Re: [PATCH v5 1/8] Kconfig: import kconfig.h from Linux 4.3

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 23/01/16 17:14, Jonathan Creekmore wrote:
> Shannon Zhao writes:
>
>> From: Shannon Zhao <shannon.zhao@linaro.org>
>>
>> To support using CONFIG_ options in C/CPP expressions, import kconfig.h
>> from the Linux v4.3 tag (commit id
>> 6a13feb9c82803e2b815eca72fa7a9f5561d7861).
>>
>> CC: Doug Goldstein <cardoe@cardoe.com>
>> Signed-off-by: Shannon Zhao <shannon.zhao@linaro.org>
>> ---
>>  xen/include/xen/config.h  |  2 +-
>>  xen/include/xen/kconfig.h | 54 +++++++++++++++++++++++++++++++++++++++++++++++
>>  2 files changed, 55 insertions(+), 1 deletion(-)
>>  create mode 100644 xen/include/xen/kconfig.h
>>
>> diff --git a/xen/include/xen/config.h b/xen/include/xen/config.h
>> index 7595599..eeb49db 100644
>> --- a/xen/include/xen/config.h
>> +++ b/xen/include/xen/config.h
>> @@ -7,7 +7,7 @@
>>  #ifndef __XEN_CONFIG_H__
>>  #define __XEN_CONFIG_H__
>>
>> -#include <generated/autoconf.h>
>> +#include <xen/kconfig.h>
>>
>>  #ifndef __ASSEMBLY__
>>  #include <xen/compiler.h>
>> diff --git a/xen/include/xen/kconfig.h b/xen/include/xen/kconfig.h
>> new file mode 100644
>> index 0000000..d68a7ed
>> --- /dev/null
>> +++ b/xen/include/xen/kconfig.h
>> @@ -0,0 +1,54 @@
>> +#ifndef __XEN_KCONFIG_H
>> +#define __XEN_KCONFIG_H
>> +
>> +#include <generated/autoconf.h>
>> +
>> +/*
>> + * Helper macros to use CONFIG_ options in C/CPP expressions. Note that
>> + * these only work with boolean and tristate options.
>> + */
>> +
>> +/*
>> + * Getting something that works in C and CPP for an arg that may or may
>> + * not be defined is tricky.  Here, if we have "#define CONFIG_BOOGER 1"
>> + * we match on the placeholder define, insert the "0," for arg1 and generate
>> + * the triplet (0, 1, 0).  Then the last step cherry picks the 2nd arg (a one).
>> + * When CONFIG_BOOGER is not defined, we generate a (... 1, 0) pair, and when
>> + * the last step cherry picks the 2nd arg, we get a zero.
>> + */
>> +#define __ARG_PLACEHOLDER_1 0,
>> +#define config_enabled(cfg) _config_enabled(cfg)
>> +#define _config_enabled(value) __config_enabled(__ARG_PLACEHOLDER_##value)
>> +#define __config_enabled(arg1_or_junk) ___config_enabled(arg1_or_junk 1, 0)
>> +#define ___config_enabled(__ignored, val, ...) val
>> +
>> +/*
>> + * IS_BUILTIN(CONFIG_FOO) evaluates to 1 if CONFIG_FOO is set to 'y', 0
>> + * otherwise. For boolean options, this is equivalent to
>> + * IS_ENABLED(CONFIG_FOO).
>> + */
>> +#define IS_BUILTIN(option) config_enabled(option)
>> +
>> +/*
>> + * IS_MODULE(CONFIG_FOO) evaluates to 1 if CONFIG_FOO is set to 'm', 0
>> + * otherwise.
>> + */
>> +#define IS_MODULE(option) config_enabled(option##_MODULE)
>> +
>> +/*
>> + * IS_REACHABLE(CONFIG_FOO) evaluates to 1 if the currently compiled
>> + * code can call a function defined in code compiled based on CONFIG_FOO.
>> + * This is similar to IS_ENABLED(), but returns false when invoked from
>> + * built-in code when CONFIG_FOO is set to 'm'.
>> + */
>> +#define IS_REACHABLE(option) (config_enabled(option) || \
>> +		 (config_enabled(option##_MODULE) && config_enabled(MODULE)))
>> +
>> +/*
>> + * IS_ENABLED(CONFIG_FOO) evaluates to 1 if CONFIG_FOO is set to 'y' or 'm',
>> + * 0 otherwise.
>> + */
>> +#define IS_ENABLED(option) \
>> +	(IS_BUILTIN(option) || IS_MODULE(option))
>> +
>> +#endif /* __XEN_KCONFIG_H */
> I am not sure that the complexity of this file is necessary since Xen
> does not support loadable modules. Essentially, IS_ENABLED(CONFIG_FOO)
> is as simple as #ifdef CONFIG_FOO.

I would like to be able to convert some of our "#ifdef CONFIG_FOO" code
into "if ( IS_ENABLED(CONFIG_FOO) )" to reduce the quantity of code-rot
in often-disabled options.

However, I agree that we don't want all the module complexity.  i.e.
IS_ENABLED() is the only one of these we need.

~Andrew