linux-kernel - Re: [PATCH] RISC-V-fixes: relocate DTB if it's outside memory region

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Date:   Thu, 28 Apr 2022 14:48:12 -0700 (PDT)
From:   Palmer Dabbelt <palmer@...belt.com>
To:     Conor.Dooley@...rochip.com
CC:     mick@....forth.gr, linux-riscv@...ts.infradead.org,
        linux-kernel@...r.kernel.org,
        Paul Walmsley <paul.walmsley@...ive.com>, aou@...s.berkeley.edu
Subject:     Re: [PATCH] RISC-V-fixes: relocate DTB if it's outside memory region

On Thu, 24 Mar 2022 02:37:29 PDT (-0700), Conor.Dooley@...rochip.com wrote:
> 
> On 22/03/2022 13:28, Nick Kossifidis wrote:
>> In case the DTB provided by the bootloader/BootROM is before the kernel
>> image or outside /memory, we won't be able to access it through the
>> linear mapping, and get a segfault on setup_arch(). Currently OpenSBI
>> relocates DTB but that's not always the case (e.g. if FW_JUMP_FDT_ADDR
>> is not specified), and it's also not the most portable approach since
>> the default FW_JUMP_FDT_ADDR of the generic platform relocates the DTB
>> at a specific offset that may not be available. To avoid this situation
>> copy DTB so that it's visible through the linear mapping.
>> 
>> Signed-off-by: Nick Kossifidis <mick@....forth.gr>
> 
> Albeit in a backport, I tested this on a PolarFire SoC based board.
> So I guess, Tested-by: Conor Dooley <conor.dooley@...rochip.com>

My scripts don't pick these up unless they're on their own line, not 
sure if that's too conservative but it's on purpose.  I just sort of 
stumbled into this one so it's in.

> 
> And a lot cleaner than using create_pgd_mapping in setup_vm_final to do it :)
> 
> Thanks,
> Conor.
> 
>> ---
>>   arch/riscv/mm/init.c | 21 +++++++++++++++++++--
>>   1 file changed, 19 insertions(+), 2 deletions(-)
>> 
>> diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
>> index 0d588032d..697a9aed4 100644
>> --- a/arch/riscv/mm/init.c
>> +++ b/arch/riscv/mm/init.c
>> @@ -206,8 +206,25 @@ static void __init setup_bootmem(void)
>>   	 * early_init_fdt_reserve_self() since __pa() does
>>   	 * not work for DTB pointers that are fixmap addresses
>>   	 */
>> -	if (!IS_ENABLED(CONFIG_BUILTIN_DTB))
>> -		memblock_reserve(dtb_early_pa, fdt_totalsize(dtb_early_va));
>> +	if (!IS_ENABLED(CONFIG_BUILTIN_DTB)) {
>> +		/*
>> +		 * In case the DTB is not located in a memory region we won't
>> +		 * be able to locate it later on via the linear mapping and
>> +		 * get a segfault when accessing it via __va(dtb_early_pa).
>> +		 * To avoid this situation copy DTB to a memory region.
>> +		 * Note that memblock_phys_alloc will also reserve DTB region.
>> +		 */
>> +		if (!memblock_is_memory(dtb_early_pa)) {
>> +			size_t fdt_size = fdt_totalsize(dtb_early_va);
>> +			phys_addr_t new_dtb_early_pa = memblock_phys_alloc(fdt_size, PAGE_SIZE);
>> +			void *new_dtb_early_va = early_memremap(new_dtb_early_pa, fdt_size);
>> +
>> +			memcpy(new_dtb_early_va, dtb_early_va, fdt_size);
>> +			early_memunmap(new_dtb_early_va, fdt_size);
>> +			_dtb_early_pa = new_dtb_early_pa;
>> +		} else
>> +			memblock_reserve(dtb_early_pa, fdt_totalsize(dtb_early_va));
>> +	}
>>   
>>   	early_init_fdt_scan_reserved_mem();
>>   	dma_contiguous_reserve(dma32_phys_limit);