Ok--so now that you pointed out that DEFAULT_RAW_BUFFER is returned as a pointer and CONFIG_ALLOCATE_RAW_LV_BUFFER is not, I can see there is a significant difference between the two.
They are both pointers; the only difference is who allocates them (Canon code or our code).
As I recall the whole issue why the Digic IV LVState cameras weren't working properly was because nobody was able to find the DEFAULT_RAW_BUFFER on these cameras.
No, the whole issue was (and still is) proper sync to avoid corrupted frames (in all video modes).
The sync issue has two components. An important one was frame size in 10/12-bit modes (which affected EDMAC behavior - it went out of sync), which was solved by RAW_SLURP. From current reports, this was not sufficient to solve all camera models x video modes. The remaining issue is
probably that RAW_SLURP does not always start transferring the raw data at the right moment (like Canon's lv_save_raw hopefully does). We need to find when to call raw_lv_vsync: once for every LiveView frame - we are already doing that - but when exactly? This time we have to sync with Canon's image capture process. The newer models (60D and newer) already do that (evfReadOutDoneInterrupt).
The buffer does not affect sync at all (it's just a place in memory where the data is going to be stored) and the EDMAC channel must not conflict with anything else, but that's common sense. These were required by RAW_SLURP. BTW - changing that 0x1000 is not going to make things any better or worse, as long as the allocation succeeds.
It is also possible to solve the frame size issue without RAW_SLURP (and therefore without a custom buffer and without changing the EDMAC channel), with cache patches. That will keep the other shortcomings of lv_save_raw (side effects in x5/x10 on most old models) and the backend for cache patching is still quite fragile in my opinion, so I'd prefer to avoid this route.
The keys for solving the sync issue are in that big LVState diagram, the debug logs
from LiveView (dm-spy-experiments branch; for models other than 500D you will have to find
some stubs) or
EDMAC activity logs/graphs (currently captured for 50D, but incomplete). I
hope to be able (but cannot promise) to solve the sync issues after analyzing all these logs.
All the logs have to be captured in 3 LiveView modes (see
#1450) in order to have something to compare. Repeat for all video modes (1080p, 720p, 5x, whatever else your camera has). Repeat for all camera models (not just 50D). The two logging tasks are also for camera models already present on the Experiments page (for double-checking and for having something to compare against).