On Sun, Jan 18, 2004 at 08:50:06PM +0100, Dimitri Torfs wrote:
> dma_map_single() is supposed to be called on a buffer that exactly
> starts and ends on a cacheline boundary, otherwise "bad things"
> (e.g. overwrite of data that was written by device, ...) (especially
> on dma non-coherent systems) may happen.
> So what should be done when dma_map_single is not called
> with a sane (ptr, size) argument ?
> - is the driver (caller) considered buggy and should we return a 0
> return-value ?
> - is the driver (caller) considered buggy but we do the mapping
> anyway, hoping that the driver has not/will not touched/touch
> the boundary cachelines ?
The driver is considered buggy; dma_map_single's behaviour is undefined so
it's perfectly ok if it paints neighbour's cat pink ;-)
> - should we take appropriate actions to make sure the
> cache-effects do not come into play (e.g. by using some kind of
> bounce buffer) ?
Technically bounce buffers can be handled inside dma_map_single & co but
it's not a good idea. Better set the appropriate flags so higher levels
can allocate memory with the appropriate GFP_* flags and thereby hopefully
avoid overly frequent buffer bouncing.