Re: [PATCH 1/6] HID: hid-input: Add offhook and ring LEDs for headsets

Re: [PATCH 1/6] HID: hid-input: Add offhook and ring LEDs for headsets

[Benjamin Tissoires]

Hi Maxim,

On Sun, Jul 4, 2021 at 12:02 AM Maxim Mikityanskiy <maxtram95@gmail.com> wrote:
>
> A lot of USBHID headsets available on the market have LEDs that indicate
> ringing and off-hook states when used with VoIP applications. This
> commit exposes these LEDs via the standard sysfs interface.
>
> Signed-off-by: Maxim Mikityanskiy <maxtram95@gmail.com>
> ---
>  drivers/hid/hid-input.c                | 2 ++
>  drivers/input/input-leds.c             | 2 ++
>  include/uapi/linux/input-event-codes.h | 2 ++
>  3 files changed, 6 insertions(+)
>
> diff --git a/drivers/hid/hid-input.c b/drivers/hid/hid-input.c
> index 4286a51f7f16..44b8243f9924 100644
> --- a/drivers/hid/hid-input.c
> +++ b/drivers/hid/hid-input.c
> @@ -798,6 +798,8 @@ static void hidinput_configure_usage(struct hid_input *hidinput, struct hid_fiel
>                 case 0x4b:  map_led (LED_MISC);     break;    /*   "Generic Indicator"        */
>                 case 0x19:  map_led (LED_MAIL);     break;    /*   "Message Waiting"          */
>                 case 0x4d:  map_led (LED_CHARGING); break;    /*   "External Power Connected" */
> +               case 0x17:  map_led (LED_OFFHOOK);  break;    /*   "Off-Hook"                 */
> +               case 0x18:  map_led (LED_RING);     break;    /*   "Ring"                     */
>
>                 default: goto ignore;
>                 }
> diff --git a/drivers/input/input-leds.c b/drivers/input/input-leds.c
> index 0b11990ade46..bc6e25b9af25 100644
> --- a/drivers/input/input-leds.c
> +++ b/drivers/input/input-leds.c
> @@ -33,6 +33,8 @@ static const struct {
>         [LED_MISC]      = { "misc" },
>         [LED_MAIL]      = { "mail" },
>         [LED_CHARGING]  = { "charging" },
> +       [LED_OFFHOOK]   = { "offhook" },

I am pretty sure this also needs to be reviewed by the led folks.
Adding them in Cc.

Cheers,
Benjamin

> +       [LED_RING]      = { "ring" },
>  };
>
>  struct input_led {
> diff --git a/include/uapi/linux/input-event-codes.h b/include/uapi/linux/input-event-codes.h
> index 225ec87d4f22..dd785a5b5076 100644
> --- a/include/uapi/linux/input-event-codes.h
> +++ b/include/uapi/linux/input-event-codes.h
> @@ -925,6 +925,8 @@
>  #define LED_MISC               0x08
>  #define LED_MAIL               0x09
>  #define LED_CHARGING           0x0a
> +#define LED_OFFHOOK            0x0b
> +#define LED_RING               0x0c
>  #define LED_MAX                        0x0f
>  #define LED_CNT                        (LED_MAX+1)
>
> --
> 2.32.0
>

Re: [PATCH 1/6] HID: hid-input: Add offhook and ring LEDs for headsets

[Jiri Kosina]

On Tue, 6 Jul 2021, Benjamin Tissoires wrote:

