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.
So how about if we trigger a udev change event on the hid device instead
when this happens ? That seems like a less invasive change on the kernel
side and then libratbag could listen for these change events?
3. Is there a bugreport open about this somewhere?
Yes, https://github.com/libratbag/libratbag/issues/785
The reason we do this is because otherwise we would loose the first
packets when the device is turned on by key press. When a device is
turned on we would have to create the device node, and the packets
received while we are creating the device node would be lost.
I don't believe that this is the reason, we only create hid child
devices for devices reported by the receiver, but some of the non
unifying hid receiver send a list of all devices paired, rather
then the ones which are actually connected at that time.
IIRC from my chats with Benjamin and Peter this is the reason, but
please correct me if I'm wrong.
Could be that we can distinguish between "paired" and "connected"
and that we are enumerating "paired" but not (yet) "connected"
devices already because of what you say, I've not touched this
code in a while.
This could solved by buffering those packets, but that is a bad solution as
it would mess up the timings.
At the moment the created node includes both normal HID and vendor
usages. To solve this problem, I propose that instead of creating a
single device node that contains all usages, we create one for normal
HID, which would exist all the time, and one for the vendor usage,
which would go away when the device disconnects. >
This slight behavior change will affect userspace. Two hidraw nodes
would be created instead of one. We need to make sure the current
userspace stacks interfacing with this would be able to properly handle
such changes.
What do you think of this approach? Anyone has a better idea?
The suggested approach sounds fragile and like it adds complexity to
an already not simple driver.
I understand, that is totally reasonable. I am working on a CI for the
driver if that helps.
It would be helpful to first describe the actual problem you are trying
to fix (rather then suggesting a solution without clearly defining the
problem) and then we can see from there.
I though I described it good enough in the first paragraph but I guess
not, sorry. You should be able to get it from my comments above, if not
please let me know :)
No problem, I have enough context now. I personally like my udev change
event idea, which seems more KISS. But ultimately this is Benjamin's call.
Regards,
Hans
