[patch] fix add_bounce_page() for BUS_DMA_KEEP_PG_OFFSET

[Ian Lepore]

On Fri, 2015-04-24 at 13:34 +0200, Svatopluk Kraus wrote:
> The add_bounce_page() can be called from _bus_dmamap_load_phys().
> Client virtual address is zero (not valid) in that case. So, client
> physical address must be used to get an offset when
> BUS_DMA_KEEP_PG_OFFSET flag is set. Note that client physical address
> is always valid.
> 
> Svatopluk Kraus

The only user of BUS_DMA_KEEP_PG_OFFSET in the system is USB.  If the
flag didn't exist, we could do bouncing with a whole lot more
efficiency, at least in the ARM world.  I wonder if it would be possible
to just eliminate the flag (and of course, USB's need for it, which I
fully admit I don't understand even a little bit).

-- Ian