Re: [9fans] usb serial on raspberry pi (again)

[Richard Miller <9fans@hamnavoe.com>]

> I cannot get the usb serial interfaces we use to work. I have never managed to get
> usb serial adapters to work with the raspberry pi, though never had problems on x86 boxen.
> ...
>
> Text into the pi works fine, but outgoing text seems stalled until text is received,
> I can type ahead and when I receive data the queued outgoing data is sent.

Hi Steve -

I just had a look at this (as a bit of displacement activity from more
stressful tasks...).  First of all I dug out an old silabs usbserial
adapter and quickly confirmed that I see the same symptoms as you.
(I had never thought of using a serial dongle on a pi, since the
uart gpio pins have always served my purposes - I always connect
to FPGAs or other microprocessors which are happy with 3.3V)

By watching changes in the file descriptor r/w count in the 'ps' output
for the usbd subprocess, I could see that the first byte of data was
being written to the usb ep, but the write was hanging in the kernel
till the next read completion.  I noted that this particular dongle
uses the same ep number for both input and output, which is unusual
(and was never properly allowed for in the original design of plan 9 usb).

Next I looked in the bcm usb adapter driver (usbdwc.c) for possible
interference between reads and writes on the same ep number, and
the answer was glaringly obvious.  Each ep has a single Epio structure,
shared by read and write sides, with a QLock in it.  My failure of vision.

The fix isn't as obvious (to me) as the diagnosis.  Maybe it's as simple
as having separate read and write QLocks.  I need to study the code for
a while to assure myself that it's safe to do that.  I wrote it so long
ago that I frankly can't recall why there's a lock at all.

You're welcome to have a look too and offer suggestions!

-- Richard