Hi,
On 2/6/20 6:01 PM, Benjamin Tissoires wrote:
On Thu, Feb 6, 2020 at 4:42 PM Filipe Laíns <lains@xxxxxxxxxxxxx> 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.
Ack.
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.
Ack.
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?
Yes, that is a good idea :) I did not know this was possible but it
seems like a better approach.
Not a big fan of that idea personally. This will add yet an other
kernel API that we have to maintain.
On Filipe's side, the hotplug support is something that has been
around for quite a long time now, so we can safely expect applications
to handle it properly.
The suggested udev event change would just require a small change
to the existing hotplug handling, currently it responds to udev
"add" and "remove" events. With my suggested change in the "add"
path it will get an error because the device is not connected and
then stop adding the device. Combine this with treating "change"
events as "add" events and that is all that has to change on the
libratbag side.
This assumes that duplicate add events are already filtered out,
which one has to do anyways to avoid coldplug enumeration vs
hotplug races.
As for yet another kernel API to maintain, udev change events
already are an existing kernel API, what would be new is the hidpp
driver; and just the hidpp driver emitting them.
All that is needed on the kernel side for this is to make the following
call when we detect a device moves from the paired to the connected state:
kobject_uevent(&hdev->dev.kobj, KOBJ_CHANGE);
And there even seems to be a precedent for this, drivers/hid/hid-wiimote-core.c
already does this for what seems to be similar reasons.
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.
Filipe is correct here.
For unifying devices, we can have up to 6 devices paired to a
receiver, 3 can be used at the same time (connected).
For the cheap receivers, we can enumerate 2 paired devices, but they
are not necessarily connected too.
Historically, when I first wrote the hid-logitech-hidpp driver, I
wanted to not export a non connected device. But as mentioned by
Filipe, this was posing issues mainly for keyboards, because generally
the first thing you type on a keyboard is your password, and you don't
necessarily have the feedback to see which keys you typed.
So we (Nestor and I) decided to almost always create the input nodes
when the device was not connected. The exceptions are when we need
some device communication to set up the input node: so just for the
touchpads.
Ok.
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.
That is correct. Paired doesn't mean connected.
We create nodes for all paired devices, no matter if they are connected
or not.
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.
OTOH, this is what we have been trying to do in the kernel for years
now: have one single node per application/usage, so we can rely on
some valid data from the user space.
I don't think the complexity of the driver should be a problem here.
Yes, it's a complex one, but introducing a new API for that is a no
from me.
udev change events are not "adding a new API" there are a well known
API using e.g. for monitor plug/unplug in the drm subsys, etc. Yes
using them in the HID subsys this way is somewhat new.
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.
Yes, I don't know about the application details (I'll have to find out
:P) but it makes more sense to me. It avoids breaking the userspace
behavior.
The udev change doesn't "break" userspace, but it is a new API. And
that means nightmare from the application point of view:
How do they know that the new API will be used? There is a high chance
they won't, so for backward compatibility they will start listening to
the hidraw node to match the current kernel behavior, and then we
would just have added a new API for nothing.
I agree that finding out if the udev change events are supported
is a bit of a challenge from userspace.
But if I understood you correctly, then libratbag currently does
not keep listening to detect the connect, but rather atm this just
does not work, in which case it does not need to know if the new API
is there it can just assume; and even if it does need to know it
check the kernel-version number for that. Not pretty but that is
e.g. what libusb does to detect if certain "undetectable" features
are there, which admittedly is not ideal.
Benjamin, what do you think?
My point of view is:
- don't add a new kernel API
Again I believe calling udev change events "a new kernel API"
is exaggerating things a bit.
- rely on existing and supported user space behavior
- ultimately, teach user space how to deal with the current situation
So right now I think Filipe's proposal is the best bad solution. I
would rank the udev event as worse than Filipe's solution because that
involves both userspace and kernel space changes.
The udev solution might require changes on both sides, but they
are very small easily reviewable changes. Anyways as I said this
is your call.
However, the proposal to add/remove the HID++ hidraw node on
connect/disconnect really doesn't appeal to me because I am pretty
sure we will have the same kind of issues that we are facing with
keyboards. There might be an application that listens to the connect
HID++ notification and turns the light on in the room whenever the
mouse reconnects (and turns it off when the mouse disconnects because
that means you left the room).
So right now, as I am writing this, I think we should split the HID++
node into its own hidraw node. This will allow application to listen
to this node without being a keylogger as we will be filtering the key
events in the actual input and the other hidraw nodes.
