[Q] I built a custom kernel for my phone, but how do I test it? - Android Q&A, Help & Troubleshooting

I'm trying to make a custom kernel for the Samsung Gravity SMART using sources from the unofficial CM9 port for the Gio/Ace (which both use very similar hardware to the Gravity Smart). What I've succeeded at so far:
1) Acquiring the source, both for the CM9 kernel and for the stock Froyo kernel
2) Searching for the device-dependent references in the stock kernel
3) Porting those changes over to the CM9 kernel
4) Getting everything to actually compile
5) Acquiring the stock CM7.2 source
6) Creating device makefiles (i.e. BoardConfig.mk, etc)
7) Building CM7.2 using kernel built in step #4
8) Putting the various img files into an Odin-compatible .tar.md5
9) Actually flashing to the phone.
And for all that work, I got.. a soft-bricked phone (fortunately, I have an Odin file for stock firmware). The phone boots to a SAMSUNG logo (not flashing/throbbing).
So my question is: how the heck do I figure out what's actually failing? Is there a bootloader to unlock, and if so how do I unlock it?

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I strongly suspect I'm dealing with a bootloader problem. If I unpack the stock boot.img, edit init.rc with a commented line, repack the boot.img and try flashing it, I get the same behavior (stuck at SAMSUNG).
Any tips for unlocking the bootloader?

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The good news is that it doesn't seem to be a locked bootloader--at least, I figured out how to flash a customized boot.img that contains a stock kernel and a modified ramdisk.
The bad news is that my kernel STILL doesn't boot, and I still have no idea how to approach troubleshooting/debugging it. Any tips?

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Connect your phone to USB, go in the SDK folder and start ddms.bat. Let it load the GUI, then start your phone. At this point it should come up in the dalvik debug monitor.
Select your device on the left side and see in real time what errors it pops-up

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DeuXGod said:
Connect your phone to USB, go in the SDK folder and start ddms.bat. Let it load the GUI, then start your phone. At this point it should come up in the dalvik debug monitor.
Select your device on the left side and see in real time what errors it pops-up
Click to expand...
Click to collapse
thats a nice piece of info, can you elaborate some more?

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"lightbox_error": "The requested content cannot be loaded. Please try again later.",
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"lightbox_full_screen": "Full screen",
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That's pretty much it. You can also request screenshots, explore files inside on the phone, request logcats and explore process logs.
It's a nifty tool

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Thanks for suggesting DDMS, unfortunately it doesn't provide any useful information. It looks like it's just not booting the kernel at all.
I think the problem has to do with the bootloader--researching other, similar phones, the boot loader firmware is tightly coupled with the kernel, particularly the modules that actually interact with the flash filesystem. Since those modules are Samsung-proprietary, my only choice is to either back-port as many features from newer kernels into the stock kernel source, or figure out how to hack the boot loaders to work with a newer revision kernel. It would be a lot simpler if Samsung released a GB update, but fat chance of that ever happening.