linux-kernel - Re: [PATCH] HID: logitech-dj: add support for the static device in the Powerplay mat/receiver

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Message-ID: <5a93677bd6238a9d7928e4fd16a8915faeb5f19e.camel@archlinux.org>
Date:   Wed, 12 Feb 2020 13:56:58 +0000
From:   Filipe Laíns <lains@...hlinux.org>
To:     Jiri Kosina <jikos@...nel.org>
Cc:     Benjamin Tissoires <benjamin.tissoires@...hat.com>,
        linux-input@...r.kernel.org, linux-kernel@...r.kernel.org,
        Nestor Lopez Casado <nlopezcasad@...itech.com>
Subject: Re: [PATCH] HID: logitech-dj: add support for the static device in
 the Powerplay mat/receiver

On Wed, 2020-02-12 at 14:50 +0100, Jiri Kosina wrote:
> On Wed, 15 Jan 2020, Filipe Laíns wrote:
> 
> > The Logitech G Powerplay has a lightspeed receiver with a static HID++
> > device with ID 7 attached to it to. It is used to configure the led on
> > the mat. For this reason I increased the max number of devices.
> > 
> > Signed-off-by: Filipe Laíns <lains@...hlinux.org>
> > ---
> >  drivers/hid/hid-logitech-dj.c | 9 +++++++--
> >  1 file changed, 7 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/hid/hid-logitech-dj.c b/drivers/hid/hid-logitech-dj.c
> > index bb50d6e7745b..79294b873057 100644
> > --- a/drivers/hid/hid-logitech-dj.c
> > +++ b/drivers/hid/hid-logitech-dj.c
> > @@ -16,11 +16,11 @@
> >  #include <asm/unaligned.h>
> >  #include "hid-ids.h"
> >  
> > -#define DJ_MAX_PAIRED_DEVICES			6
> > +#define DJ_MAX_PAIRED_DEVICES			7
> >  #define DJ_MAX_NUMBER_NOTIFS			8
> >  #define DJ_RECEIVER_INDEX			0
> >  #define DJ_DEVICE_INDEX_MIN			1
> > -#define DJ_DEVICE_INDEX_MAX			6
> > +#define DJ_DEVICE_INDEX_MAX			7
> >  
> >  #define DJREPORT_SHORT_LENGTH			15
> >  #define DJREPORT_LONG_LENGTH			32
> > @@ -980,6 +980,11 @@ static void logi_hidpp_recv_queue_notif(struct hid_device *hdev,
> >  		break;
> >  	}
> >  
> > +	/* custom receiver device (eg. powerplay) */
> > +	if (hidpp_report->device_index == 7) {
> > +		workitem.reports_supported |= HIDPP;
> > +	}
> > +
> 
> I guess we can't do anything else than to trust Logitech that they will 
> not assign conflicting device_index ID in the future to something that 
> will not be HID++, right?
> 
> Or is this properly documented somewhere?
> 
> Thanks,


CCing Nestor

Yes, we need to trust Logitech on this. There's isn't anything
documented regarding this AFAIK.

Even if Logitech released a device with index 7 that didn't support
HID++, it wouldn't be that big of an issue. We would just end up with a
hidraw node which would export a vendor page report descriptor but the
device wouldn't actually reply to anything.

Regards,
Filipe Laíns

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