> > A lot of USBHID headsets available on the market have LEDs that indicate
> > ringing and off-hook states when used with VoIP applications. This
> > commit exposes these LEDs via the standard sysfs interface.
> >
> > Signed-off-by: Maxim Mikityanskiy <maxtram95@gmail.com>
> > ---
> >  drivers/hid/hid-input.c                | 2 ++
> >  drivers/input/input-leds.c             | 2 ++
> >  include/uapi/linux/input-event-codes.h | 2 ++
> >  3 files changed, 6 insertions(+)
> >
> > diff --git a/drivers/hid/hid-input.c b/drivers/hid/hid-input.c
> > index 4286a51f7f16..44b8243f9924 100644
> > --- a/drivers/hid/hid-input.c
> > +++ b/drivers/hid/hid-input.c
> > @@ -798,6 +798,8 @@ static void hidinput_configure_usage(struct hid_input *hidinput, struct hid_fiel
> >                 case 0x4b:  map_led (LED_MISC);     break;    /*   "Generic Indicator"        */
> >                 case 0x19:  map_led (LED_MAIL);     break;    /*   "Message Waiting"          */
> >                 case 0x4d:  map_led (LED_CHARGING); break;    /*   "External Power Connected" */
> > +               case 0x17:  map_led (LED_OFFHOOK);  break;    /*   "Off-Hook"                 */
> > +               case 0x18:  map_led (LED_RING);     break;    /*   "Ring"                     */
> >
> >                 default: goto ignore;
> >                 }
> > diff --git a/drivers/input/input-leds.c b/drivers/input/input-leds.c
> > index 0b11990ade46..bc6e25b9af25 100644
> > --- a/drivers/input/input-leds.c
> > +++ b/drivers/input/input-leds.c
> > @@ -33,6 +33,8 @@ static const struct {
> >         [LED_MISC]      = { "misc" },
> >         [LED_MAIL]      = { "mail" },
> >         [LED_CHARGING]  = { "charging" },
> > +       [LED_OFFHOOK]   = { "offhook" },
> 
> I am pretty sure this also needs to be reviewed by the led folks.
> Adding them in Cc.

Can we please get Ack from the LED maintainers? Thanks.

> 
> Cheers,
> Benjamin
> 
> > +       [LED_RING]      = { "ring" },
> >  };
> >
> >  struct input_led {
> > diff --git a/include/uapi/linux/input-event-codes.h b/include/uapi/linux/input-event-codes.h
> > index 225ec87d4f22..dd785a5b5076 100644
> > --- a/include/uapi/linux/input-event-codes.h
> > +++ b/include/uapi/linux/input-event-codes.h
> > @@ -925,6 +925,8 @@
> >  #define LED_MISC               0x08
> >  #define LED_MAIL               0x09
> >  #define LED_CHARGING           0x0a
> > +#define LED_OFFHOOK            0x0b
> > +#define LED_RING               0x0c
> >  #define LED_MAX                        0x0f
> >  #define LED_CNT                        (LED_MAX+1)
> >
> > --
> > 2.32.0
> >
> 

-- 
Jiri Kosina
SUSE Labs

Re: [PATCH 1/6] HID: hid-input: Add offhook and ring LEDs for headsets

[Dmitry Torokhov]

On Thu, Jul 15, 2021 at 08:57:44PM +0200, Jiri Kosina wrote:
> On Tue, 6 Jul 2021, Benjamin Tissoires wrote:
> 
> > > A lot of USBHID headsets available on the market have LEDs that indicate
> > > ringing and off-hook states when used with VoIP applications. This
> > > commit exposes these LEDs via the standard sysfs interface.
> > >
> > > Signed-off-by: Maxim Mikityanskiy <maxtram95@gmail.com>
> > > ---
> > >  drivers/hid/hid-input.c                | 2 ++
> > >  drivers/input/input-leds.c             | 2 ++
> > >  include/uapi/linux/input-event-codes.h | 2 ++
> > >  3 files changed, 6 insertions(+)
> > >
> > > diff --git a/drivers/hid/hid-input.c b/drivers/hid/hid-input.c
> > > index 4286a51f7f16..44b8243f9924 100644
> > > --- a/drivers/hid/hid-input.c
> > > +++ b/drivers/hid/hid-input.c
> > > @@ -798,6 +798,8 @@ static void hidinput_configure_usage(struct hid_input *hidinput, struct hid_fiel
> > >                 case 0x4b:  map_led (LED_MISC);     break;    /*   "Generic Indicator"        */
> > >                 case 0x19:  map_led (LED_MAIL);     break;    /*   "Message Waiting"          */
> > >                 case 0x4d:  map_led (LED_CHARGING); break;    /*   "External Power Connected" */
> > > +               case 0x17:  map_led (LED_OFFHOOK);  break;    /*   "Off-Hook"                 */
> > > +               case 0x18:  map_led (LED_RING);     break;    /*   "Ring"                     */
> > >
> > >                 default: goto ignore;
> > >                 }
> > > diff --git a/drivers/input/input-leds.c b/drivers/input/input-leds.c
> > > index 0b11990ade46..bc6e25b9af25 100644
> > > --- a/drivers/input/input-leds.c
> > > +++ b/drivers/input/input-leds.c
> > > @@ -33,6 +33,8 @@ static const struct {
> > >         [LED_MISC]      = { "misc" },
> > >         [LED_MAIL]      = { "mail" },
> > >         [LED_CHARGING]  = { "charging" },
> > > +       [LED_OFFHOOK]   = { "offhook" },
> > 
> > I am pretty sure this also needs to be reviewed by the led folks.
> > Adding them in Cc.
> 
> Can we please get Ack from the LED maintainers? Thanks.

