[Nut-upsdev] Problems with USB-serial converters -- please advise on code fix

[Vadim]

Hello!

I've ran into this annoying issue when setting up a new server. I have an APC 
SmartUPS hooked up on a USB-serial converter on /dev/ttyUSB0.

This works nicely, until:

hub 3-0:1.0: port 1 disabled by hub (EMI?), re-enabling... 
pl2303 ttyUSB0: pl2303 converter now disconnected from ttyUSB0
usb 3-1: pl2303 converter now attached to ttyUSB1

apcsmart gets into an infinite loop of this:
apcsmart[2285]: smartmode: issuing 'Y' failed: Input/output error

while upsd becomes unhappy:
upsd[2311]: Data for UPS [apc] is stale - check driver (because now it can't 
talk to the UPS)

After this, a shutdown usually follows, though my understanding is that this 
should only happen on battery power.

This is very easy to reproduce, just use an USB-serial converter, and pull it 
out of the port.


I've been looking into fixing that and would like some advice on how to 
proceed.

I tried the easiest fix possible: abort on EIO and hope I can get systemd to 
restart the driver. That so far hasn't worked, it results in a series of very 
fast restarts, and systemd giving up shortly.

So I'm going to try to make the driver reconnect on EIO. So far I have made an 
attempt, but it still quits somewhere. It also seems some changes might be 
necessary to the driver API to get this done.

So I'd like some advice first:

Is there any work on this planned already?

Do you have any preferences regarding how this should work? Eg, retry 
connection only after connection has been established? Add a configuration 
parameter to enable this mode?

How should I approach changing the driver API? For instance it seems like a 
upsdrv_initups argument to ensure it won't quit on failure may be needed.

What about other UPS models? This would imply changes in main.c, and affect 
other models, but I can only test my APC SmartUPS. Also this problem probably 
applies to USB UPSes.


Thanks!