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

[Jeff Garzik]

Andi Kleen wrote:
> 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@xxxxxxxxxx>
> > > > > Signed-off-by: Jon Mason <jdmason@xxxxxxxxxx>
> > > > 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.

I respectfully disagree with that logic.  If its a key hot path -- which 
it is, every modern network or disk I/O runs through these paths -- then 
it deserves at least _some_ consideration before adding more CPU cycles.


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

As the author noted, it was only used in early platform bring-up.  And 
simply reviewing the patch... it is clear that screwing up the 
parameters would cause massive, noticeable problems immediately -- such 
as on EM64T with swiotlb.

	Jeff


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