Question: Multiple board support is broken

[Russell King - ARM Linux]

On Sat, Dec 08, 2012 at 12:28:11PM +0400, Alexander Shiyan wrote:
> On Fri, 7 Dec 2012 16:17:54 +0000
> Russell King - ARM Linux <linux at arm.linux.org.uk> wrote:
> 
> > > Today I was tested multiple boards (not multiplatform) in the kernel
> > > and found problems with booting.
> > 
> > Err, I test this almost constantly and it works, and it's been working
> > 100% correctly for about the last 12 years.  There is no problem here.
> > 
> > > Exacly, if multiple boards defined
> > > in. config, we can proceed to boot only the last (determined by mach number).
> > 
> > The mach-types file doesn't have anything to do with which board gets chosen.
> > That file just provides the IDs, and a bunch of _optimized_ macros to
> > allow the compiler to perform optimizations.
> > 
> > Let's look at how this works today - let me pull out two entries:
> > 
> > #ifdef CONFIG_ARCH_EBSA285
> > # ifdef machine_arch_type
> > #  undef machine_arch_type
> > #  define machine_arch_type     __machine_arch_type
> > # else
> > #  define machine_arch_type     MACH_TYPE_EBSA285
> > # endif
> > # define machine_is_ebsa285()   (machine_arch_type == MACH_TYPE_EBSA285)
> > #else
> > # define machine_is_ebsa285()   (0)
> > #endif
> > 
> > #ifdef CONFIG_ARCH_NETWINDER
> > # ifdef machine_arch_type
> > #  undef machine_arch_type
> > #  define machine_arch_type     __machine_arch_type
> > # else
> > #  define machine_arch_type     MACH_TYPE_NETWINDER
> > # endif
> > # define machine_is_netwinder() (machine_arch_type == MACH_TYPE_NETWINDER)
> > #else
> > # define machine_is_netwinder() (0)
> > #endif
> > 
> > #ifndef machine_arch_type
> > #define machine_arch_type       __machine_arch_type
> > #endif
> > 
> > Now, if CONFIG_ARCH_EBSA285=y and CONFIG_ARCH_NETWINDER=n, then follow
> > through what the preprocessor does.  In the first case, machine_arch_type
> > is _not_ defined as a pre-processor symbol, so the "ifdef" is false.
> > That means machine_arch_type gets defined as MACH_TYPE_EBSA285.
> > 
> > machine_is_ebsa285() gets defined as (machine_arch_type == MACH_TYPE_EBSA285)
> > and when all the macro subsitutions occur (which happens where it's used)
> > this ends up becoming (MACH_TYPE_EBSA285 == MACH_TYPE_EBSA285).  This is
> > always true, so any code inside an if (machine_is_ebsa285()) {} block will
> > be compiled into the kernel and will be executed unconditionally - which
> > is exactly what we want.
> > 
> > Now, machine_is_netwinder() gets defined as constant (0).  This is always
> > false, so any code inside an if (machine_is_netwinder()) {} block will
> > be optimized away - exactly what we want.  This isn't affected by the
> > machine_arch_type definition.
> > 
> > Next, you can do the same thing for the CONFIG_ARCH_EBSA285=n and
> > CONFIG_ARCH_NETWINDER=y case, and you'll end up with similar results.
> > 
> > Finally for the case where CONFIG_ARCH_EBSA285=y and CONFIG_ARCH_NETWINDER=y.
> > In this case, it starts off just like the CONFIG_ARCH_EBSA285=y case above.
> > When we hit the CONFIG_ARCH_NETWINDER block a very important change happens.
> > This time machine_arch_type is already defined.
> > 
> > So, what happens is machine_arch_type first gets undefined to avoid any
> > compiler warnings about multiple definitions.  We then define
> > machine_arch_type to be the C variable __machine_arch_type.
> > 
> > This makes any references to (machine_arch_type == MACH_TYPE_WHATEVER)
> > become a runtime interpreted condition, which occurs for any machine type
> > that has its config symbol enabled.  So, machine_is_ebsa285() becomes
> > (__machine_arch_type == MACH_TYPE_EBSA285) and machine_is_netwinder()
> > becomes (__machine_arch_type == MACH_TYPE_NETWINDER), while other
> > platforms machine_is_xxx() macros remain defined to constant 0.
> > 
> > So, this all works exactly as we want.  There is no bug here.
> 
> Apparently the problem is that I have a custom symbol for their own board,
> but I specify an existing ID in the MACHINE_START. The initial boot is fine,
> because support for the ID is in the kernel, but the mach_desc is taken from
> a different machine. Below is a test program to demonstrate. In this test I
> boot kernel with ID=4 and I have a valid machines with ID=4 and 5, but symbol
> associated with ID=4 is missing. This case will a result boot a machine ID=5.
> How to solve the problem, I do not know yet...
> 
> <<< test.c
> #include <stdio.h>
> #include <string.h>
> #include <stdlib.h>
> 
> //#define CONFIG_ARCH_EBSA285 1
> #define CONFIG_ARCH_NETWINDER 1
> 
> #define MACH_TYPE_EBSA285 4
> #define MACH_TYPE_NETWINDER 5
> 
> #ifdef CONFIG_ARCH_EBSA285
> # ifdef machine_arch_type
> # undef machine_arch_type
> # define machine_arch_type __machine_arch_type
> # else
> # define machine_arch_type MACH_TYPE_EBSA285
> # endif
> # define machine_is_ebsa285() (machine_arch_type == MACH_TYPE_EBSA285)
> #else
> # define machine_is_ebsa285() (0)
> #endif
> 
> #ifdef CONFIG_ARCH_NETWINDER
> # ifdef machine_arch_type
> # undef machine_arch_type
> # define machine_arch_type __machine_arch_type
> # else
> # define machine_arch_type MACH_TYPE_NETWINDER
> # endif
> # define machine_is_netwinder() (machine_arch_type == MACH_TYPE_NETWINDER)
> #else
> # define machine_is_netwinder() (0)
> #endif
> 
> #ifndef machine_arch_type
> #define machine_arch_type __machine_arch_type
> #endif
> 
> unsigned int __machine_arch_type = MACH_TYPE_EBSA285; //From bootloader
> 
> int main(int argc, char** argv)
> {
> 	if (machine_is_ebsa285())
> 		printf("machine_is_ebsa285()\n");
> 	if (machine_is_netwinder())
> 		printf("machine_is_netwinder()\n");
> 	printf("  machine_arch_type=%u\n", machine_arch_type);
> 	printf("__machine_arch_type=%u\n", __machine_arch_type);
> 
> 	return 0;
> }
> >>>
> 
> shc at shc /home/git/test $ ./test 
> machine_is_netwinder()
>   machine_arch_type=5
> __machine_arch_type=4

That is working as designed, nothing wrong there.  You have EBSA285
support disabled (you commented out CONFIG_ARCH_EBSA285) and you have
Netwinder support enabled (CONFIG_ARCH_NETWINDER).  So,
machine_is_ebsa285() is constant false and machine_is_netwinder() is
constant true.

However, such a kernel won't boot when passed ID 5 because it won't
find the platform record in the kernel, and we explicitly error out
in that case.