arm: mvebu: Kirkwood boards are broken by commit 6fe50e3

[Jerome Forissier]

On 6/20/25 12:53, Jerome Forissier wrote:
> Hi Tony,
> 
> On 6/19/25 22:37, Tony Dinh wrote:
>> Hi Jerome,
>>
>> While fixing some ext4 file system problems on 2025.07-rc4, I ran into
>> a problem booting a few Kirkwood boards. kwboot loaded the u-boot
>> image,
>> and then u-boot execution was frozen without anything going to the
>> serial console. I did a bisect and found the commit that caused it.
>>
>> https://github.com/trini/u-boot/commit/6fe50e39508043f386fc1bd40bbc02b8a75c1940
>>
>> When I tried to reverse this commit, I had a build error like this
>> "{standard input}:24246: Error: selected processor does not support
>> `mrc p15,1,r3,c15,c1,0' in Thumb mode"
>>
>> Currently, the only way to boot Kirkwood boards is to disable either
>> CONFIG_LTO or CONFIG_SYS_THUMB_BUILD or both.
>>
>> I'm using the pogo_v4 and nsa325 boards as test beds. Please let me
>> know if I can help with testing to troubleshoot this regression.
>>
>> All the best,
>> Tony
> 
> The problem seems to be that some arm32 instructions are inlined
> inside thumb code and that's not allowed on this CPU without some
> interworking instruction (bx, blx etc.). For example, consider
> board_init_r(), is is mainly 16-bit instructions (Thumb):
> 
> 006186a0 <board_init_r>:
> board_init_r():
> /home/jerome/work/u-boot/common/board_r.c:799
>   6186a0:       b5f0            push    {r4, r5, r6, r7, lr}
>   6186a2:       b0ab            sub     sp, #172        @ 0xac
> get_gd():
> /home/jerome/work/u-boot/./arch/arm/include/asm/global_data.h:127
>   6186a4:       464a            mov     r2, r9
> ...
> 
> but then suddenly:
> 
> /home/jerome/work/u-boot/arch/arm/mach-kirkwood/cpu.c:242
>   619aae:       9b15            ldr     r3, [sp, #84]   @ 0x54
> writefr_extra_feature_reg():
> /home/jerome/work/u-boot/./arch/arm/include/asm/arch/cpu.h:100
>   619ab0:       ee2f3f11        mcr     15, 1, r3, cr15, cr1, {0}
>                 ^^^^^^^^
>                 32-bit arm instruction
> 
> I would expect some call to a *_from_thumb() veneer, such as in:
> 
> 006009dc <do_bootm_qnxelf>:
> [...]
> /home/jerome/work/u-boot/boot/bootm_os.c:379
>   600a02:       9501            str     r5, [sp, #4]
> /home/jerome/work/u-boot/boot/bootm_os.c:384
>   600a04:       f04a fc7c       bl      64b300 <__dcache_status_from_thumb>
> 
> 
> 0064b130 <__dcache_enable_from_thumb>:
> __dcache_enable_from_thumb():
>   64b130:       4778            bx      pc
>   64b132:       e7fd            b.n     64b130 <__dcache_enable_from_thumb>
>   64b134:       e59fc000        ldr     ip, [pc]        @ 64b13c <__dcache_enable_from_thumb+0xc>
>   64b138:       e08cf00f        add     pc, ip, pc
>   64b13c:       fffffb28        .word   0xfffffb28
> 
> We may need to move the inline functions with mrc/mcr to a .S file,
> making sure it's built with -marm on CPUs that do not support Thumb2.
> Then hopefully the proper interworking sequence would be inserted by the
> linker as is the case when Thumb is calling into object files resulting
> from .c files built with -marm (for example: arch/arm/mach-kirkwood/cpu.c
> which has CFLAGS_cpu.o := -marm).
> 
> In the meantime, could you try the following patch?
> 
> diff --git a/common/Makefile b/common/Makefile
> index 35991562a12..a509675335a 100644
> --- a/common/Makefile
> +++ b/common/Makefile
> @@ -19,6 +19,8 @@ obj-y += version.o
>  # # boards
>  obj-y += board_f.o
>  obj-y += board_r.o
> +CFLAGS_board_f.o += -marm
> +CFLAGS_board_r.o += -marm
>  obj-$(CONFIG_DISPLAY_BOARDINFO) += board_info.o
>  obj-$(CONFIG_DISPLAY_BOARDINFO_LATE) += board_info.o
> 
> I think it should resolve the issue, but is does also increase the
> size of the binary (+4 KiB). So the .S solution would definitely be
> better if it works IMO. I'll try to propose something ASAP, will
> likely be on Monday, feel free to give it a try by yourself though :)

Or maybe this which is a bit less radical:

diff --git a/common/Makefile b/common/Makefile
index 35991562a12..f0b7eadf7d2 100644
--- a/common/Makefile
+++ b/common/Makefile
@@ -19,6 +19,8 @@ obj-y += version.o
 # # boards
 obj-y += board_f.o
 obj-y += board_r.o
+CFLAGS_REMOVE_board_f.o := $(LTO_CFLAGS)
+CFLAGS_REMOVE_board_r.o := $(LTO_CFLAGS)
 obj-$(CONFIG_DISPLAY_BOARDINFO) += board_info.o
 obj-$(CONFIG_DISPLAY_BOARDINFO_LATE) += board_info.o

Still results in +1 KiB.

-- 
Jerome