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. 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. > > > 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. > > > > > 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. > > > > 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. > > > > > > 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. > > Benjamin, what do you think? My point of view is: - don't add a new kernel API - 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. 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. But then, applications will have to learn how to listen to the HID++ node, especially given that they are talking HID++ in the first place. Cheers, Benjamin
