Re: [PATCH] staging: fbtft: use ARRAY_SIZE() in NUMARGS macro

[Joyeta Modak]

Thank you for the feedback and the question.

I checked every write_reg() across all fbtft drivers and found that
the largest number of arguments is 129 in write_reg(par,
MIPI_DCS_WRITE_LUT,...)
As COUNT_ARGS() in args.h only supports up to 15, it is not a safe fit here.

However, the kernel test robot reported a problem with my
implementation as the __must_be_array() check in ARRAY_SIZE() requires
the array to be a compile time constant expression and thus breaks the
call at several places.(example par->bgr)

I tried to reproduce this locally on my system using both GCC and
Clang with ARCH=um on x86_64 but could not reproduce the build
failure.

Since the original sizeof() based approach had no such errors flagged,
I am thinking of dropping the ARRAY_SIZE() approach.

Any other feedback is appreciated. Thanks again.

On Wed, Jun 24, 2026 at 5:01 PM Andy Shevchenko
<andriy.shevchenko@xxxxxxxxx> wrote:
>
> On Wed, Jun 24, 2026 at 01:08:04PM +0530, Joyeta Modak wrote:
> > NUMARGS() computes the number of arguments by dividing the size of a
> > temporary int array by sizeof(int). Using the standard ARRAY_SIZE()
> > macro is the correct way to count array elements in the kernel, and
> > ARRAY_SIZE() also provides a __must_be_array() compile time check. There
> > are no functional changes.
>
> ...
>
> > -#define NUMARGS(...)  (sizeof((int[]){__VA_ARGS__}) / sizeof(int))
> > +#define NUMARGS(...)  ARRAY_SIZE(((int[]){__VA_ARGS__}))
> >
> >  #define write_reg(par, ...)                                            \
> >       ((par)->fbtftops.write_register(par, NUMARGS(__VA_ARGS__), __VA_ARGS__))
>
> What is the maximum parameters .write_register() takes in practice in the
> fbtft drivers? If it's less than or equal to 15, we may use args.h instead.
>
> --
> With Best Regards,
> Andy Shevchenko
>
>


-- 
Regards,
Joyeta Modak