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

[Attilio Rao]

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.

Attilio


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