalone executable in CE 6.0, we no longer can use this back door but only interface exported in NKGlobal which just provides enough for OAL but no more. Accessing filesystem or using sync object to communicate to other drivers or application is even not an option. Sounds like the kernel lock itself up; of course, OAL is in kernel space, you can still do whatever you want to hack into kernel, but once again, it is not only make it a dirty solution but also fragile. So isn't there an elegant solution?
Let's see how a debug message print out. In private\winceos\COREOS\nk\kernel\printf.c, the OutputDebugStringW is the one for pumping out the messages; most of the code is for error handling and serialization but what really interesting is the following code piece
if (g_cInterruptsOff) {
OEMWriteDebugString ((unsigned short *)str);
} else {
g_pNKGlobal->pfnWriteDebugString ((unsigned short *)str);
}
CELOG_OutputDebugString(dwActvProcId, dwCurThId, str);
It outputs the message to default debug output (is redirected to KITL when available) or OAL when needed but note that highlight part, it also invokes CELOG_OutputDebugString. Follow the thread to private\winceos\COREOS\nk\logger\CeLogInstrumentation.c, this function dump whatever input to CELOG. So whatever the debug message is we always got a clone in CELOG. General speaking, all of the debug message is logged to CELOG already, so what you need to do is using celogflush.exe with CELZONE_DEBUG zone, and then viewing the data using the by Readlog tool. Here are some information about these tools
CELOG - http://msdn.microsoft.com/en-us/library/ee479818.aspx
READLOG - http://msdn.microsoft.com/en-us/library/ee481220.aspx
Also for advanced reader, I encourage you to dig into private\winceos\COREOS\nk\celog\celogdll, the source of CELOG.DLL and use it as a starting point to create a more lightweight debug message logger for your own device!
