Re: Questions comparing 1394 and Ethernet [was Re: Recommended 1394 chipsets?]

Steve Kinneberg wrote:

>I don't think that SCSI replies will ever get matched directly to the
>inteded buffer by the hardware. The base 1394 protocol has absolutely
>no knowledge of SCSI anything. SBP-2 is solely responsible for SCSI <->
>1394 translation. In effect, SBP-2 tunnels SCSI transactions through
>the 1394 link.
>
>To give you an idea how FireWire relates to other protocols that use it,
>consider the following.
>
>In a normal SCSI setup, you have the block layer sitting on top of the
>SCSI protocol layer, which in turn sits on top of the SCSI hardware
>driver which talks to the real SCSI device.
>
>In a FireWire environment, the SBP-2 protocol driver takes the place of
>the SCSI hardware level driver and sits on top of the core IEEE 1394
>driver. The core IEEE 1394 driver then sits on top of the hardware
>level driver (in this case the OHCI driver).
>
>The same goes for the eth1394 driver. It takes the place of a NIC
>driver and sits on top of the core IEEE 1394 driver.
>
>
Thanks for the overview. I took a look at sbp2.c; looks like the SCSI
midlayer scatter/gather lists are passed along to the 1394 driver
without extra copies during block transfers. So, even though 1394
doesn't speak SCSI, it seems to support some higher-level demultiplexing
during DMA.

However, eth1394.c does perform a few copies on receive. Linux doesn't
yet have a coherent zero-copy/domain transfer API story, so it's
challenging to take full advantage of NIC features on receive.
Performance would probably improve if precise layering is relaxed, and
TCP/UDP flow IDs passed down to the 1394 link layer.

Looks like 1394 might be fun to play with, even though it doesn't work
out of the box for what I originally wanted to do.

Alan

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Received on Sun Jun 6 01:41:40 2004