linux-kernel - Re: [PATCH 0/3] Highmem support for 32-bit RISC-V

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Date:   Sun, 3 May 2020 22:20:39 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     afzal mohammed <afzal.mohd.ma@...il.com>
Cc:     Russell King <linux@...linux.org.uk>,
        Alan Kao <alankao@...estech.com>,
        Eric Lin <tesheng@...estech.com>, Gary Guo <gary@...yguo.net>,
        alex@...ti.fr, David Abdurachmanov <david.abdurachmanov@...il.com>,
        Anup Patel <Anup.Patel@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Steven Price <steven.price@....com>, atish.patra@....com,
        yash.shah@...ive.com, Albert Ou <aou@...s.berkeley.edu>,
        Palmer Dabbelt <palmer@...belt.com>,
        Greentime Hu <green.hu@...il.com>, zong.li@...ive.com,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...e.de>,
        Logan Gunthorpe <logang@...tatee.com>,
        linux-riscv@...ts.infradead.org
Subject: Re: [PATCH 0/3] Highmem support for 32-bit RISC-V

On Sun, May 3, 2020 at 4:50 PM afzal mohammed <afzal.mohd.ma@...il.com> wrote:
>
> Hi Arnd,
>
> > On Tue, Apr 14, 2020 at 09:29:46PM +0200, Arnd Bergmann wrote:
>
> > > Another thing to try early is to move the vmlinux virtual address
> > > from the linear mapping into vmalloc space. This does not require
> > > LPAE either, but it only works on relatively modern platforms that
> > > don't have conflicting fixed mappings there.
>
> i have started by attempting to move static kernel mapping from lowmem
> to vmalloc space. At boot the execution so far has went past assembly
> & reached C, to be specific, arm_memblock_init [in setup_arch()],
> currently debugging the hang that happens after that point.

Ah, good start. Which SoC platform are you running this on? Just making
sure that this won't conflict with static mappings later.

One problem I see immediately in arm_memblock_init() is that it uses
__pa() to convert from virtual address in the linear map to physical,
but now you actually pass an address that is in vmalloc rather than
the linear map. There are certainly more problems like this to come.

> To make things easier in the beginning, ARM_PATCH_PHYS_VIRT is disabled &
> platform specific PHYS_OFFSET is fed, this is planned to be fixed once
> it boots.
>
> [ i will probably start a new thread or hopefully RFC on LAKML ]

Ok, makes sense.

     Arnd