linux-kernel - Re: e1000 driver 2.6.18 - how to waste processor cycles

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Message-ID: <4807377b0611081701i26ee7ce0k1f822dbbe52c2c8@mail.gmail.com>
Date:	Wed, 8 Nov 2006 17:01:44 -0800
From:	"Jesse Brandeburg" <jesse.brandeburg@...il.com>
To:	"Jeff V. Merkey" <jmerkey@...eranetworks.com>
Cc:	"Linux kernel" <linux-kernel@...r.kernel.org>,
	"NetDEV list" <netdev@...r.kernel.org>
Subject: Re: e1000 driver 2.6.18 - how to waste processor cycles

included netdev...

On 11/8/06, Jeff V. Merkey <jmerkey@...eranetworks.com> wrote:
>
> Is there a good reason the skb refill routine in e1000_alloc_rx_buffers
> needs to go and touch and remap skb memory
> on already loaded descriptors/  This seems extremely wasteful of
> processor cycles when refilling the ring buffer.
>
> I note that the archtiecture has changed and is recycling buffers from
> the rx_irq routine and when the routine is called
> to refill the ring buffers, a lot of wasteful and needless calls for
> map_skb is occurring.

we have to unmap the descriptor (or at least do
pci_dma_sync_single_for_cpu / pci_dma_sync_single_for_device) because
the dma API says we can't be guaranteed the cacheable memory is
consistent until we do one of the afore mentioned pci dma ops.

we have to do *something* before we access it.  Simplest path is to
unmap it and then recycle/map it.

If you can show that it is faster to use pci_dma_sync_single_for_cpu
and friends I'd be glad to take a patch.

Hope this helps,
  Jesse
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