Dealing with limited resources and DMA Engine copies

Dealing with limited resources and DMA Engine copies

[Olof Johansson]

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?


-Olof

RE: Dealing with limited resources and DMA Engine copies

[Sosnowski, Maciej]

>---------- Original message ----------
>From: Olof Johansson <olof@lixom.net>
>Date: Feb 14, 2008 3:38 AM
>Subject: Dealing with limited resources and DMA Engine copies
>To: shannon.nelson@intel.com, dan.j.williams@intel.com
>Cc: netdev@vger.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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