I do not think we should be adding more LED bits to the input
subsystem/events; this functionality should be routed purely though LED
subsystem. input-leds is a bridge for legacy input functionality
reflecting it onto the newer LED subsystem.

Thanks.

-- 
Dmitry

Re: [PATCH 1/6] HID: hid-input: Add offhook and ring LEDs for headsets

[Pavel Machek]

[-- Attachment #1: Type: text/plain, Size: 1671 bytes --]

On Thu 2021-07-15 13:39:26, Dmitry Torokhov wrote:
> On Thu, Jul 15, 2021 at 08:57:44PM +0200, Jiri Kosina wrote:
> > On Tue, 6 Jul 2021, Benjamin Tissoires wrote:
> > 
> > > > A lot of USBHID headsets available on the market have LEDs that indicate
> > > > ringing and off-hook states when used with VoIP applications. This
> > > > commit exposes these LEDs via the standard sysfs interface.

> > > > diff --git a/drivers/input/input-leds.c b/drivers/input/input-leds.c
> > > > index 0b11990ade46..bc6e25b9af25 100644
> > > > --- a/drivers/input/input-leds.c
> > > > +++ b/drivers/input/input-leds.c
> > > > @@ -33,6 +33,8 @@ static const struct {
> > > >         [LED_MISC]      = { "misc" },
> > > >         [LED_MAIL]      = { "mail" },
> > > >         [LED_CHARGING]  = { "charging" },
> > > > +       [LED_OFFHOOK]   = { "offhook" },
> > > 
> > > I am pretty sure this also needs to be reviewed by the led folks.
> > > Adding them in Cc.
> > 
> > Can we please get Ack from the LED maintainers? Thanks.
> 
> I do not think we should be adding more LED bits to the input
> subsystem/events; this functionality should be routed purely though LED
> subsystem. input-leds is a bridge for legacy input functionality
> reflecting it onto the newer LED subsystem.

If we do it purely through the LED subsystem, will it get trickier to
associate the devices?

Anyway, it is a headset. What does headset have to do with input
subsystem? Sounds like sound device to me... And we already have a
"micmute" LED which sounds quite similar to the "offhook" LED... no?

Best regards,
								Pavel
-- 
http://www.livejournal.com/~pavelmachek

[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 195 bytes --]

Re: [PATCH 1/6] HID: hid-input: Add offhook and ring LEDs for headsets

[Maxim Mikityanskiy]

