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

[Attilio Rao]

On Fri, May 30, 2014 at 5:03 PM, John Baldwin <jhb at freebsd.org> wrote:
> 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().

Did you notice that I *removed* the destroy not *added*?

Attilio


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