Re: RISC-V reserved memory problems

[Alexandre Ghiti]

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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