On Fri, Jul 16, 2021 at 1:49 AM Pavel Machek <pavel@ucw.cz> wrote:
>
> On Thu 2021-07-15 13:39:26, Dmitry Torokhov wrote:
> > On Thu, Jul 15, 2021 at 08:57:44PM +0200, Jiri Kosina wrote:
> > > On Tue, 6 Jul 2021, Benjamin Tissoires wrote:
> > >
> > > > > A lot of USBHID headsets available on the market have LEDs that indicate
> > > > > ringing and off-hook states when used with VoIP applications. This
> > > > > commit exposes these LEDs via the standard sysfs interface.
>
> > > > > diff --git a/drivers/input/input-leds.c b/drivers/input/input-leds.c
> > > > > index 0b11990ade46..bc6e25b9af25 100644
> > > > > --- a/drivers/input/input-leds.c
> > > > > +++ b/drivers/input/input-leds.c
> > > > > @@ -33,6 +33,8 @@ static const struct {
> > > > >         [LED_MISC]      = { "misc" },
> > > > >         [LED_MAIL]      = { "mail" },
> > > > >         [LED_CHARGING]  = { "charging" },
> > > > > +       [LED_OFFHOOK]   = { "offhook" },
> > > >
> > > > I am pretty sure this also needs to be reviewed by the led folks.
> > > > Adding them in Cc.
> > >
> > > Can we please get Ack from the LED maintainers? Thanks.
> >
> > I do not think we should be adding more LED bits to the input
> > subsystem/events; this functionality should be routed purely though LED
> > subsystem. input-leds is a bridge for legacy input functionality
> > reflecting it onto the newer LED subsystem.

I'm a bit confused by this answer. I wasn't aware that input-leds was
some legacy stuff. Moreover, hid-input only handles LEDs through
input-leds, it doesn't use any modern replacement. So, does your
answer mean I need to implement this replacement? If so, I anticipate
some issues with this approach:

1. hid-input will handle different LEDs in different ways, which will
make code complicated and error-prone. There will be two parallel
implementations for LEDs.

2. A lot of code will be similar with input-leds, but not shared/reused.

3. New driver callbacks may be needed if drivers want to override the
default behavior, like they do with input_mapping/input_mapped.

4. If some hypothetical input device is a headset, but not HID, it
won't be able to reuse the LED handling code. With input-leds it
wouldn't be a problem.

5. (Minor) LED_MUTE is already there. If we handle LED_OFFHOOK and
LED_RING in a different way, it would be confusing.

Let's discuss the architecture before I write any new code, if we are
going to take this way. However, to me, input-leds looks like a better
fit: the implementation is much simpler and follows existing patterns,
and it integrates well with drivers and hid-input. If we throw away
input-leds, we'll have to do its job ourselves, and if we throw it
away only for part of LEDs, the code will likely be ugly.

> If we do it purely through the LED subsystem, will it get trickier to
> associate the devices?

I agree with this point. With the current approach, it's easy to look
up all LEDs of an input device. If the suggested approach makes it
hard, it's a serious drawback.

> Anyway, it is a headset. What does headset have to do with input
> subsystem? Sounds like sound device to me...

That's right, the main function of a headset is of course sound, but
such headsets also have buttons (vol up/down, mic mute, answer the
call) and LEDs (mic muted, ringing, offhook). The sound "subdevice"
(sorry, I'm not really familiar with USB terminology) is handled by
snd-usb-audio, and the buttons/LEDs are handled by usbhid.

Two examples of such headsets are mentioned in commit messages in this
patch series. Such headsets are usually "certified for skype for
business", but of course can be used with a variety of other VoIP
applications. The goal of this series is to provide a standard
interface between headsets and userspace applications, so that VoIP
applications could react to buttons and set LED state, making Linux
more ready for desktop.

> And we already have a
> "micmute" LED which sounds quite similar to the "offhook" LED... no?

These two are different. A typical headset has three LEDs: micmute,
ring and offhook (ring and offhook are often one physical LED, which
blinks in the ring state and glows constantly in the offhook state).

Offhook indicates that a call is in progress, while micmute shows that
the microphone is muted. These two states are orthogonal and may
happen in any combination. On the tested devices, offhook state didn't
affect sound, it was just a logical indication of an active call.

If you are interested in more details, I can describe the behavior of
the headsets that I tested (some info is actually in the commit
messages), but the short answer is that micmute and offhook are two
different physical LEDs with completely different functions.

