Re: [PATCH] spi: work around clang bug in SPI_BPW_RANGE_MASK()

[Arnd Bergmann <arnd@xxxxxxxx>]

On Thu, Mar 7, 2019 at 2:09 PM Rasmus Villemoes
<linux@xxxxxxxxxxxxxxxxxx> wrote:
>
> On 07/03/2019 11.56, Arnd Bergmann wrote:
> > Clang-8 evaluates both sides of a ?: expression to check for
> > valid arithmetic even in the side that is never taken. This
> > results in a build warning:
> >
> > drivers/spi/spi-sh-msiof.c:1052:24: error: shift count >= width of type [-Werror,-Wshift-count-overflow]
> >         .bits_per_word_mask = SPI_BPW_RANGE_MASK(8, 32),
> >                               ^~~~~~~~~~~~~~~~~~~~~~~~~
> >
> > Change it to shift one less than we want, and then shift one
> > more bit afterwards. This should give the correct result for
> > all valid input, since it has to be in the range 1..32 anyway.
>
> Why not use GENMASK which is provided by the same header that #defines BIT?

It might be an options, but

- I had not thought of it
- It looks like it would have the same problem with shifting right by
32 bits (?)
- it seems to have slightly different semantics from SPI_BPW_RANGE_MASK(),
  counting the bits from 0 instead of 1.

I tried this version now, which doesn't produce any warnings as far as I can
tell, but I'm not convinced that it's actually correct. Can you have a look?

#define SPI_BPW_RANGE_MASK(min, max) GENMASK((min) - 1, (max) - 1)

       Arnd