ARM: mmu_early_enable

Lior Weintraub liorw at
Thu Aug 17 00:32:16 PDT 2023

But the UART is set on a total different memory space.
The SRAM where barebox runs and also given as membase and memsize is in the region of 0xC0_0000_0000
The UART is in the region of 0xD0_0030_7000.

According the tarmac log, it looks like access to this location caused the exception:
    3505306 ns  ES  (000000c0000814ac:910003fd) O el3h:	mov	x29,	sp 	(console_putc)
                    R X29 (AARCH64) 000000c0 00377ac0
    3505306 ns  ES  (000000c0000814b0:f9400460) O el3h:	ldr	x0,	[x3,	#8] 	(console_putc)
                    LD 000000c0000825d0  ........ ........ 000000d0 00307000   S:c0000825d0
                    R X0 (AARCH64) 000000d0 00307000
    3505307 ns  ES  (000000c0000814b4:d63f0040) O el3h:	blr	x2 	(console_putc)
                    R X30 (AARCH64) 000000c0 000814b8
    3505371 ns  ES  (000000c000000b5c:b9000001) O el3h:	str	w1,	[x0] 	(spider_serial_putc)
                    EXC [0x200] Synchronous Current EL with SP_ELx
                    R FAR_EL3 (AARCH64) 000000c0 00000b5c
                    R ESR_EL3 (AARCH64) 8600000f
                    R CPSR 200003cd
                    R SPSR_EL3 (AARCH64) 200003cd
                    R ELR_EL3 (AARCH64) 000000c0 00000b5c
    3505443 ns  ES  (000000c004000a00:14000586) O el3h:	b	c004002018	<exception_entry> 	(Vectors)
                    EXC [0x200] Synchronous Current EL with SP_ELx
                    R FAR_EL3 (AARCH64) 000000c0 04000a00
                    R ESR_EL3 (AARCH64) 8600000e
                    R CPSR 200003cd
                    R SPSR_EL3 (AARCH64) 200003cd
                    R ELR_EL3 (AARCH64) 000000c0 04000a00

BTW, In addition to the UART, there seems to be another issue.
The Vectors themselves are located in our ROM location which also resides on a different memory area (0xC0_0400_0000 space).
Once the UART access caused an exception, the jump to Vectors caused another exception so we are in a loop.

Looks like a catch22 to me.
On one hand the barebox wanted to start clean and disabled the MMU but on the other hand the mmu_early_enable sets a partial MMU which causes exceptions.
What do you think needs to be the best solution here?


> -----Original Message-----
> From: Sascha Hauer <s.hauer at>
> Sent: Thursday, August 17, 2023 10:18 AM
> To: Lior Weintraub <liorw at>
> Cc: barebox at
> Subject: Re: ARM: mmu_early_enable
> CAUTION: External Sender
> On Thu, Aug 17, 2023 at 06:22:50AM +0000, Lior Weintraub wrote:
> > Hi Sascha,
> >
> > I think I found an issue with the CONFIG_MMU feature.
> > When the code under barebox_pbl_start calls mmu_early_enable, the MMU
> > is set such that only the given SRAM is defined (membase, memsize).
> > But then, if DEBUG_LL is in use and the function pr_debug is called we
> > get an exception because the UART address is not included in the MMU.
> That shouldn't happen. See the code in mmu_early_enable():
>         early_remap_range(0, 1UL << (BITS_PER_VA - 1), MAP_UNCACHED);
>         early_remap_range(membase, memsize - OPTEE_SIZE, MAP_CACHED);
>         early_remap_range(membase + memsize - OPTEE_SIZE, OPTEE_SIZE,
> The first line maps the whole address space uncached in a flat 1:1
> mapping. The second and third lines map the SDRAM (SRAM in your case)
> cached.
> Your availabe memory is quite small (3MiB) and by skipping the
> relocation your SRAM layout is not standard. Could it be that something
> overwrites your page tables?
> Sascha
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