On 02/04/2013 04:11 PM, Linus Walleij wrote:
On Mon, Feb 4, 2013 at 9:33 PM, Mark Brown
<broonie@xxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
On Mon, Feb 04, 2013 at 09:29:46PM +0100, Linus Walleij wrote:
On Mon, Feb 4, 2013 at 8:22 PM, Cyril Chemparathy <cyril@xxxxxx> wrote:
Based on our experience with fitting multiple subsystems on top of this
DMA-Engine driver, I must say that the DMA-Engine interface has proven
to be a less than ideal fit for the network driver use case.
The first problem is that the DMA-Engine interface expects to "push"
completed traffic up into the upper layer as a part of its callback.
This doesn't fit cleanly with NAPI, which expects to "pull" completed
traffic from below in the NAPI poll. We've somehow kludged together a
solution around this, but it isn't very elegant.
I cannot understand the actual technical problem from the above
paragraphs though. dmaengine doesn't have a concept of pushing
nor polling, it basically copies streams of words from A to B, where
A/B can be a device or a buffer, nothing else.
The thing you're looking for sounds more like an adapter on top
of dmaengine, which can surely be constructed, some
drivers/dma/dmaengine-napi.c or whatever.
Broadly speaking what NAPI wants is to never get any callbacks from the
hardware (or DMAs). It wants to wake up periodically, take a look at
what packets have been read by the hardware and process them. The goal
is to have the DMAs sitting and running without disturbing the processor
at all after the first packet has been handled.
OK we should definately be able to encompass that in dmaengine
quite easily.
So I think the above concerns are moot. The callback we can
set on cookies is entirely optional, and it's even implemented by
each DMA engine, and some may not even support it but *require*
polling, and then it won't even be implemented by the driver.
Which probably stems from the original design of the dmaengine
API, which was for TCP networking acceleration, mainly.
Cyril, just stack up the cookies and take a sweep over them to see
which ones are baked when the NAPI poll comes in -> problem
solved.
You're assuming that cookies complete in order. That is not necessarily
true.
Thanks
-- Cyril.
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