On Tue, Aug 06, 2024 at 09:12:24PM -0700, Dmitry Torokhov wrote:

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2. Can we consider using something other than EV_KEY? For example we
could define EV_MSC/MSC_PROG_KEY and EV_MSC/MSG_PROG_VAL pair to allow
transmitting key number and state of keys that do not have pre-defined
meaning. Here we are losing event deduplication and ability to query
full keyboard state, but I wonder how important that is for the devices
in question.

The same problem rears its head in the EV_ABS and EV_REL range, so
fixing it for EV_KEY doesn't necessarily fix it for those.

MSC_PROG_KEY/VAL pairs would make it difficult to send two button
updates in the same frame without an SYN_MT_REPORT equivalent.

I do not think that frame notion is that important for keys. It is
typically important for a pointing device state.

true, I remember a conversation years back that frames aren't
consistently implemented in keyboard drivers anyway which is the reason
libinput sends (most) EV_KEY events immediately instad of waiting for a
SYN_REPORT.

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All in all, we've had people using this patch (but increasing KEY_MAX to a
whopping 0x4ff) for the past few years with no adverse effects. I've been
using a custom Linux kernel with this patch on my Arch machine since about
May, and didn't notice anything, even when compiling with debug flags and
following and filtering dmesg.

So here's the thing I'm most curious about. Is this something, you'd just
want to resolve differently, to make it nicer and more logical, or is this
really something that would break everything and doing it in this way will
never be allowed/merged? That would make a lot of us sad :(

We need to figure out not only how to handle your class of devices, but
also allow extending number of keys that do have certain semantic
meaning. Peter raised a lot of questions that we need to answer.

But I wonder, these devices with large number of buttons that do not
have predefined meaning - do they have to be a single input device? Can
we create N input devices if we exceed the "trigger happy" range, all of
them mapping to "trigger happy"? That would mean that userspace would
keep track of key assignment on per-device basis.

We already split HID devices on per-apllication (not userspace but HID
application) basis, and also when there are several USB interfaces.

Honestly, I'd vote against this.
re-combining input devices into a single device in userspace is a pain.
The split per application in HID is mostly fine because they're
usually physically different devices but I recently ran into the issue
with the uclogic drivers where various features are split across
event nodes. Thse devices have the ring on one event node, the buttons
on another, etc. Nothing in (my) userspace is currently set up for this
and it'd require a major rework in many places to be able to handle this
properly. And it requires that rework in every userspace stack, possibly
special-cased on a vendor id. In the end it was easier (re-)writing BPFs
to get the expected event node behaviour than dealing with the split.

A device that arbitrarily splits makes this even more difficult - which
one of the event nodes has buttons 1-20 and which one has 21-40? We'd need
some other magic somewhere (e.g. hiding in uniq) and some digging around
in udev to figure out which ones are part of the same device.

I'd rather not go with a simple-for-now solution that makes everything
in userspace more complicated, forever.

Cheers,
  Peter