Re: Make the hid-logitech-dj driver remove the HID++ nodes when the device disconnects

["Filipe Laíns" <lains@archlinux.org>]

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On Fri, 2020-02-07 at 10:03 +1000, Peter Hutterer wrote:
> On 7/2/20 3:01 am, Benjamin Tissoires wrote:
> > On Thu, Feb 6, 2020 at 4:42 PM Filipe Laíns <lains@archlinux.org> wrote:
> > > On Thu, 2020-02-06 at 13:13 +0100, Hans de Goede wrote:
> > > > Hi,
> > > > 
> > > > On 2/6/20 12:51 PM, Filipe Laíns wrote:
> > > > > On Thu, 2020-02-06 at 12:30 +0100, Hans de Goede wrote:
> > > > > > HI,
> > > > > > 
> > > > > > On 2/6/20 12:14 PM, Filipe Laíns wrote:
> > > > > > > Hello,
> > > > > > > 
> > > > > > > Right now the hid-logitech-dj driver will export one node for each
> > > > > > > connected device, even when the device is not connected. That causes
> > > > > > > some trouble because in userspace we don't have have any way to know if
> > > > > > > the device is connected or not, so when we try to communicate, if the
> > > > > > > device is disconnected it will fail.
> > > > > > 
> > > > > > I'm a bit reluctant to make significant changes to how the
> > > > > > hid-logitech-dj driver works. We have seen a number of regressions
> > > > > > when it was changed to handle the non unifying receivers and I would
> > > > > > like to avoid more regressions.
> > > > > > 
> > > > > > Some questions:
> > > > > > 1. What is the specific use case where you are hitting this?
> > > > > 
> > > > > For example, in libratbag we enumerate the devices and then probe them.
> > > > > Currently if the device is not connected, the communication fails. To
> > > > > get the device to show up we need to replug it, so it it triggers udev,
> > > > > or restart the daemon.
> > > > 
> > > > Thanks, that is exactly the sort of context to your suggested changes
> > > > which I need.
> > > > 
> > > > > > 2. Can't the userspace tools involved by modified to handle the errors
> > > > > > they are getting gracefully?
> > > > > 
> > > > > They can, but the approaches I see are not optimal:
> > > > >     - Wait for HID events coming from the device, which could never
> > > > > happen.
> > > > >     - Poll the device until it wakes up.
> > > > 
> > > > I guess we do get some (other or repeated?) event when the device does
> > > > actually connect, otherwise your suggested changes would not be possible.
> > > 
> > > No, I was thinking to just send the HID++ version identification
> > > routine and see if the device replies.
> > 
> > Hmm, to continue on these questions:
> > - yes, the current approach is to have the users of the HID++ device
> > try to contact the device, get an error from the receiver, then keep
> > the hidraw node open until we get something out of it, and then we can
> > start talking to it
> > - to your question Hans, when a device connects, it emits a HID++
> > notification, which we should be relaying in the hidraw node. If not,
> > well then starting to receive a key or abs event on the input node is
> > a pretty good hint that the device connected.
> > 
> > So at any time, the kernel knows which devices are connected among
> > those that are paired, so the kernel knows a lot more than user space.
> > 
> > The main problem Filipe is facing here is that we specifically
> > designed libratbag to *not* keep the device nodes opened, and to not
> > poll on the input events. The reason being... we do not want libratbag
> > to be considered as a keylogger.
> 
> I'm wondering - can we really get around this long-term? Even if we have 
> a separate HID++ node and/or udev change events and/or some other 
> notification, in the end you still have some time T between that event 
> and userspace opening the actual event node. Where the first key event 
> wakes up the physical keyboard, you're now racing.

Yes but it doesn't really matter in this case. We would only be
potentially losing HID++ events, which are not that important, unlike
normal input events. In fact, libratbag does not care about HID++
events, they are just ignored.

We would still have the same issue, yes, except here we don't really
care.

> So the separate HID++ node works as long as libratbag *only* listens to 
> that node, as soon as we need to start caring about a normal event it 
> won't work any longer.

You mean when libratbag starts caring about normal input events? What
is the point of that? Why would we need to do that? Also, as Benjamin
pointed out, that would classify as a keylogger.

Cheers,
Filipe Laíns 

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