On-stack allocation of DMA S/G lists

[Stanislav Sedov]

On Mon, 6 Aug 2012 10:26:06 -0400
John Baldwin <jhb at freebsd.org> mentioned:

> On Thursday, July 12, 2012 8:26:05 am John Baldwin wrote:
> > On Sunday, July 08, 2012 7:05:16 am Peter Jeremy wrote:
> > > BTW(2): Whilst studying busdma_machdep.c for arm and mips, I've
> > > noticed they appear to potentially allocate substantial kernel stack
> > > under some conditions as several bus_dma(9) functions include:
> > >     bus_dma_segment_t dm_segments[dmat->nsegments];
> > > What prevents this overflowing the kernel stack?
> > 
> > That does seem dubious.  x86 stores the array in the tag instead.
> 
> I have an untested patch to change bus-dma on arm and mips to allocate a
> dynamic S/G list in each DMA tag on first use instead of using on-stack
> allocation (which I think is rather bogus).  Can folks review and test this 
> patch please?  Thanks.
> 
> http://www.FreeBSD.org/~jhb/patches/arm_mips_dynamic_dma_segs.patch
> 

Seems to work fine for me on ARM.
I had to initialize mflags to 0 in one place to get it compiling though.

-- 
Stanislav Sedov
ST4096-RIPE

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