I am recently working on kernel upgrade from 2.6.23.17 to 2.6.27-rc5 for our VoIP SoCs.
The following are lessons I learned this time.
* use git to help migrate the patches, as we manage patches in our system, not by git. So, we may use git to help migrate the patches.
* get a kernel git tree (git fetch)
* check out a branch for your current kernel version (git branch, git checkout)
* patch the git tree with your patches (patch, git commit)
* rebase the git tree to the newer kernel (git rebase)
* resolve conflict, for each resolved conflict, remember your changes (git rerere)
* once resolved all conflicts, generate new patches for the new kernel (git format-patch)
* to get your new kernel built for the platform at first
* use make oldconfig to migrate the kernel configurations
* keep all the old configuration as much as possible
* turn on most of the kernel debug option, like early_printk, printk_with_time, spinlock, etc.
* If it doesn't boot, there may be a lot of reasons
* check kernel dump, especially the dumped code, disassemble the whole kernel to figure out where the dump happened
* there are usually a big change in each major version of kernel, such as timer change in mips architecture, path and inclusion changes in arm architecture, sd/mmc infrastructure changes, be very careful.
