netdev - Re: NAPI and TX

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Message-Id: <1241792678.3194.5.camel@achroite>
Date:	Fri, 08 May 2009 15:24:38 +0100
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	Krzysztof Halasa <khc@...waw.pl>
Cc:	netdev@...r.kernel.org
Subject: Re: NAPI and TX

On Fri, 2009-05-08 at 14:32 +0200, Krzysztof Halasa wrote:
> Hi,
> 
> Could NAPI or something similar be used for TX in addition to RX?
> Perhaps some driver already does that?

Yes, several drivers do this.  Mostly they receive a single interrupt
for both RX and TX completions and use a single NAPI context.  PCI NICs
using MSI-X can generate distinct IRQs.

> I'm specifically thinking of IXP4xx Ethernet (and possibly WAN), those
> are 266-667 MHz ARM XScale CPUs and the interrupts handling just
> transmitted sk_buffs are a pain. Could I delay those interrupts
> somehow?

Since there are separate IRQs for RX and TX completions then you need to
set up a second NAPI context (struct napi_struct) for TX completion
handling.  Note that TX completions do not count against the "budget" so
your TX poll function should never need to return early.

Ben.

-- 
Ben Hutchings, Senior Software Engineer, Solarflare Communications
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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