netdev - RE: [PATCH v2] net/macb: Use non-coherent memory for rx buffers

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Date:	Wed, 5 Dec 2012 09:35:39 -0000
From:	"David Laight" <David.Laight@...LAB.COM>
To:	"Nicolas Ferre" <nicolas.ferre@...el.com>
Cc:	"David S. Miller" <davem@...emloft.net>, <netdev@...r.kernel.org>,
	<linux-arm-kernel@...ts.infradead.org>,
	<linux-kernel@...r.kernel.org>,
	"Joachim Eastwood" <manabian@...il.com>,
	"Jean-Christophe PLAGNIOL-VILLARD" <plagnioj@...osoft.com>,
	"Havard Skinnemoen" <havard@...nnemoen.net>
Subject: RE: [PATCH v2] net/macb: Use non-coherent memory for rx buffers

> If I understand well, you mean that the call to:
> 
> 		dma_sync_single_range_for_device(&bp->pdev->dev, phys,
> 				pg_offset, frag_len, DMA_FROM_DEVICE);
> 
> in the rx path after having copied the data to skb is not needed?
> That is also the conclusion that I found after having thinking about
> this again... I will check this.

You need to make sure that the memory isn't in the data cache
when you give the rx buffer back to the MAC.
(and ensure the cpu doesn't read it until the rx is complete.)
I've NFI what that dma_sync call does - you need to invalidate
the cache lines.

> For the CRC, my driver is not using the CRC offloading feature for the
> moment. So no CRC is written by the device.

I was thinking it would matter if the MAC wrote the CRC into the
buffer (even though it was excluded from the length).
It doesn't - you only need to worry about data you've read.

> > I was wondering if the code needs to do per page allocations?
> > Perhaps that is necessary to avoid needing a large block of
> > contiguous physical memory (and virtual addresses)?
> 
> The page management seems interesting for future management of RX
> buffers as skb fragments: that will allow to avoid copying received data.

Dunno - the complexities of such buffer loaning schemes often
exceed the gain of avoiding the data copy.
Using buffers allocated to the skb is a bit different - since
you completely forget about the memory once you pass the skb
upstream.

Some quick sums indicate you might want to allocate 8k memory
blocks and split into 5 buffers.

	David

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