Re: [PATCH] iio: tools: Use proper macro to detect _Float16 support

[Jonathan Cameron <jic23@kernel.org>]

On Mon, 21 Sep 2026 15:33:55 +0400
"Ivan A. Melnikov" <iv@altlinux.org> wrote:

> On Sat, Sep 19, 2026 at 11:39:45AM +0200, Joshua Crofts wrote:
> > On Fri, 18 Sep 2026 20:13:02 +0400
> > "Ivan A. Melnikov" <iv@altlinux.org> wrote:
> >   
> > > __FLT16_MAX__ is an internal compiler macro. GCC defines it if
> > > _Float16 is supported by the target in general, but the support
> > > may depend on the exact target options. For example, on i586
> > > without -msse2 I get the following error from GCC15:
> > > 
> > >   iio_generic_buffer.c: In function 'print2byte':
> > >   iio_generic_buffer.c:141:17: error: invalid conversion from type '_Float16' without option '-msse2'
> > >     141 |                 printf("%05f ", ((float)converter.f + info->offset) * info->scale);
> > >         |                 ^~~~~~
> > > 
> > > To fix that, use the public macro without double underscores.
> > > 
> > > Fixes: cdd445d4b2a9 ("iio: tools: Add support for floating-point types in buffer scan elements")
> > > Signed-off-by: Ivan A. Melnikov <iv@altlinux.org>
> > > ---
> > >  tools/iio/iio_generic_buffer.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/tools/iio/iio_generic_buffer.c b/tools/iio/iio_generic_buffer.c
> > > index 000193612aad..9ea93984e0e9 100644
> > > --- a/tools/iio/iio_generic_buffer.c
> > > +++ b/tools/iio/iio_generic_buffer.c
> > > @@ -131,7 +131,7 @@ static void print2byte(uint16_t input, struct iio_channel_info *info)
> > >  		printf("%05f ", ((float)input + info->offset) * info->scale);
> > >  		break;
> > >  	case 'f': {
> > > -#if defined(__FLT16_MAX__)
> > > +#if defined(FLT16_MAX)
> > >  		union {
> > >  			uint16_t u;
> > >  			_Float16 f;  
> > 
> > One Sashiko issue:
> > 
> > Does this unintentionally disable _Float16 support for all architectures?  
> 
> Indeed, it does. Sorry, my patch is incorrect. I must have completely
> messed up all my checks on Friday.
> 
> > Since the tools build environment uses gnu11 compilation flags without
> > explicitly defining __STDC_WANT_IEC_60559_TYPES_EXT__, FLT16_MAX is
> > undefined across all architectures. This causes the preprocessor to skip
> > this block entirely, causing the tool to always print 
> > <unsupported 2-byte float> for 16-bit float data, even on platforms where 
> > native support previously worked.
> > 
> > If FLT16_MAX were actually defined, the compiler would still parse the
> > _Float16 block and throw the same missing -msse2 compilation error mentioned
> > in the commit message. Is there an alternative way to check for hardware
> > support without completely disabling the feature?  
> 
> Apparently, the only correct way to detect _Float16 is to try compiling
> some code that uses it. For that, a feature can be added to tools/build.
> 
> On other hand, x86 without SSE2 is the only platform I could find where
> __FLT16_MAX__ is defined, but _Float16 does not work, so a simple
> platform-specific workaround would also work and would be much less
> elaborate.
> 
> WDYT?
I would do it properly.  Avoids problems with what is defined for architectures
changing in the future.

Thanks,

Jonathan

>