netdev - RE: [PATCH net-next v3 0/3] enic: Add support for rx

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Date:	Thu, 14 Aug 2014 09:34:50 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Govindarajulu Varadarajan' <_govind@....com>,
	"davem@...emloft.net" <davem@...emloft.net>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
CC:	"ben@...adent.org.uk" <ben@...adent.org.uk>,
	"stephen@...workplumber.org" <stephen@...workplumber.org>,
	"ssujith@...co.com" <ssujith@...co.com>,
	"benve@...co.com" <benve@...co.com>
Subject: RE: [PATCH net-next v3 0/3] enic: Add support for rx_copybreak

From: Govindarajulu Varadarajan
> dma_map_single()/dma_unmap_single() is more expensive than alloc_skb & memcpy
> for smaller packets. By doing this we can reuse the dma buff which is already
> mapped. This is very useful when iommu is on. The default skb copybreak value
> is 256.
> 
> When iommu is on, we can go much higher than 256. All the drivers that supports
> rx_copybreak provides module parameter to change this value. Since module
> parameter is the least preferred way for changing driver values, this series
> adds ethtool support for setting rx_copybreak.

Is there any mileage in having a system-wide default for rx_copybreak?
I'd have thought that the value is really driver independent since it
(mostly) depends on the comparative cost of dma_map and copy operations.

The same also applies to any equivalent copy done during transmit.
(Not sure if I've seen any drivers that keep a permanently mapped
buffer for transmitting small fragments.)

	David



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