netdev - RE: Dealing with limited resources and DMA Engine copies

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Date:	Wed, 20 Feb 2008 13:01:01 -0000
From:	"Sosnowski, Maciej" <maciej.sosnowski@...el.com>
To:	<olof@...om.net>
Cc:	<netdev@...r.kernel.org>,
	"Nelson, Shannon" <shannon.nelson@...el.com>,
	"Williams, Dan J" <dan.j.williams@...el.com>
Subject: RE: Dealing with limited resources and DMA Engine copies

>---------- Original message ----------
>From: Olof Johansson <olof@...om.net>
>Date: Feb 14, 2008 3:38 AM
>Subject: Dealing with limited resources and DMA Engine copies
>To: shannon.nelson@...el.com, dan.j.williams@...el.com
>Cc: netdev@...r.kernel.org
>
>Hi,
>
>My DMA Engine has a limited resource: It's got a descriptor ring, so
>it's not always possible to add a new descriptor to it (i.e. it might
be
>full). While allocating a huge ring will help, eventually I'm sure I
>will hit a case where it'll overflow.
>
>I thought this was going to be taken care of automatically by the fact
>that you return your max(?) number of descriptors in the channel
>allocation function, but it looks like that value is discarded in
>dma_client_chan_alloc().
>
>So, I just got a couple of spurious:
>dma_cookie < 0
>dma_cookie < 0
>
>...on the console and the connection terminated. Looks like that came
>from tcp_recvmsg(). Ouch.
>
>How about falling back to the cpu-based copy in case of failure? Or
would
>you prefer that I sleep locally in my driver and wait on a descriptor
>slot to open up?
>

I have taken a closer look at this in the code. It seems to be a good
idea to withdraw for a while from ioat-dma copy in case of "dma_cookie <
0" error to let the ring free some descriptors.
I will be able to work on it next week probably. As soon as I have some
stable results, I will get back to you with it.

Maciej


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