Linker set issues with ath(4) HALs

[Kostik Belousov]

On Sat, Mar 05, 2011 at 07:50:05PM +1100, Peter Jeremy wrote:
> I have a Atheros AR5424 and so, based on the 8.2-STABLE i386 NOTES
> and some rummaging in the sources, I tried to build a kernel with:
> 
> device		ath		# Atheros pci/cardbus NIC's
> device		ath_ar5212	# HAL for Atheros AR5212 and derived chips
> device		ath_rate_sample	# SampleRate tx rate control for ath
> 
> and this died during the kernel linking with:
> linking kernel.debug
> ah.o(.text+0x23c): In function `ath_hal_rfprobe':
> /usr/src/sys/dev/ath/ath_hal/ah.c:142: undefined reference to `__start_set_ah_rf
> s'
> ah.o(.text+0x241):/usr/src/sys/dev/ath/ath_hal/ah.c:142: undefined reference to  `__stop_set_ah_rfs'
> ah.o(.text+0x25a):/usr/src/sys/dev/ath/ath_hal/ah.c:142: undefined reference to  `__stop_set_ah_rfs'
> 
> Following a suggestion by a friend, I changed that to:
> 
> device		ath		# Atheros pci/cardbus NIC's
> options AH_SUPPORT_AR5416
> device		ath_hal		# Atheros HAL
> device		ath_rate_sample	# SampleRate tx rate control for ath
> 
> and it worked.  Normally, I would leave it at that but I'd like to
> understand what is actually going on...
> 
> In both cases, ah.o contains the following 4 references:
>          U __start_set_ah_chips
>          U __start_set_ah_rfs
>          U __stop_set_ah_chips
>          U __stop_set_ah_rfs
> generated by:
> /* linker set of registered chips */
> OS_SET_DECLARE(ah_chips, struct ath_hal_chip);
> /* linker set of registered RF backends */
> OS_SET_DECLARE(ah_rfs, struct ath_hal_rf);
> 
> These symbols do not appear in any other .o files, though there are a
> variety of other __{start,stop}_set_* symbols - all of which show up
> as 'A' (absolule) values in the final kernel.
> 
> My questions are:
> How are these linker set references resolved?  I can't find anything
> that defines these symbols - either in .o files or in ldscript files.
> 
> In the first case, there are 2 pairs of undefined linker set variables
> but the linker only reports one pair as unresolved.  Why don't both
> sets show up as resolved or unresolved?  Why does using the generic
> "ath_hal", rather than the hardware-specific HAL fix the problem?

Linker synthesizes the symbols assuming the following two conditions are
met:
- the symbols are referenced;
- there exists an ELF section named `set_ah_rfs'.
It assigns the (relocated) start of the section to __start_<sectionname>,
and end to __stop_<sectionname>.

Most likely, omitting the option causes some SET_ENTRY() macro, which put
a symbol into linker set, to be ommitted. Then, no section is created
and linker does not synthesizes the missed symbols.
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