Re: [PATCH] HID: hid-input: avoid splitting keyboard, system and consumer controls

Hi Benjamin,

On Thu, Jan 14, 2021 at 10:23:02AM +0100, Benjamin Tissoires wrote:

Hi Dmitry,

On Thu, Jan 14, 2021 at 7:24 AM Dmitry Torokhov
[off-list ref] wrote:

quoted

A typical USB keyboard usually splits its keys into several reports:

- one for the basic alphanumeric keys, modifier keys, F<n> keys, six pack
  keys and keypad. This report's application is normally listed as
  GenericDesktop.Keyboard
- a GenericDesktop.SystemControl report for the system control keys, such
  as power and sleep
- Consumer.ConsumerControl report for multimedia (forward, rewind,
  play/pause, mute, etc) and other extended keys.
- additional output, vendor specific, and feature reports

Splitting each report into a separate input device is wasteful and even
hurts userspace as it makes it harder to determine the true capabilities
(set of available keys) of a keyboard, so let's adjust application
matching to merge system control and consumer control reports with
keyboard report, if one has already been processed.

Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
---
 drivers/hid/hid-input.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/drivers/hid/hid-input.c b/drivers/hid/hid-input.c
index f797659cb9d9..df45d8d07dc2 100644
--- a/drivers/hid/hid-input.c
+++ b/drivers/hid/hid-input.c

@@ -1851,6 +1851,16 @@ static struct hid_input *hidinput_match_application(struct hid_report *report)
        list_for_each_entry(hidinput, &hid->inputs, list) {
                if (hidinput->application == report->application)
                        return hidinput;
+
+               /*
+                * Keep SystemControl and ConsumerControl applications together
+                * with the main keyboard, if present.
+                */
+               if ((report->application == HID_GD_SYSTEM_CONTROL ||
+                    report->application == HID_CP_CONSUMER_CONTROL) &&
+                   hidinput->application == HID_GD_KEYBOARD) {

I am not fundamentally against the patch, but I think that if the
device exposes first a HID_CP_CONSUMER_CONTROL and then a
HID_GD_KEYBOARD we will end up with 2 different input nodes. We likely
need to "convert" HID_GD_SYSTEM_CONTROL and HID_CP_CONSUMER_CONTROL to
HID_GD_KEYBOARD when creating the hidinput.

While it is technically possible that consumer control or system control
comes first, before main keyboard report, in reality all keyboards that
I have seen so far have the main keyboard report first, so I opted not
to handle the uncommon case to keep the code simple.

I could add the above as a comment, and we could wait to see if there
are devices that are exceptions to the common practice, or I can go and
try to implement the conversion if you feel it is required. Please let
me know.

Note that we will still end up with 2+ input nodes if a device uses
several USB interfaces, but we can't really do much about such cases
(well, short of having a specialized driver claiming additional
interfaces while probing the first one, but that is really outside of
scope of this patch).

Thanks!

-- 
Dmitry