[U-Boot] [PATCH V7] POST cleanup.

[Wolfgang Denk]

Dear Michael Zaidman,

In message <d520c6ef298416a03789ebfa4e05e257b5331693.1284965175.git.michael.zaidman at gmail.com> you wrote:
> - Revives POST for blackfin arch;
> - Removes redundant code:
>      arch/blackfin/lib/post.c
>      arch/powerpc/cpu/ppc4xx/commproc.c
>      arch/powerpc/cpu/mpc512x/common.c
> - fixes up the post_word_{load|store} usage.

Unfortunately it turns out that the code now contains a few nasty
bugs...

...
>  #define CONFIG_SYS_GBL_DATA_OFFSET   (CONFIG_SYS_INIT_RAM_END - CONFIG_SYS_GBL_DATA_SIZE)
> -#define CONFIG_SYS_POST_WORD_ADDR    (CONFIG_SYS_GBL_DATA_OFFSET - 0x4)
> -#define CONFIG_SYS_INIT_SP_OFFSET    CONFIG_SYS_POST_WORD_ADDR
> +#define CONFIG_SYS_INIT_SP_OFFSET    (CONFIG_SYS_GBL_DATA_OFFSET - 0x4)

This is a seriously broken design, as it sneaks in storage for a
variable in a storage location where it is not expected.

The "official" layout is that we have CONFIG_SYS_INIT_RAM_BYTES
available; the top CONFIG_SYS_GBL_DATA_SIZE bytes (now
GENERATED_GBL_DATA_SIZE) are used for global data, and the part below
is used for the stack.  No other room is reserved there.

Shifting down the stack by 4 bytes as it's done here causes that the
stack is not correctly aligned any more, which may cause really nasty
subsequent errors.

But it's even worse.

> diff --git a/include/configs/mpc5121-common.h b/include/configs/mpc5121-common.h
> index 96fab20..afae1ab 100644
> --- a/include/configs/mpc5121-common.h
> +++ b/include/configs/mpc5121-common.h
...
> -#define CONFIG_SYS_POST_WORD_ADDR	(CONFIG_SYS_GBL_DATA_OFFSET - 0x4)
> -#define CONFIG_SYS_INIT_SP_OFFSET	CONFIG_SYS_POST_WORD_ADDR
> +#define CONFIG_SYS_INIT_SP_OFFSET	(CONFIG_SYS_GBL_DATA_OFFSET - 0x4)

There the same is done, but what happens actually?

Have a look how the stack setup gets implemented in
"arch/powerpc/cpu/mpc512x/start.S":

...
244 in_flash:
245         lis     r1, (CONFIG_SYS_INIT_RAM_ADDR + CONFIG_SYS_GBL_DATA_OFFSET)@h
246         ori     r1, r1, (CONFIG_SYS_INIT_RAM_ADDR + CONFIG_SYS_GBL_DATA_OFFSET)@l
247
248         li      r0, 0           /* Make room for stack frame header and */
249         stwu    r0, -4(r1)      /* clear final stack frame so that      */
250         stwu    r0, -4(r1)      /* stack backtraces terminate cleanly   */
...

As you can see, the code does not use CONFIG_SYS_INIT_SP_OFFSET at
all; instead it performs a calculation which should be redundant, but
in the current code it means that the location of the POST_WORD is
right in the initial stack.

I did not check if the code for other processors has similar issues.


"Reserving" private storage like that is bad, as other involved
parties probably have no knowledge of such a private reservation.


Why do we not simply reserve a word in the global data structure instead?


This bug needs pretty urgent fixing.


Best regards,

Wolfgang Denk

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