linux-kernel - Re: [PATCH] Input: reserve 2 events code because of HID

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Date:   Fri, 7 Sep 2018 10:35:05 -0700
From:   Dmitry Torokhov <dmitry.torokhov@...il.com>
To:     Benjamin Tissoires <benjamin.tissoires@...il.com>
Cc:     Jiri Kosina <jikos@...nel.org>, Harry Cutts <hcutts@...omium.org>,
        Peter Hutterer <peter.hutterer@...-t.net>,
        linux-input@...r.kernel.org, linux-kernel@...r.kernel.org,
        Benjamin Tissoires <benjamin.tissoires@...hat.com>
Subject: Re: [PATCH] Input: reserve 2 events code because of HID

On Fri, Sep 07, 2018 at 10:51:15AM +0200, Benjamin Tissoires wrote:
> From: Benjamin Tissoires <benjamin.tissoires@...hat.com>
> 
> Prior to commit 190d7f02ce8e ("HID: input: do not increment usages when
> a duplicate is found") from the v4.18 kernel, HID used to shift the
> event codes if a duplicate usage was found. This ended up in a situation
> where a device would export a ton of ABS_MISC+n event codes, or a ton
> of REL_MISC+n event codes.
> 
> This is now fixed, however userspace needs to detect those situation.
> Fortunately, ABS_MISC+1 was never assigned a code, and so libinput
> can detect fake multitouch devices from genuine ones by checking is
> ABS_MISC+1 is set.
> 
> Now that we have REL_WHEEL_HI_RES, libinput won't be able to differentiate
> true high res mice from some other device in a pre-v4.18 kernel.
> 
> Set in stone that the ABS_MISC+1 and REL_MISC+1 are reserved and should not
> be used so userspace can properly work around those old kernels.
> 
> Signed-off-by: Benjamin Tissoires <benjamin.tissoires@...hat.com>
> ---
> 
> Hi,
> 
> while reviewing my local tree, I realize that we might want to be able
> to differentiate older kernels from new ones that export REL_WHEEL_HI_RES.
> 
> I know Dmitry was against adding several REL_MISC, so I hope just moving
> REL_WHEEL_HI_RES by one and reserving the faulty event codes would be good
> this time.
> 
> This patch applies on top of the branch for-4.20/logitech-highres from
> Jiri's tree. It should go through Jiri's tree as well.

Hm, this is a bit ugly, bit I guess we could spare an event code.

Acked-by: Dmitry Torokhov <dmitry.torokhov@...il.com>

> 
> Cheers,
> Benjamin
> 
>  include/uapi/linux/input-event-codes.h | 13 ++++++++++++-
>  1 file changed, 12 insertions(+), 1 deletion(-)
> 
> diff --git a/include/uapi/linux/input-event-codes.h b/include/uapi/linux/input-event-codes.h
> index 29fb891ea337..30149939249a 100644
> --- a/include/uapi/linux/input-event-codes.h
> +++ b/include/uapi/linux/input-event-codes.h
> @@ -708,7 +708,12 @@
>  #define REL_DIAL		0x07
>  #define REL_WHEEL		0x08
>  #define REL_MISC		0x09
> -#define REL_WHEEL_HI_RES	0x0a
> +/*
> + * 0x0a is reserved and should not be used.
> + * It was used by HID as REL_MISC+1 and usersapce needs to detect if
> + * the next REL_* event is correct or is just REL_MISC + n.
> + */
> +#define REL_WHEEL_HI_RES	0x0b
>  #define REL_MAX			0x0f
>  #define REL_CNT			(REL_MAX+1)
>  
> @@ -745,6 +750,12 @@
>  
>  #define ABS_MISC		0x28
>  
> +/*
> + * 0x29 is reserved and should not be used.
> + * It was used by HID as ABS_MISC+1 and usersapce needs to detect if
> + * the next ABS_* event is correct or is just ABS_MISC + n.
> + */
> +
>  #define ABS_MT_SLOT		0x2f	/* MT slot being modified */
>  #define ABS_MT_TOUCH_MAJOR	0x30	/* Major axis of touching ellipse */
>  #define ABS_MT_TOUCH_MINOR	0x31	/* Minor axis (omit if circular) */
> -- 
> 2.14.3
> 

-- 
Dmitry