Re: DMA capable Ethernet driver... (zero-copy)?

[Gary Thomas <gary at mlbassoc dot com>]

Nick Garnett wrote:
> "Lars Povlsen" <lpovlsen@vitesse.com> writes:
>
> > Hello!
> >
> > On working on a.k.a. Ethernet for eCos I was initially triggered by
> > seeing a stack-allocated Ethernet header (redboot/stand-alone) to
> > investigate other Ethernet drivers... I was sad to see only copying
> > send() implementations until I saw the following comment in the eCos
> > docs:
> >
> > "Note: In future, this function may be extended so that the data need
> > not be copied by having the function return a "disposition" code (done,
> > send pending, etc). At this point, you should move the data to some
> > "safe" location before returning."
> >
> > Is this really the state of affairs? Copy no matter that the hardware
> > may have all kinds of fancy scatter/gather frame DMA?
>
>
> Direct DMA from the buffers should work since they should not be
> released until the driver calls the tx_done() function. A key is
> passed back and forth so that the calls can be properly matched. As an
> example the Intel 82544 driver does direct DMA transmission.
>
> I have to admit that I'm not entirely sure what that note means, since
> the functionality it describes is already mostly present in the
> send()/tx_done() interface. Perhaps Gary, who probably wrote it
> several years ago, could explain what he meant.

The comment was mostly an indication of what the hardware driver
*could* do.  As Nick said, the framework is in place to let the
driver do whatever it wants with the data.  If you want to use
zero-copy and your hardware can handle it, the 'key' with callback
mechanism allows for the data buffers to be held (not reused) by
the stacks until the transmission is complete.  Achieving zero
copy on input is a bit trickier and the framework doesn't quite
cut it - there would need to be a way to create incoming buffers
before hand that the driver could be instructed to use and the
receive callback would then just indicate that the data had arrived.
At this time though, the middle layer (eth_drv.c and friends) does
not support this.


--
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Gary Thomas                 |  Consulting for the
MLB Associates              |    Embedded world
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