Dear Simon Glass,
In message 1382800457-26608-1-git-send-email-sjg@chromium.org you wrote:
Many parts of the U-Boot code base are sprinkled with #ifdefs. This makes different boards compile different versions of the source code, meaning that we must build all boards to check for failures. It is easy to misspell an #ifdef and there is not as much checking of this by the compiler. Multiple dependent #ifdefs are harder to do than with if..then..else. Variable declarations must be #idefed as well as the code that uses them, often much later in the file/function. #ifdef indents don't match code indents and have their own separate indent feature. Overall, excessive use of #idef hurts readability and makes the code harder to modify and refactor. For people coming newly into the code base, #ifdefs can be a big barrier.
The use of #ifdef in U-Boot has possibly got a little out of hand. In an attempt to turn the tide, this series includes a patch which provides a way to make CONFIG macros available to C code without using the preprocessor. This makes it possible to use standard C conditional features such as if/then instead of #ifdef. A README update exhorts compliance.
As mentioned before, I'm really interested in seeing something like this going into mainline, but I have some doubts about the actual implementation.
To summarize: Your current proposal was to convert code snippets like this:
#ifdef CONFIG_VERSION_VARIABLE setenv("ver", version_string); /* set version variable */ #endif
into
if (autoconf_version_variable()) setenv("ver", version_string); /* set version variable */
By chance I ran about "include/linux/kconfig.h" in the Linux kernel tree, which provides (among other things) the IS_ENABLED() macro that implements essentially the very same feature. Using this, the same code would be written as:
if (IS_ENABLED(CONFIG_VERSION_VARIABLE)) setenv("ver", version_string); /* set version variable */
I agree that this does not solve some of the isses that have been raised about this change (indentation level increses - which may in turn require reformatting of bigger parts of the code; code becomes less readable), but on the other hand it avoids the need for a new autoconf header file, and it should be possible to introduce this easily step by step.
And I really like the idea of re-using existing code that is already known to Linux hackers, especially as we we are currently having our eyes on the Kconfig stuff anyway.
What do you think?
Best regards,
Wolfgang Denk