Re: I need advice with UPS connection. (ping)

[Filipe Laíns]

On Mon, 2021-11-22 at 15:13 -0500, Alan Stern wrote:
> On Mon, Nov 22, 2021 at 11:25:26AM -0500, David Niklas wrote:
> > Ok, I first edited the kernel to return -ENOMEM like you suggested but
> > the UPS still disconnected. I then edited it again to re-add the 1060
> > byte request and the UPS still disconnected.
> > 
> > I'm attaching the usbmon traces.
> > If you need any additional info I'll do my best to provide it.
> 
> Holy cow!  I just realized what's going on.  And these little changes 
> we've been messing around with have nothing to do with it.
> 
> For the first time, I looked at the timestamps in the usbmon traces.  It 
> turns out that the disconnects occur several seconds after the kernel 
> retrieves the HID report descriptor from the device.  Under normal 
> conditions we would expect to see report packets coming in from the 
> device, starting just a fraction of a second after the descriptor is 
> received.  But that isn't happening in the Linux traces, whereas it does 
> happen in the Windows pcap log.
> 
> I would guess that the UPS is programmed to disconnect itself 
> electronically from the USB bus if it doesn't get any requests for 
> reports within a couple of seconds.  That certainly would explain what 
> you've been seeing.  I can't imagine why it would be programmed to 
> behave this way, but companies have been known to do stranger things.
> 
> As for why the kernel doesn't try to get the reports...  That's a little 
> harder to answer.  Maybe Jiri or Benjamin will know something about it.  
> 
> The UPS's vendor ID is 0d9f (POWERCOM) and the product ID is 0004.  Now, 
> the drivers/hid/hid-quirks.c file contains a quirk entry for 0d9f:0002 
> (product POWERCOM_UPS), which is probably an earlier model of the same 
> device, or a very similar device.  This quirk entry is in the 
> hid_ignore_list; it tells the HID core not to handle the device at all.
> 
> I don't know why that quirk entry is present, and furthermore, it can't 
> directly affect what is happening with your device because the product 
> IDs are different.  Still, it is an indication that something strange is 
> going on behind the scenes.
> 
> Perhaps there is no kernel driver for these UPS devices?  Perhaps the 
> intention is that some user program will handle all the communication 
> when one of them is detected?  A quick search on Google turns up 
> usbhid-ups, part of Network USB Tools (NUT) -- maybe you need to 
> install that package in order to use the device.
> 
> I don't know what the full story is.  With luck, Jiri or Benjamin can 
> help more.
> 
> Alan Stern

hid-generic should be able to handle these devices, but UPSes are much less
tested than normal input peripherals, so it's not that surprising that there may
be some unexpected weirdness. FWIW, I have two UPSes, one works OOTB and the
other doesn't, I have been meaning to investigate the issue. However, David's
case seems to me like a hardware quirk, but that's just my guess.

Cheers,
Filipe Laíns

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