linux-kernel - Re: [PATCH v2 5/6] iio: light: ROHM BU27034 Ambient Light Sensor

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Message-ID: <4beef812-8f4f-3857-c814-efd9173d49e6@gmail.com>
Date:   Sun, 5 Mar 2023 15:10:38 +0200
From:   Matti Vaittinen <mazziesaccount@...il.com>
To:     Jonathan Cameron <jic23@...nel.org>
Cc:     Matti Vaittinen <matti.vaittinen@...rohmeurope.com>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Paul Gazzillo <paul@...zz.com>,
        Zhigang Shi <Zhigang.Shi@...eon.com>,
        Shreeya Patel <shreeya.patel@...labora.com>,
        Dmitry Osipenko <dmitry.osipenko@...labora.com>,
        linux-kernel@...r.kernel.org, linux-iio@...r.kernel.org
Subject: Re: [PATCH v2 5/6] iio: light: ROHM BU27034 Ambient Light Sensor

On 3/4/23 22:17, Jonathan Cameron wrote:
> On Thu, 2 Mar 2023 12:58:59 +0200
> Matti Vaittinen <mazziesaccount@...il.com> wrote:
> 
> As per other branch of the thread.
> 
> 	ch0 = max(1, le16_to_cpu(res[0]);
>  > would be cleaner.

I tried this out. Comparing u16 to literal 1 results comparison of 
values with different sizes:

./include/linux/minmax.h:20:28: warning: comparison of distinct pointer 
types lacks a cast
   (!!(sizeof((typeof(x) *)1 == (typeof(y) *)1)))
                             ^
./include/linux/minmax.h:26:4: note: in expansion of macro ‘__typecheck’
    (__typecheck(x, y) && __no_side_effects(x, y))
     ^~~~~~~~~~~
./include/linux/minmax.h:36:24: note: in expansion of macro ‘__safe_cmp’
   __builtin_choose_expr(__safe_cmp(x, y), \
                         ^~~~~~~~~~
./include/linux/minmax.h:74:19: note: in expansion of macro ‘__careful_cmp’
  #define max(x, y) __careful_cmp(x, y, >)
                    ^~~~~~~~~~~~~
drivers/iio/light/rohm-bu27034.c:1057:8: note: in expansion of macro ‘max’
   ch0 = max(1, ch0);


I could work around this by doing:

const u16 min_ch_val = 1;

...

ch0 = max(min_ch_val, le16_to_cpu(res[0]));

but I think that would really be obfuscating the meaning. I assume

ch0 = max((u16)1, le16_to_cpu(res[0]));

might work too - but to me it's pretty ugly.

The more I am looking at this, the stronger I feel we should really just 
write this as it was. Check if res[0] contains the only unsafe data 
"!res[0]" - and if yes, set it to 1. The comment above it will clarify 
it to a reader wondering what happens.

I will leave it like it was in v2 for v3. If you still feel strong about 
it then we need to continue rubbing it.

Yours,
	-- Matti

-- 
Matti Vaittinen
Linux kernel developer at ROHM Semiconductors
Oulu Finland

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