linux-kernel - Re: [PATCH] input: s6sy761: fix coordinate read bit shift

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Date:   Fri, 5 Mar 2021 13:55:48 +0200
From:   Andi Shyti <andi@...zian.org>
To:     Caleb Connolly <caleb@...nolly.tech>
Cc:     Andi Shyti <andi@...zian.org>,
        Dmitry Torokhov <dmitry.torokhov@...il.com>,
        ~postmarketos/upstreaming@...ts.sr.ht, phone-devel@...r.kernel.org,
        linux-input@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] input: s6sy761: fix coordinate read bit shift

Hi Caleb,

> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA256

Please clean up the commit message.

> The touch coordinates are read by shifting a value left by 3,
> this is incorrect and effectively causes the coordinates to
> be half of the correct value.
> 
> Shift by 4 bits instead to report the correct value.
> 
> This matches downstream examples, and has been confirmed on my
> device (OnePlus 7 Pro).

The real reason is that from the register we get:

       byte 3             byte 2             byte 1
+--------+--------+ +-----------------+ +-----------------+
|        |        | |                 | |                 |
| X[3:0] | Y[3:0] | |     Y[11:4]     | |     X[11:4]     |
|        |        | |                 | |                 |
+--------+--------+ +-----------------+ +-----------------+

and the 12 bit values have to fit in a 16bit variable.

The upper 8 bits (in event[2] and event[1] need to be shifted
left by '4' and not by '3' in order to leave space to the lower
4 bits (in event[3]).

> 
> Signed-off-by: Caleb Connolly <caleb@...nolly.tech>
> ---
>  drivers/input/touchscreen/s6sy761.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/input/touchscreen/s6sy761.c b/drivers/input/touchscreen/s6sy761.c
> index b63d7fdf0cd2..85a1f465c097 100644
> --- a/drivers/input/touchscreen/s6sy761.c
> +++ b/drivers/input/touchscreen/s6sy761.c
> @@ -145,8 +145,8 @@ static void s6sy761_report_coordinates(struct s6sy761_data *sdata,
>  	u8 major = event[4];
>  	u8 minor = event[5];
>  	u8 z = event[6] & S6SY761_MASK_Z;
> -	u16 x = (event[1] << 3) | ((event[3] & S6SY761_MASK_X) >> 4);
> -	u16 y = (event[2] << 3) | (event[3] & S6SY761_MASK_Y);
> +	u16 x = (event[1] << 4) | ((event[3] & S6SY761_MASK_X) >> 4);
> +	u16 y = (event[2] << 4) | (event[3] & S6SY761_MASK_Y);

the devil knows how that '3' has ended up there :)

Thanks for catching it!

Reviewed-by: Andi Shyti <andi@...zian.org>

Andi