>
> Best regards,
>                                                                 Pavel
> --
> http://www.livejournal.com/~pavelmachek

Re: [PATCH 1/6] HID: hid-input: Add offhook and ring LEDs for headsets

[Maxim Mikityanskiy]

Dmitry, what's your opinion on the points that I raised? I would like
to progress with this patch set, let's discuss the direction and sum
up the requirements.

On Fri, Jul 16, 2021 at 8:23 PM Maxim Mikityanskiy <maxtram95@gmail.com> wrote:
>
> On Fri, Jul 16, 2021 at 1:49 AM Pavel Machek <pavel@ucw.cz> wrote:
> >
> > On Thu 2021-07-15 13:39:26, Dmitry Torokhov wrote:
> > > On Thu, Jul 15, 2021 at 08:57:44PM +0200, Jiri Kosina wrote:
> > > > On Tue, 6 Jul 2021, Benjamin Tissoires wrote:
> > > >
> > > > > > A lot of USBHID headsets available on the market have LEDs that indicate
> > > > > > ringing and off-hook states when used with VoIP applications. This
> > > > > > commit exposes these LEDs via the standard sysfs interface.
> >
> > > > > > diff --git a/drivers/input/input-leds.c b/drivers/input/input-leds.c
> > > > > > index 0b11990ade46..bc6e25b9af25 100644
> > > > > > --- a/drivers/input/input-leds.c
> > > > > > +++ b/drivers/input/input-leds.c
> > > > > > @@ -33,6 +33,8 @@ static const struct {
> > > > > >         [LED_MISC]      = { "misc" },
> > > > > >         [LED_MAIL]      = { "mail" },
> > > > > >         [LED_CHARGING]  = { "charging" },
> > > > > > +       [LED_OFFHOOK]   = { "offhook" },
> > > > >
> > > > > I am pretty sure this also needs to be reviewed by the led folks.
> > > > > Adding them in Cc.
> > > >
> > > > Can we please get Ack from the LED maintainers? Thanks.
> > >
> > > I do not think we should be adding more LED bits to the input
> > > subsystem/events; this functionality should be routed purely though LED
> > > subsystem. input-leds is a bridge for legacy input functionality
> > > reflecting it onto the newer LED subsystem.
>
> I'm a bit confused by this answer. I wasn't aware that input-leds was
> some legacy stuff. Moreover, hid-input only handles LEDs through
> input-leds, it doesn't use any modern replacement. So, does your
> answer mean I need to implement this replacement? If so, I anticipate
> some issues with this approach:
>
> 1. hid-input will handle different LEDs in different ways, which will
> make code complicated and error-prone. There will be two parallel
> implementations for LEDs.
>
> 2. A lot of code will be similar with input-leds, but not shared/reused.
>
> 3. New driver callbacks may be needed if drivers want to override the
> default behavior, like they do with input_mapping/input_mapped.
>
> 4. If some hypothetical input device is a headset, but not HID, it
> won't be able to reuse the LED handling code. With input-leds it
> wouldn't be a problem.
>
> 5. (Minor) LED_MUTE is already there. If we handle LED_OFFHOOK and
> LED_RING in a different way, it would be confusing.
>
> Let's discuss the architecture before I write any new code, if we are
> going to take this way. However, to me, input-leds looks like a better
> fit: the implementation is much simpler and follows existing patterns,
> and it integrates well with drivers and hid-input. If we throw away
> input-leds, we'll have to do its job ourselves, and if we throw it
> away only for part of LEDs, the code will likely be ugly.
>
> > If we do it purely through the LED subsystem, will it get trickier to
> > associate the devices?
>
> I agree with this point. With the current approach, it's easy to look
> up all LEDs of an input device. If the suggested approach makes it
> hard, it's a serious drawback.
>
> > Anyway, it is a headset. What does headset have to do with input
> > subsystem? Sounds like sound device to me...
>
> That's right, the main function of a headset is of course sound, but
> such headsets also have buttons (vol up/down, mic mute, answer the
> call) and LEDs (mic muted, ringing, offhook). The sound "subdevice"
> (sorry, I'm not really familiar with USB terminology) is handled by
> snd-usb-audio, and the buttons/LEDs are handled by usbhid.
>
> Two examples of such headsets are mentioned in commit messages in this
> patch series. Such headsets are usually "certified for skype for
> business", but of course can be used with a variety of other VoIP
> applications. The goal of this series is to provide a standard
> interface between headsets and userspace applications, so that VoIP
> applications could react to buttons and set LED state, making Linux
> more ready for desktop.
>
> > And we already have a
> > "micmute" LED which sounds quite similar to the "offhook" LED... no?
>
> These two are different. A typical headset has three LEDs: micmute,
> ring and offhook (ring and offhook are often one physical LED, which
> blinks in the ring state and glows constantly in the offhook state).
>
> Offhook indicates that a call is in progress, while micmute shows that
> the microphone is muted. These two states are orthogonal and may
> happen in any combination. On the tested devices, offhook state didn't
> affect sound, it was just a logical indication of an active call.
>
> If you are interested in more details, I can describe the behavior of
> the headsets that I tested (some info is actually in the commit
> messages), but the short answer is that micmute and offhook are two
> different physical LEDs with completely different functions.
>
> >
> > Best regards,
> >                                                                 Pavel
> > --
> > http://www.livejournal.com/~pavelmachek

