Messages in this thread | | | Subject | Re: [PATCHv2 net 1/2] sfc: Delete EFX_PAGE_IP_ALIGN, equivalent to NET_IP_ALIGN | From | Ben Hutchings <> | Date | Tue, 14 May 2013 14:48:43 +0100 |
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On Tue, 2013-05-14 at 09:32 +0100, David Laight wrote: > > The two architectures that define CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS > > (powerpc and x86) now both define NET_IP_ALIGN as 0, so there is no > > need for this optimisation any more. > > Hmmm.... even on x86 there will be a measurable cost > in misaligned accesses - at least for some workloads.
When they cross cache-line boundaries, yes.
Maybe napi_get_frags() should be adding the 2 byte offset on all architectures, as the skbs it allocates are never RX DMA buffers.
> If the DMA is able to write to a mis-aligned buffer and > still perform aligned burst transfers mid-frame then > 4n+2 aligning the rx buffer should be a win even on x86.
I don't think so.
> Note to hardware engineers: add an option to write two > bytes of junk before the rx data :-)
There is some hardware with that option.
Ben.
-- Ben Hutchings, Staff Engineer, Solarflare Not speaking for my employer; that's the marketing department's job. They asked us to note that Solarflare product names are trademarked.
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