Re: [Libusb-win32-devel] How Fast?

[Dan E. <Dan...@di...>]

I haven't seen any example code, I just worked it out from the API.
Basically you need 2 buffers and flags to mark if the buffer is in
transit or not, and when a buffer becomes full (we typically use 16k)
you call usb_submit_async, and then if the other buffer was in transit,
wait for it to become free by calling usb_reap_async so that it can be
filled again.

I'm not personally aware of any high speed host controllers which can't
achieve that speed - i.e. no we didn't use anything special.=20

Dan.

Brad Kingston wrote:
> Dan,
>=20
> Thanks for the prompt reply. Is there an example on the website, or
> somewhere else, of the double buffering and async call technique?=20
>=20
> The target hardware we are trying to communicate with sounds similar
> to your setup where we have an FPGA consuming the data via DMA,
> except it is not using NetChip hardware. I don't know what USB
> hardware it is using as it is being custom built by a subcontractor.
> It is good to know that such high rates are possible, since that is
> what I'm trying to prove to them.    =20
>=20
> Did you use any special host controller hardware on the PC in order
> to get the 260Mbps rate? I read something in the list archives that
> some controllers are slower than others. =20
>=20
> Thanks.
> Brad
>=20
>=20
>> From: "Dan Ellis" <Dan...@di...>
>> Reply-To: lib...@li...
>> To: <lib...@li...>
>> Subject: Re: [Libusb-win32-devel] How Fast?
>> Date: Fri, 17 Nov 2006 13:42:57 -0000
>>=20
>> Brad Kingston wrote:
>>> What is the practical sustained speed limit I should expect when
>>> using the usb_bulk_read and usb_bulk_write calls? I'm seeing a limit
>>> of about 20MB/sec when talking to a PXI NetChip peripheral card and
>>> another custom board. I'm not sure if its the libusb side or the
>>> peripheral side that is limiting me to 20MB/sec.
>>>=20
>>> In a tight loop that sends 1GB of data, I send 150MB in each
>>> usb_bulk_write call in order to get up to the 20MB/sec speed I'm
>>> seeing. As I drop the size down for each write, the sustained rate
>>> decreases also.
>>=20
>> We've managed to get sustained 260Mbps (32.5MB/s) doing bulk writes
>> to a NetChip NET2272 with it operating in 16 bit DMA mode (and using
>> libusb with 2 buffers and async transfers). We had an FPGA consuming
>> the data from the DMA. I don't think the NetChip could achieve that
>> rate on the PCI card.=20
>>=20
>> We never tried bulk reads.
>>=20
>> The bus won't accommodate more traffic than that - there was no
>> NAKing going on, the gaps between the transactions looked on the
>> analyzer to be about long enough for the data to be read over PCI
>> ready for the next transaction, so I think PCI is the limiting
>> factor. It's possible that a read could get more data bandwidth out
>> of the bus.=20
>>=20
>> In 8 bit mode this drops to 180Mbps.
>>=20
>>> From a software perspective, it's quite important to use double
>> buffering and the async calls so that you keep the controller busy
>> with work to do. On Linux which doesn't have an aynchronous
>> interface to libusb (yet!) I'm struggling to get more than 140Mbps.
>>=20
>> --
>> Dan Ellis
>> Senior Development Engineer
>> DisplayLink (UK) Ltd.
>>=20
>>=20
>>
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