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

[Andrzej Hajda <a.hajda@samsung.com>]

W dniu 30.09.2021 o 00:54, Lucas De Marchi pisze:
> 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@intel.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?


I think it is just matter of disallowing such construct in places where 
compiler expects constant expression.

If accepted, the expression is apparently evaluated in compile time. See 
[1].

[1]: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69960#c18


>
>>
>> - 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.


I've spotted in include/generated/autoconf.h following line:

#define CONFIG_ILLEGAL_POINTER_VALUE 0xdead000000000000

It corresponds to following kconfig entry:

config ILLEGAL_POINTER_VALUE
        hex
        default 0 if X86_32
        default 0xdead000000000000 if X86_64

Grepping shows more: grep -r --include=Kconfig -3 -P '^\s*hex' .

Anyway do you really need to handle undefined case? If not, the macro 
can stay simple, w/o hacky constructs.


Regards

Andrzej


>
> 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 */
>