netdev - RE: [PATCH 11/13] net: mvneta: implement rx

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Date:	Fri, 17 Jan 2014 09:28:13 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Willy Tarreau' <w@....eu>, David Miller <davem@...emloft.net>
CC:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"thomas.petazzoni@...e-electrons.com" 
	<thomas.petazzoni@...e-electrons.com>,
	"gregory.clement@...e-electrons.com" 
	<gregory.clement@...e-electrons.com>
Subject: RE: [PATCH 11/13] net: mvneta: implement rx_copybreak

From: Willy Tarreau 
..
> Ah that's an interesting trick! We don't have an IOMMU on this platform
> so the call is cheap. However, it relies on an I/O barrier to wait for
> a cache snoop completion. From what I read, it's not needed when going
> to the device. But when going to the CPU for the Rx case, we're calling
> it for every packet which is counter productive because I'd like to do
> it only once when entering the rx loop and avoid any other call during
> the loop. I measured that I could save 300 ns per packet by doing this
> (thus slightly modifying the DMA API to add a new dma_io_barrier function).
> 
> I think it could be interesting (at least for this platform, I don't know
> if other platforms work with cache coherent DMA which only require to
> verify end of snooping), to have two distinct set of DMA calls, something
> like this :
> 
>     rx_loop(napi, budget)
>     {
>          dma_wait_for_snoop_completion(dev);
>          ...
>          for_each_rx_packet(pkt) {
>              dma_sync_single_range_for_cpu_unless_completed(dev, pkt->addr);
>              /* use packet */
>          }
>     }

You'd need to scan the rx ring for completed entries before the waiting
for the snoop to complete - and then only process those entries.
(Or read a block of rx ring entries...)

Otherwise you might try to process a rx that finished after you
waited for the snoop to complete.

	David



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