RE: [PATCH] Enable '-Werror' by default for all kernel builds

[David Laight <David.Laight@ACULAB.COM>]

From: Arnd Bergmann
> Sent: 08 September 2021 10:50
...
> > > I don't know the hardware, so I can not answer the ioremap() question.
> >
> > Yes it should.  But this driver dates back to 2.1.110, when only
> > half of the architectures already had ioremap().
> 
> How does  mvme16x even create the mapping? Is this a virtual address
> that is hardwired to the bus or do you have a static mapping somewhere?
> I see two other drivers accessing the nvram here
> 
> arch/m68k/mvme16x/config.c:static MK48T08ptr_t volatile rtc = (MK48T08ptr_t)MVME_RTC_BASE;
> arch/m68k/mvme16x/rtc.c:        volatile MK48T08ptr_t rtc = (MK48T08ptr_t)MVME_RTC_BASE;
> 
> The same trick should work here, just create a local variable with a
> volatile pointer and read from that.

Or define a C 'extern' for the actual data and get the linker script
to assign a fixed value to the symbol.
(Although that does pollute the global namespace.)

An alternative is to use an asm statement so the compiler
cannot track the actual value.
Something like:

#define launder(x) asm volatile( "" : "+r" (x))

	MK48T08ptr_t rtc = (void *)MVME_RTC_BASE;
	launder(rtc);

That also works a bit like READ_ONCE() except that is works
on a value that is (hopefully) already in a register rather
that during the read from memory.
Useful when the compiler's 'value tracking' pessimises code.

	David

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