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

[Nick Garnett <nickg at ecoscentric dot com>]

"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.

> 
> I guess I may DMA the frame directly out - spinning on completion. Ouch!

No need to spin or even wait in the send() function. You just return
once the transmission has been started. Eventually an interrupt will
indicate completion, and cause the deliver function to be called, in
which you call tx_done().


-- 
Nick Garnett                                 eCos Kernel Architect
http://www.ecoscentric.com            The eCos and RedBoot experts