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

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Date:   Fri, 05 Mar 2021 17:54:23 +0000
From:   Caleb Connolly <caleb@...nolly.tech>
To:     Andi Shyti <andi@...zian.org>
Cc:     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 Andi,

On 05/03/2021 11:55 am, Andi Shyti wrote:
> Hi Caleb,
>
>> -----BEGIN PGP SIGNED MESSAGE-----
>> Hash: SHA256
> Please clean up the commit message.
oops! Sorry
>
>> 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]).
Thanks for clarifying, sorry for my rather naive commit message.
>> 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>

Regards,

Caleb

>
> Andi