I have a working gp8psk driver (it's part of dvb-usb) compiled in 2.6.18-7.3 environment.
and it works fine with DM500HD (and similar) receivers.

I tried to recompile gp8psk driver for a newer kernel version used in DMM images - 2.6.18-7.4.
And I can not. Compilation aborts on ringbufer functions use.

Looks like DMM changed dvb_ringbuffer.h and dvb_ringbuffer.c files in this latest kernel version.
But they provided ONLY header file (dvb_ringbuffer.h).
New dvb_ringbuffer.h file has different prototypes for the same function used in older dvb_ringbuffer.h.
As a result, dvb-core.o can not be compiled.
surely, I can not recompile my gp8psk driver (it depends on dvb-core).

Is dvb_ringbuffer.c going to be published?
and possibly all other changes in the code required to compile dvb-core.
Right now my compilation stops with this error:
dmxdev.c:101: error: too many arguments to function 'dvb_ringbuffer_read'
I don't know what else is wrong with the source after that.

PS: I tried to compile my driver against older dvb-core folder (which I copied from 2.6.18-7.3).
I was able to compile the driver, and even mount it. But as soon as I started usbtuner, kernel crashed.

hmm, interesting.
Thanks for clarification.
I do see that bitbake_folder/dm500hd/build/tmp/work/dm500hd-oe-linux/v4l-dvb-modules-0.0+hgXXXXXXXXX-r1/v4l-dvb/linux/drivers/media/dvb/dvb-core contains patched dvb_ringbuffer.h and dvb_ringbuffer.h files.

Th fact that in bitbake_folder/dm500hd/build/tmp/work/dm500hd-oe-linux/linux-dm500hd-2.6.18-r7.1/linux-2.6.18/drivers/media/dvb/dvb-core I see already patched dvb_ringbuffer.h, but dvb_ringbuffer.c is still unpatched through me off my feet.
Somehow, the source code is partially patched in that folder (it's already not the same as in standard 2.6.18 tree).

I thought that source files will be patched in that folder (which I used) before the compilation.

simple copying of the content of
bitbake_folder/dm500hd/build/tmp/work/dm500hd-oe-linux/v4l-dvb-modules-0.0+hg20100904-r1/v4l-dvb/linux/drivers/media/dvb/dvb-core
into
bitbake_folder/dm500hd/build/tmp/work/dm500hd-oe-linux/linux-dm500hd-2.6.18-r7.1/linux-2.6.18/drivers/media/dvb/dvb-core
did not help.

Kconfig are way different and "make menuconfig" does not allow to select a driver anymore.
Had to fiddle with Kconfig files, hoping to do a "brute force" compiling.

And now I'm stuck with new functions not defined in standard 2.6.18 yet.
Like memdup_user() function call (used in dvb-core/dvb_demux.c)
This function was defined in later kernel versions (and declared in linux/string.h header).

Looks like string.h was back ported as well. But I can not find it (the one which is in bitbake_folder/dm500hd/build/tmp/work/dm500hd-oe-linux/linux-dm500hd-2.6.18-r7.1/linux-2.6.18/include/linux folder is old).

Do you have any tips on how to compile a modified dvb driver in YOUR environment?

added:
it would be probably more straightforward if I asked the simple question first.
I used to compile my dvb-usb-gp8psk.ko driver using dvb tree in (in older version, 2.6.18-7.3)
bitbake_folder/build-dm800/tmp/work/linux-dm500hd-2.6.18-r5/linux-2.6.18/drivers/media/dvb/

Now I made some backporting into dvb-usb-gp8pskto accommodate new s2api calls (and frontend.h definitions).
How do I compile it using your new v4l tree?
I tried to copy it into bitbake_folder/dm500hd/build/tmp/work/dm500hd-oe-linux/linux-dm500hd-2.6.18-r7.1/linux-2.6.18/drivers/
Obviously, it did not work.
What's the correct way to do that?