Messages in this thread | | | Date | Thu, 25 May 2006 05:58:43 -0400 | From | Jeff Garzik <> | Subject | Re: [PATCH 2/4] x86-64: Calgary IOMMU - move valid_dma_direction into the callers |
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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@il.ibm.com> >>> Signed-off-by: Jon Mason <jdmason@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...
>> _Why_ do we need this at all? > > It was helpful for us during the dma-ops work and Calgary bringup and > Andi requested that we move it from Calgary to common code. I think > we're fine with dropping it if that's the consensus, but it did catch > a few bugs early on and the cost is tiny.
Key phrase: "early on"
These checks will only be useful during _early_ development of a new DMA platform. For _100%_ of the real world users, these checks are useless. Not 99%, 100%.
Jeff
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