On 7/2/20 3:01 am, Benjamin Tissoires wrote:

On Thu, Feb 6, 2020 at 4:42 PM Filipe Laíns [off-list ref] wrote:

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On Thu, 2020-02-06 at 13:13 +0100, Hans de Goede wrote:

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Hi,

On 2/6/20 12:51 PM, Filipe Laíns wrote:

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On Thu, 2020-02-06 at 12:30 +0100, Hans de Goede wrote:

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HI,

On 2/6/20 12:14 PM, Filipe Laíns wrote:

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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.

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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.

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.

Cheers,
   Peter