> With ISA busmasters, the kernel has to double-buffer the IO memory for
> the 16MB+ area, which makes the whole idea with DMA fail completely.
> The extra copy is _worse_ than the CPU doing the reading by hand in
> the first place.
That should not happen if the kernel was compiled with the 16 MB RAM
limit. Until I grepped the kernel source, I thought that config option
would get rid of the double-buffer overhead.
Bounce buffers will only be used if you actually have more than 16MB and if the
actual data transfer is taking place to an area that cannot be DMA'ed to
directly. You don't even need to set the 16M option.
Leonard