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

On Thu, Jan 28, 2021 at 05:37:26PM +0300, Dan Carpenter wrote:

On Thu, Jan 28, 2021 at 02:07:05PM +0100, Michał Mirosław wrote:

quoted

On Thu, Jan 28, 2021 at 12:57:12PM +0300, Dan Carpenter wrote:

quoted

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

[...]

quoted

   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.

Is there some kind of prize for avoiding if statements??

Less LOC and an occassional puzzle, of course. :-) Considering your
examples below I can see where the check came from. I wouldn't object
to a patch changing the code or adding a comment on what it does (it
maps a selected nibble value (0..15) to a byte (1..255) spreading the
values evenly).

quoted

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.

Yeah.  The rule is just a simple heuristic of a logical negate used
with a bitwise operation.

[...]

Maybe it could differentiate between "|= !x" and "&= !x", the second one
being more suspicious? I must say that 'x | !x' idiom looks obvious
as other sigle-operator-letter misspellings ('x | ~x' and 'x || !x')
beg the question of why the constant wasn't used directly?

Best Regards
Michał Mirosław