Re: [PATCH 1/6] HID: hid-input: Add offhook and ring LEDs for headsets

[Jiri Kosina]

On Mon, 9 Aug 2021, Maxim Mikityanskiy wrote:

> Dmitry, what's your opinion on the points that I raised? I would like
> to progress with this patch set, let's discuss the direction and sum
> up the requirements.

Friendly ping on this one :) Thanks.

-- 
Jiri Kosina
SUSE Labs

Re: [PATCH 1/6] HID: hid-input: Add offhook and ring LEDs for headsets

[Dmitry Torokhov]

Hi,

On Fri, Jul 16, 2021 at 08:23:02PM +0300, Maxim Mikityanskiy wrote:

Sorry for disappearing for a while.

> On Fri, Jul 16, 2021 at 1:49 AM Pavel Machek <pavel@ucw.cz> wrote:
> >
> > On Thu 2021-07-15 13:39:26, Dmitry Torokhov wrote:
> > > On Thu, Jul 15, 2021 at 08:57:44PM +0200, Jiri Kosina wrote:
> > > > On Tue, 6 Jul 2021, Benjamin Tissoires wrote:
> > > >
> > > > > > A lot of USBHID headsets available on the market have LEDs that indicate
> > > > > > ringing and off-hook states when used with VoIP applications. This
> > > > > > commit exposes these LEDs via the standard sysfs interface.
> >
> > > > > > diff --git a/drivers/input/input-leds.c b/drivers/input/input-leds.c
> > > > > > index 0b11990ade46..bc6e25b9af25 100644
> > > > > > --- a/drivers/input/input-leds.c
> > > > > > +++ b/drivers/input/input-leds.c
> > > > > > @@ -33,6 +33,8 @@ static const struct {
> > > > > >         [LED_MISC]      = { "misc" },
> > > > > >         [LED_MAIL]      = { "mail" },
> > > > > >         [LED_CHARGING]  = { "charging" },
> > > > > > +       [LED_OFFHOOK]   = { "offhook" },
> > > > >
> > > > > I am pretty sure this also needs to be reviewed by the led folks.
> > > > > Adding them in Cc.
> > > >
> > > > Can we please get Ack from the LED maintainers? Thanks.
> > >
> > > I do not think we should be adding more LED bits to the input
> > > subsystem/events; this functionality should be routed purely though LED
> > > subsystem. input-leds is a bridge for legacy input functionality
> > > reflecting it onto the newer LED subsystem.
> 
> I'm a bit confused by this answer. I wasn't aware that input-leds was
> some legacy stuff.

Yes, input-leds provides bridge from legacy leds defined in input
subsystem over to proper LED subsystem that we have now. Now that we
have proper LED subsystem all new LEDs should be introduced via it.
Especially given that some of the LEDs defined in HID have little
relation to input (Printer Out of Paper, Battery OK, RGB LEDs, etc).

