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

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Date:   Wed, 29 Sep 2021 23:08:18 +0200
From:   Andrzej Hajda <a.hajda@...sung.com>
To:     Lucas De Marchi <lucas.demarchi@...el.com>,
        intel-gfx@...ts.freedesktop.org
Cc:     Daniel Vetter <daniel.vetter@...el.com>,
        dri-devel@...ts.freedesktop.org,
        Masahiro Yamada <masahiroy@...nel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/3] drm/i915/utils: do not depend on config being
 defined


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@...el.com>
> ---
>   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,

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

It is probably OK for private macro, but it can hurt in kconfig.h, 
especially the 2nd issue


Regards

Andrzej

>   
>   #endif /* !__I915_UTILS_H */