Date:   Thu, 9 Mar 2023 12:51:59 +0000
From:   Conor Dooley <conor.dooley@microchip.com>
To:     Alexandre Ghiti <alex@ghiti.fr>
CC:     Mike Rapoport <rppt@kernel.org>, Conor Dooley <conor@kernel.org>,
        <palmer@dabbelt.com>, <linux-riscv@lists.infradead.org>,
        <linux-kernel@vger.kernel.org>, <devicetree@vger.kernel.org>,
        <frowand.list@gmail.com>, <robh+dt@kernel.org>,
        <mick@ics.forth.gr>, <paul.walmsley@sifive.com>,
        <aou@eecs.berkeley.edu>, <Valentina.FernandezAlanis@microchip.com>,
        <Daire.McNamara@microchip.com>
Subject: Re: RISC-V reserved memory problems
Message-ID: <b797bd15-ef3d-4d28-9aad-ffe0a32aa0b0@spud>
References: <8e10bf15-9fa9-fe90-1656-35bf3e87e7f8@microchip.com>
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 <Y9wytv5KSt1ca+td@spud>
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On Thu, Mar 09, 2023 at 01:45:05PM +0100, Alexandre Ghiti wrote:
>=20
> On 3/7/23 12:35, Mike Rapoport wrote:
> > Hi Conor,
> >=20
> > Sorry for the delay, somehow this slipped between the cracks.
> >=20
> > On Thu, Feb 02, 2023 at 10:01:26PM +0000, Conor Dooley wrote:
> > > Hullo Palmer, Mike & whoever else may read this,
> > >=20
> > > Just reviving this thread from a little while ago as I have been in t=
he
> > > 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.
> >=20
> > Could be I'm shooting in the dark :)
>=20
>=20
> 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 virt=
ual
> addresses, which is the problem.
>=20
> So to me, the solution is:
>=20
> - 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)
>=20
> - 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 mapp=
ing
> from setup_vm_final (which is what Mike proposes, we should use the fixmap
> region to have the same virtual addresses)
>=20
> 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.

Thanks Alex!
Conor.

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