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

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Date:	Thu, 09 Nov 2006 01:46:52 -0700
From:	"Jeffrey V. Merkey" <jmerkey@...eranetworks.com>
To:	Jesse Brandeburg <jesse.brandeburg@...il.com>
CC:	"Jeff V. Merkey" <jmerkey@...eranetworks.com>,
	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

Jesse Brandeburg wrote:

> 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.

In the case I am referring to, the memory is already mapped with a 
previous call, which means it may be getting
mapped twice.

Jeff

>
> 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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