Re: drivers/hwmon/jc42.c:477 jc42_readable_reg() warn: always true condition '(reg >= 0) => (0-u32max >= 0)'

Re: drivers/hwmon/jc42.c:477 jc42_readable_reg() warn: always true condition '(reg >= 0) => (0-u32max >= 0)'

[Martin Blumenstingl]

Hi Guenter et al.,

On Thu, Dec 22, 2022 at 3:36 PM kernel test robot <lkp@intel.com> wrote:
[...]
>    475  static bool jc42_readable_reg(struct device *dev, unsigned int reg)
>    476  {
>  > 477          return (reg >= JC42_REG_CAP && reg <= JC42_REG_DEVICEID) ||
>    478                  reg == JC42_REG_SMBUS;
The bot is right: we can omit "reg >= JC42_REG_CAP" as it's already
covered by the fact that:
- the reg variable is unsigned, which means the lower limit is zero
- reg <= JC42_REG_DEVICEID covers the upper limit

Before I send a patch I'd like to hear if removal of "reg >=
JC42_REG_CAP" makes sense to other people.


Best regards,
Martin

Re: drivers/hwmon/jc42.c:477 jc42_readable_reg() warn: always true condition '(reg >= 0) => (0-u32max >= 0)'

[Guenter Roeck]

On Thu, Dec 22, 2022 at 10:20:13PM +0100, Martin Blumenstingl wrote:
> Hi Guenter et al.,
> 
> On Thu, Dec 22, 2022 at 3:36 PM kernel test robot <lkp@intel.com> wrote:
> [...]
> >    475  static bool jc42_readable_reg(struct device *dev, unsigned int reg)
> >    476  {
> >  > 477          return (reg >= JC42_REG_CAP && reg <= JC42_REG_DEVICEID) ||
> >    478                  reg == JC42_REG_SMBUS;
> The bot is right: we can omit "reg >= JC42_REG_CAP" as it's already
> covered by the fact that:
> - the reg variable is unsigned, which means the lower limit is zero
> - reg <= JC42_REG_DEVICEID covers the upper limit
> 
> Before I send a patch I'd like to hear if removal of "reg >=
> JC42_REG_CAP" makes sense to other people.
> 

The bot keeps complaining about it. Yes, it is technically unnecessary,
but I left it in on purpose to indicate that JC42_REG_CAP is the first
register and that it wasn't forgotten. Any modern C compiler notices
that the check is unnecessary and drops it, so there is no runtime penalty.

This is one of those situations where I'd like to have a means to tell
the checker to please stop complaining.

Guenter

Re: drivers/hwmon/jc42.c:477 jc42_readable_reg() warn: always true condition '(reg >= 0) => (0-u32max >= 0)'

[Martin Blumenstingl]

Hi Guenter,

On Fri, Dec 23, 2022 at 12:41 AM Guenter Roeck <linux@roeck-us.net> wrote:
>
> On Thu, Dec 22, 2022 at 10:20:13PM +0100, Martin Blumenstingl wrote:
> > Hi Guenter et al.,
> >
> > On Thu, Dec 22, 2022 at 3:36 PM kernel test robot <lkp@intel.com> wrote:
> > [...]
> > >    475  static bool jc42_readable_reg(struct device *dev, unsigned int reg)
> > >    476  {
> > >  > 477          return (reg >= JC42_REG_CAP && reg <= JC42_REG_DEVICEID) ||
> > >    478                  reg == JC42_REG_SMBUS;
> > The bot is right: we can omit "reg >= JC42_REG_CAP" as it's already
> > covered by the fact that:
> > - the reg variable is unsigned, which means the lower limit is zero
> > - reg <= JC42_REG_DEVICEID covers the upper limit
> >
> > Before I send a patch I'd like to hear if removal of "reg >=
> > JC42_REG_CAP" makes sense to other people.
> >
>
> The bot keeps complaining about it. Yes, it is technically unnecessary,
> but I left it in on purpose to indicate that JC42_REG_CAP is the first
> register and that it wasn't forgotten. Any modern C compiler notices
> that the check is unnecessary and drops it, so there is no runtime penalty.
Thanks for your feedback. Since I had to double check the bot's
complaint I'll just keep this as-is (and not send any patch for this
at all).

> This is one of those situations where I'd like to have a means to tell
> the checker to please stop complaining.
I see, in some cases this may be an actual logic error (for example:
accidentally using an unsigned data type instead of a signed one).


Best regards,
Martin