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

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Message-ID: <b797bd15-ef3d-4d28-9aad-ffe0a32aa0b0@spud>
Date:   Thu, 9 Mar 2023 12:51:59 +0000
From:   Conor Dooley <conor.dooley@...rochip.com>
To:     Alexandre Ghiti <alex@...ti.fr>
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 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.

Thanks Alex!
Conor.

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