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

[Andy Shevchenko <andriy.shevchenko@intel.com>]

On Fri, Jun 26, 2026 at 02:27:25PM +0530, Joyeta Modak wrote:

Please, do not top-post!

> 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.

How "um" is anyhow related to the real world cases? Try to compile your stuff
with real ARCH settings, like x86_64.

> 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@intel.com> 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