USB transfers in device drivers

[O'Connor, Daniel]

> On 2 May 2019, at 18:06, Hans Petter Selasky <hps at selasky.org> wrote:
> 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?
> 
> 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.

OK, thanks.
To be honest I would much prefer to work out why this particular hardware & software seem to drop the ball for such a long time - 50msec without the kernel getting to schedule something (on a basically idle system) is quite perplexing to me.

>>> 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.

OK, yes, has to be reliable :)

>> 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.

I wondered about this too, probably easier than instrumenting the EHCI driver I suppose.

Sadly I don't have a USB analyser and even if I did the system in question is in another country :(

--
Daniel O'Connor
"The nice thing about standards is that there
are so many of them to choose from."
 -- Andrew Tanenbaum