Re: [PATCH v2 2/3] drm/i915/utils: do not depend on config being defined

[Lucas De Marchi]

On Wed, Sep 29, 2021 at 11:08:18PM +0200, Andrzej Hajda wrote:
> W dniu 29.09.2021 o 20:33, Lucas De Marchi pisze:
> > Like the IS_ENABLED() counterpart, we can make IS_CONFIG_NONZERO() to
> > return the right thing when the config is not defined rather than a
> > build error, with the limitation that it can't be used on preprocessor
> > context.
> >
> > The trick here is that macro names can't start with a number or dash, so
> > we stringify the argument and check that the first char is a number != 0
> > (or starting with a dash to cover negative numbers). Except for -O0
> > builds the strings are all eliminated.
> >
> > Taking CONFIG_DRM_I915_REQUEST_TIMEOUT in
> > drivers/gpu/drm/i915/gem/i915_gem_context.c as example, we have the
> > following output of the preprocessor:
> >
> > old:
> >   if (((20000) != 0) &&
> > new:
> >   if (( ("20000"[0] > '0' && "20000"[0] < '9') || "20000"[0] == '-' ) &&
> >
> > New one looks worse, but is also eliminated from the object:
> >
> > $ size drivers/gpu/drm/i915/gem/i915_gem_context.o.*
> >     text    data     bss     dec     hex filename
> >    52021    1070     232   53323    d04b drivers/gpu/drm/i915/gem/i915_gem_context.o.new
> >    52021    1070     232   53323    d04b drivers/gpu/drm/i915/gem/i915_gem_context.o.old
> >
> > Signed-off-by: Lucas De Marchi <lucas.demarchi@xxxxxxxxx>
> > ---
> >   drivers/gpu/drm/i915/i915_utils.h | 6 +++++-
> >   1 file changed, 5 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/gpu/drm/i915/i915_utils.h b/drivers/gpu/drm/i915/i915_utils.h
> > index 02bbfa4d68d3..436ce612c46a 100644
> > --- a/drivers/gpu/drm/i915/i915_utils.h
> > +++ b/drivers/gpu/drm/i915/i915_utils.h
> > @@ -28,6 +28,7 @@
> >   #include <linux/list.h>
> >   #include <linux/overflow.h>
> >   #include <linux/sched.h>
> > +#include <linux/stringify.h>
> >   #include <linux/types.h>
> >   #include <linux/workqueue.h>
> >
> > @@ -469,6 +470,9 @@ static inline bool timer_expired(const struct timer_list *t)
> >    *
> >    * Returns 0 if @config is 0, 1 if set to any value.
> >    */
> > -#define IS_CONFIG_NONZERO(config) ((config) != 0)
> > +#define IS_CONFIG_NONZERO(config) (						\
> > +	(__stringify_1(config)[0] > '0' && __stringify_1(config)[0] < '9') ||	\
> > +	__stringify_1(config)[0] == '-'						\
> > +)
>
>
> Quite clever trick, but I see two issues:
>
> - gcc < 8.1 treats expressions with string indices (ex. "abc"[0]) as
> non-constant expressions, so they cannot be used everywhere, for example
> in global variable initializations,

ugh, that would kill the idea - having the strings and additional
runtime checks would not be good. Maybe if we check with
__builtin_constant_p() and do the simpler expansion if it's not
constant?

> - it does not work with hex (0x1) or octal values (01)

indeed, but I guess that would be fixable by checking (s[0] == '0' && s[1] == '\0')?
However, it seems kconfig doesn't support setting int options to hex or
octal.

If I try an hex value in menuconfig it says "You have made an invalid entry."
If I try editing .config or setting via scripts/config --set-val, it
just gets reset when trying to generate include/generated/autoconf.h

Lucas De Marchi

> It is probably OK for private macro, but it can hurt in kconfig.h,
> especially the 2nd issue
>
>
> Regards
>
> Andrzej
>
> >   #endif /* !__I915_UTILS_H */