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

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
[off-list ref] wrote:

quoted

On Wed, Jun 24, 2026 at 01:08:04PM +0530, Joyeta Modak wrote:

quoted

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.

...

quoted

quoted

-#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