> Moreover, hid-input only handles LEDs through
> input-leds, it doesn't use any modern replacement. So, does your
> answer mean I need to implement this replacement?

Yes.

> If so, I anticipate
> some issues with this approach:
> 
> 1. hid-input will handle different LEDs in different ways, which will
> make code complicated and error-prone. There will be two parallel
> implementations for LEDs.

Yes you will need to route currently defined input LEDs as they are now
to keep compatibility with existing userspace, and new LEDs should be
registered directly.

> 
> 2. A lot of code will be similar with input-leds, but not shared/reused.

Hmm, not really. I mean you will need to call to register LEDs and
toggle them, but that's the same as any other driver registering LEDs.

> 
> 3. New driver callbacks may be needed if drivers want to override the
> default behavior, like they do with input_mapping/input_mapped.

OK.

> 
> 4. If some hypothetical input device is a headset, but not HID, it
> won't be able to reuse the LED handling code. With input-leds it
> wouldn't be a problem.

We have a lot of non-HID touchpads, touchscreens, mice, joysticks, etc
that do it for all other data. LEDs are not different.

> 
> 5. (Minor) LED_MUTE is already there. If we handle LED_OFFHOOK and
> LED_RING in a different way, it would be confusing.

Not to userspace that uses LED subsystem. And I admit that adding LEDs
beyond keyboard indicators was a mistake (but we did not have proper LED
subsystem back then).

> 
> Let's discuss the architecture before I write any new code, if we are
> going to take this way. However, to me, input-leds looks like a better
> fit: the implementation is much simpler and follows existing patterns,
> and it integrates well with drivers and hid-input. If we throw away
> input-leds, we'll have to do its job ourselves, and if we throw it
> away only for part of LEDs, the code will likely be ugly.

I think it will be OK. You just need to note how each led should be
routed, and then call appropriate API when handling events.

> 
> > If we do it purely through the LED subsystem, will it get trickier to
> > associate the devices?
> 
> I agree with this point. With the current approach, it's easy to look
> up all LEDs of an input device. If the suggested approach makes it
> hard, it's a serious drawback.

You already need to deal with composite devices and figure a way to
associate all parts of them. And you already need to locate LED devices
associated with the input device because you are supposed to interface
via LED subsystem and not input (i.e. new applications should not be
using EVIOCGLED and writing EV_LED to evdev to control LEDs).

> 
> > Anyway, it is a headset. What does headset have to do with input
> > subsystem? Sounds like sound device to me...
> 
> That's right, the main function of a headset is of course sound, but
> such headsets also have buttons (vol up/down, mic mute, answer the
> call) and LEDs (mic muted, ringing, offhook). The sound "subdevice"
> (sorry, I'm not really familiar with USB terminology) is handled by
> snd-usb-audio, and the buttons/LEDs are handled by usbhid.
> 
> Two examples of such headsets are mentioned in commit messages in this
> patch series. Such headsets are usually "certified for skype for
> business", but of course can be used with a variety of other VoIP
> applications. The goal of this series is to provide a standard
> interface between headsets and userspace applications, so that VoIP
> applications could react to buttons and set LED state, making Linux
> more ready for desktop.
> 
> > And we already have a
> > "micmute" LED which sounds quite similar to the "offhook" LED... no?
> 
> These two are different. A typical headset has three LEDs: micmute,
> ring and offhook (ring and offhook are often one physical LED, which
> blinks in the ring state and glows constantly in the offhook state).
> 
> Offhook indicates that a call is in progress, while micmute shows that
> the microphone is muted. These two states are orthogonal and may
> happen in any combination. On the tested devices, offhook state didn't
> affect sound, it was just a logical indication of an active call.
> 
> If you are interested in more details, I can describe the behavior of
> the headsets that I tested (some info is actually in the commit
> messages), but the short answer is that micmute and offhook are two
> different physical LEDs with completely different functions.
> 
> >
> > Best regards,
> >                                                                 Pavel
> > --
> > http://www.livejournal.com/~pavelmachek

Thanks.

-- 
Dmitry