USB transfers in device drivers

[Hans Petter Selasky]

On 2019-05-02 10:22, O'Connor, Daniel wrote:
> 
> 
>> On 2 May 2019, at 06:15, Hans Petter Selasky <hps at selasky.org> wrote:
>> On 2019-05-01 10:34, O'Connor, Daniel wrote:
>>> I don't have a solid hypothesis for the failures as yes but one thing I'd like to make sure is that the USB stack is keeping the USB hardware busy with pending requests - does anyone know if the USB FIFO code does that automatically?
>>
>> Only the XHCI driver supports HW based double buffering of BULK transfers.
> 
> Ahh interesting - is that a ECHI hardware limitation or a driver one?

Hi,

It is an EHCI hardware "limitation". It is possible to queue up more 
jobs with the EHCI, but it ends that you get a race with the hardware 
you'll need to catch. I think it is related to how receiving short 
packets are handled.

> 
>> I suppose you are using BULK. Else you will need to use ISOCHRONOUS transfers.
> 
> Yes it's using bulk transfers.
> 
> I did consider isochronous transfers when I started this project but I wasn't sure if there would be enough bandwidth (but perhaps I read the spec wrong). I imagine there would be enough for this data rate but we have others at higher speeds (eg 35MB/sec).

If you want reliable data transfer, BULK is the best.

> 
> Related to bandwidth - are there any statistics gathered about how busy a port is?

No, but I wanted to add a text-graph frontent to usbdump to collect this 
info realtime. Else use a USB wire-analyzer.

--HPS