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Date:   Sun, 11 Jun 2023 07:27:08 +0800
From:   Woody Zhang <woodylab@foxmail.com>
To:     Conor Dooley <conor@kernel.org>
Cc:     Alexandre Ghiti <alexghiti@rivosinc.com>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] riscv: reserve DTB before possible memblock allocation
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On Sat, Jun 10, 2023 at 04:41:17PM +0100, Conor Dooley wrote:
>On Thu, Jun 08, 2023 at 09:49:44AM +0200, Alexandre Ghiti wrote:
>> On Wed, Jun 7, 2023 at 8:17 PM Conor Dooley <conor@kernel.org> wrote:
>> >
>> > +CC Alex, you should take a look at this patch.
>> >
>> > On Wed, Jun 07, 2023 at 09:35:19PM +0800, Woody Zhang wrote:
>> > > It's possible that early_init_fdt_scan_reserved_mem() allocates memory
>> > > from memblock for dynamic reserved memory in `/reserved-memory` node.
>> > > Any fixed reservation must be done before that to avoid potential
>> > > conflicts.
>> > >
>> > > Reserve the DTB in memblock just after early scanning it.
>> >
>> > The rationale makes sense to me, I am just wondering what compelling
>> > reason there is to move it away from the memblock_reserve()s for the
>> > initd and vmlinux? Moving it above early_init_fdt_scan_reserved_mem()
>> > should be the sufficient minimum & would keep things together.
>
>> Thanks Conor.
>> 
>> So the patch looks good to me.
>> 
>> But I find this fragile:
>> 
>> - we do not check memblock_reserve() return value to make sure the
>> reservation really happened (and quickly looking at the code, I'm not
>> even sure it returns an error if the region was already allocated).
>> - we have to make sure no memblock allocation happens before setup_bootmem().
>> - we also have to check that no fixed memblock_reserve() happens after.
>> 
>> The last 2 points may sound natural, but we'll have to take great care
>> when adding some code around here. I'm working on an "early boot
>> document" and I'll add something about that, but a runtime thing would
>> be way better IMO.
>> 
>> You can add:
>> 
>> Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
>
>btw Alex/Woody, what is the appropriate Fixes tag here?

In ef69d2559fe9 ("riscv: Move early dtb mapping into the fixmap region"),
Alex move early_init_fdt_scan_reserved_mem to setup_bootmem() to prevent
memory allocations before of reservations. But it should not be put before
DTB reservation.


Woody