[discuss] Re: [PATCH 2/4] x86-64: Calgary IOMMU - move valid_dma_direction into the callers

Andi Kleen ak at suse.de
Fri May 26 09:57:01 CEST 2006


On Thursday 25 May 2006 11:58, Jeff Garzik wrote:
> Muli Ben-Yehuda wrote:
> > On Thu, May 25, 2006 at 12:35:07AM -0400, Jeff Garzik wrote:
> >> Jon Mason wrote:
> >>> >From Andi Kleen's comments on the original Calgary patch, move
> >>> valid_dma_direction into the calling functions.
> >>>
> >>> Signed-off-by: Muli Ben-Yehuda <muli at il.ibm.com>
> >>> Signed-off-by: Jon Mason <jdmason at us.ibm.com>
> >> Even though BUG_ON() includes unlikely(), this introduces additional 
> >> tests in very hot paths.
> > 
> > Are they really very hot? I mean if you're calling the DMA API, you're
> > about to frob the hardware or have already frobbed it - does this
> > check really matter?
> 
> When you are adding a check that will _never_ be hit in production, to 
> the _hottest_ paths in the kernel, you can be assured it matters...

pci_dma_* shouldn't be that hot. Or at least IO usually has so much
overhead that some more bugging shouldn't matter.

On the other hand if the problem of passing wrong parameters here
isn't common I would be ok with dropping them.

-Andi



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