Re: [bug report] Input: elants_i2c - add support for eKTF3624

["Michał Mirosław" <mirq-linux@rere.qmqm.pl>]

On Thu, Jan 28, 2021 at 12:57:12PM +0300, Dan Carpenter wrote:
> Hello Michał Mirosław,
> 
> The patch 9517b95bdc46: "Input: elants_i2c - add support for
> eKTF3624" from Jan 24, 2021, leads to the following static checker
> warning:
> 
> 	drivers/input/touchscreen/elants_i2c.c:966 elants_i2c_mt_event()
> 	warn: should this be a bitwise negate mask?
> 
> drivers/input/touchscreen/elants_i2c.c
[...]
>    963                                  w = buf[FW_POS_WIDTH + i / 2];
>    964                                  w >>= 4 * (~i & 1);
>    965                                  w |= w << 4;
>    966                                  w |= !w;
>                                         ^^^^^^^^
> 
> This code is just very puzzling.  I think it may actually be correct?
> The boring and conventional way to write this would be to do it like so:
> 
> 	if (!w)
> 		w = 1;

It could also be written as:

	w += !w;

or:
	w += w == 0;

while avoiding conditional.

But, in this case, the warning is bogus. Because w | ~w == all-ones (always),
it might as well suggested to write:

	w = -1;

or:
	w = ~0;

making the code broken.

Best Regards
Michał Mirosław