svn commit: r266775 - head/sys/x86/x86

[John Baldwin]

On Friday, May 30, 2014 10:54:06 am Attilio Rao wrote:
> On Tue, May 27, 2014 at 11:31 PM, Scott Long <scottl at freebsd.org> wrote:
> > Author: scottl
> > Date: Tue May 27 21:31:11 2014
> > New Revision: 266775
> > URL: http://svnweb.freebsd.org/changeset/base/266775
> >
> > Log:
> >   Eliminate the fake contig_dmamap and replace it with a new flag,
> >   BUS_DMA_KMEM_ALLOC.  They serve the same purpose, but using the flag
> >   means that the map can be NULL again, which in turn enables significant
> >   optimizations for the common case of no bouncing.
> 
> While I think this is in general a good idea, unfortunately our
> drivers do so many dumb things when freeing DMA allocated buffers that
> having a NULL map is going to cause some "turbolence" and make such
> bugs more visible.
> An example is with ATA, where I think this fix is needed:
> http://www.freebsd.org/~attilio/dmamem_free-ata.patch
> 
> Otherwise, what can happen with bounce buffers, is that the allocated
> memory via contig malloc was not going to be freed anytime.
> 
> I tried to look around and I found questionable (read broken) code in
> basically every driver which allocates DMA buffers, so I really don't
> feel I want to fix the majority of our drivers. I just think such
> paths are not excercised enough to be seen in practice often or the
> bugs just get unnoticed.

Eh, many maps for static allocations were already NULL and have been for a 
long time.  This is nothign new.  Plus, the diff you posted has a bug 
regardless of explicitly destroying a map created by bus_dmamem_alloc().

-- 
John Baldwin