Change in flashrom[master]: programmer_table: move each entry to the associated programmer source

[Angel Pons (Code Review)]

Angel Pons has posted comments on this change. ( https://review.coreboot.org/c/flashrom/+/52946 )

Change subject: programmer_table: move each entry to the associated programmer source
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Patch Set 12:

(1 comment)

File programmer.h:

https://review.coreboot.org/c/flashrom/+/52946/comment/73431de3_961b68e3 
PS4, Line 402: #if CONFIG_DUMMY == 1
> Please just drop all these `#if`. The only difference it makes is
> whether we get a compile-time or link-time error if something goes
> wrong. I don't think it matters or even justifies the clutter.
>
> Mentioning the file name is also a bit redundant. Maybe just a single
> comment /* programmer drivers */ ?
>
> Doesn't have to be in this patch, though

CB:55387
> Thank you so much! Your long message is very helpful, I think now I actually understand: extern declaration by itself is fine, as long as all the usages are guarded.

I made CB:55386 on coreboot earlier today. It introduces a helper function to transform a compile-time error into a link-time error, which allows dropping some preprocessor usage in code.

I just forced build errors by not enabling `DEBUG_BOOT_STATE` and manually inverting the condition in src/lib/hardwaremain.c in order to show you the difference between a compile-time error and a link-time error.

Compile-time error excerpt:

src/lib/hardwaremain.c: In function 'bs_call_callbacks':
 src/lib/hardwaremain.c:279:15: error: 'struct boot_state_callback' has no member named 'location'
      bscb, bscb->location);

The `location` member is inside a text block guarded by a conditional preprocessor directive. Since the `CONFIG(DEBUG_BOOT_STATE)` condition is false, the preprocessor does not pass the contents of this text block to the compiler, and the compiler does not "see" the `location` member in the struct definition. When it reaches the `bscb->location` part, the compiler complains about the non-existent `location` member.

Link-time error excerpt:

coreboot-builds/HP_280_G2/generated/ramstage.o: in function `bscb_location':
 src/include/bootstate.h:114: undefined reference to `_dead_code_assertion_failed'

The `dead_code_t` macro inside the `bscb_location` function evaluates to some expression that calls the `_dead_code_assertion_failed` function, which is intentionally never defined (only declared). Compilation succeeds: the source code can be translated into a `hardwaremain.o` object file, which contains external definitions and references for linking. However, when the linker tries to resolve the external references, it cannot find a definition for the `_dead_code_assertion_failed` function, and bails out.

The advantage of link-time errors over compile-time errors (i.e. what CB:55386 leverages) is that the compiler can automatically optimise out the code that would cause a link-time error. CB:55386 works because `bscb_location` is never reached when the `CONFIG(DEBUG_BOOT_STATE)` condition is false, and the compiler can thus optimise `bscb_location` away along with the `_dead_code_assertion_failed` function call.