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 15:54:33 -0700
From:   Lucas De Marchi <lucas.demarchi@...el.com>
To:     Andrzej Hajda <a.hajda@...sung.com>
Cc:     intel-gfx@...ts.freedesktop.org,
        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

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@...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,

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 */