Книга: Embedded Linux Primer: A Practical, Real-World Approach
14.5.1. Early Serial Debug Output
14.5.1. Early Serial Debug Output
The first tool you might have available is CONFIG_SERIAL_TEXT_DEBUG. This Linux kernel-configuration option adds support for debug messages very early in the boot process. At the present time, this feature is limited to the PowerPC architecture, but nothing prevents you from duplicating the functionality in other architectures. Listing 14-22 provides an example of this feature in use on a PowerPC target using the U-Boot bootloader.
Listing 14-22. Early Serial Text Debug
## Booting image at 00200000 ...
Image Name: Linux-2.6.14
Created: 2005-12-19 22:24:03 UTC
Image Type: PowerPC Linux Kernel Image (gzip compressed)
Data Size: 607149 Bytes = 592.9 kB
Load Address: 00000000
Entry Point: 00000000
Verifying Checksum ... OK
Uncompressing Kernel Image ... OK
id mach(): done <== Start of messages enabled by
MMU:enter <== CONFIG_SERIAL_TEXT_DEBUG
MMU:hw init
MMU:mapin
MMU:setio
MMU:exit
setup_arch: enter
setup_arch: bootmem
arch: exit
arch: real exit
Using this feature, you can often tell where your board is getting stuck during the boot process. Of course, you can add your own early debug messages in other places in the kernel. Here is an example of its usage found in .../arch/ppc/mm/init.c :
/* Map in all of RAM starting at KERNELBASE */
if (ppc_md.progress)
ppc_md.progress("MMU:mapin", 0x301);
mapin_ram();
The AMCC Yosemite platform is an excellent example of this infrastructure. Consult the following files in the Linux source tree[99] for details of how this debugging system is implemented:
| File | Function | Purpose |
|---|---|---|
| gen550_dbg.c | gen550_init | Serial port setup, called by yosemite.c platform-initialization file |
| gen550_dbg.c | gen550_progress | Low-level serial output routine |
| ibm44x_common.c | ibm44x_platform_init | Binds platform-specific progress routine to generic ppc machine-dependent infrastructure |
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