linux-kernel - Re: RISC-V reserved memory problems

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Message-ID: <de204b7c-7c1d-bd7b-0072-d128757258e2@ghiti.fr>
Date:   Thu, 9 Mar 2023 16:12:57 +0100
From:   Alexandre Ghiti <alex@...ti.fr>
To:     Conor Dooley <conor.dooley@...rochip.com>
Cc:     Mike Rapoport <rppt@...nel.org>, Conor Dooley <conor@...nel.org>,
        palmer@...belt.com, linux-riscv@...ts.infradead.org,
        linux-kernel@...r.kernel.org, devicetree@...r.kernel.org,
        frowand.list@...il.com, robh+dt@...nel.org, mick@....forth.gr,
        paul.walmsley@...ive.com, aou@...s.berkeley.edu,
        Valentina.FernandezAlanis@...rochip.com,
        Daire.McNamara@...rochip.com
Subject: Re: RISC-V reserved memory problems

On 3/9/23 13:51, Conor Dooley wrote:
> On Thu, Mar 09, 2023 at 01:45:05PM +0100, Alexandre Ghiti wrote:
>> On 3/7/23 12:35, Mike Rapoport wrote:
>>> Hi Conor,
>>>
>>> Sorry for the delay, somehow this slipped between the cracks.
>>>
>>> On Thu, Feb 02, 2023 at 10:01:26PM +0000, Conor Dooley wrote:
>>>> Hullo Palmer, Mike & whoever else may read this,
>>>>
>>>> Just reviving this thread from a little while ago as I have been in the
>>>> area again recently...
>>> TBH, I didn't really dig deep into the issues, but the thought I had was
>>> what if DT was mapped via fixmap until the setup_vm_final() and then it
>>> would be possible to call DT methods early.
>>>
>>> Could be I'm shooting in the dark :)
>>
>> I think I understand the issue now, it's because In riscv, we establish 2
>> different virtual mappings and we map the device tree at 2 different virtual
>> addresses, which is the problem.
>>
>> So to me, the solution is:
>>
>> - to revert your previous fix, that is calling
>> early_init_fdt_scan_reserved_mem() before any call to memblock_alloc()
>> (which could result in an allocation in the area you want to reserve)
>>
>> - to map the device tree at the same virtual address, because
>> early_init_fdt_scan_reserved_mem() initializes reserved_mem with the dtb
>> mapping established in setup_vm() and uses reserved_mem with the new mapping
>> from setup_vm_final (which is what Mike proposes, we should use the fixmap
>> region to have the same virtual addresses)
>>
>> Hope that makes sense: I'll come up with something this afternoon for you to
>> test!
> Sounds good. Please give me some ELI5 commit messages if you can,
> explanations for this stuff (which I found took a lot of archaeology to
> understand) would be very welcome next time we need to go back looking
> at this stuff.

Can you give it a try here: 
https://github.com/AlexGhiti/riscv-linux/commits/dev/alex/conor_dtb_fixmap_v1 
?

That works for me but I need to carefully explain and check that's 
correct though, not upstreamable as is.

Thanks,

Alex

> Thanks Alex!
> Conor.
>
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> linux-riscv mailing list
> linux-riscv@...ts.infradead.org
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