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

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

OK, I'll just eat my own words. I think I've only really been testing on
Intel motherboard host controllers, which don't have trouble achieving
260Mb/s. I've just tried an old NEC chip on a PCI plug in card, and it's
only giving me 163Mb/s, so I guess your mileage may vary.

Dan.

Dan Ellis wrote:
> 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.    =20
>=20
> 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
>=20
> Dan.
>=20
> 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
>> 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
>> 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
>> 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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