Subject: Re: [PATCH v6] ARM: boot: Obtain start of physical memory from DTB

From: Russell King - ARM Linux admin <[email-masked]>
Date: Tue, 19 May 2020 13:27:06 +0100

On Tue, May 19, 2020 at 02:20:25PM +0200, Lukasz Stelmach wrote:
> It was <2020-05-19 wto 12:43>, when Russell King - ARM Linux admin wrote:
> > On Tue, May 19, 2020 at 01:21:09PM +0200, Geert Uytterhoeven wrote:
> >> On Tue, May 19, 2020 at 11:46 AM Russell King - ARM Linux admin
> >> <[email-masked]> wrote:
> >> > On Tue, May 19, 2020 at 11:44:17AM +0200, Geert Uytterhoeven wrote:
> >> > > On Tue, May 19, 2020 at 10:54 AM Lukasz Stelmach <[email-masked]> wrote:
> >> > > > It was <2020-04-29 śro 10:21>, when Geert Uytterhoeven wrote:
> >> > > > > Currently, the start address of physical memory is obtained by masking
> >> > > > > the program counter with a fixed mask of 0xf8000000.  This mask value
> >> > > > > was chosen as a balance between the requirements of different platforms.
> >> > > > > However, this does require that the start address of physical memory is
> >> > > > > a multiple of 128 MiB, precluding booting Linux on platforms where this
> >> > > > > requirement is not fulfilled.
> >> > > > >
> >> > > > > Fix this limitation by obtaining the start address from the DTB instead,
> >> > > > > if available (either explicitly passed, or appended to the kernel).
> >> > > > > Fall back to the traditional method when needed.
> >> > > > >
> >> > > > > This allows to boot Linux on r7s9210/rza2mevb using the 64 MiB of SDRAM
> >> > > > > on the RZA2MEVB sub board, which is located at 0x0C000000 (CS3 space),
> >> > > > > i.e. not at a multiple of 128 MiB.
> >> > > > >
> >> > > > > Suggested-by: Nicolas Pitre <[email-masked]>
> >> > > > > Signed-off-by: Geert Uytterhoeven <[email-masked]>
> >> > > > > Reviewed-by: Nicolas Pitre <[email-masked]>
> >> > > > > Reviewed-by: Ard Biesheuvel <[email-masked]>
> >> > > > > Tested-by: Marek Szyprowski <[email-masked]>
> >> > > > > Tested-by: Dmitry Osipenko <[email-masked]>
> >> > > > > ---
> >> > > >
> >> > > > [...]
> >> > > >
> >> > > > Apparently reading physical memory layout from DTB breaks crashdump
> >> > > > kernels. A crashdump kernel is loaded into a region of memory, that is
> >> > > > reserved in the original (i.e. to be crashed) kernel. The reserved
> >> > > > region is large enough for the crashdump kernel to run completely inside
> >> > > > it and don't modify anything outside it, just read and dump the remains
> >> > > > of the crashed kernel. Using the information from DTB makes the
> >> > > > decompressor place the kernel outside of the dedicated region.
> >> > > >
> >> > > > The log below shows that a zImage and DTB are loaded at 0x18eb8000 and
> >> > > > 0x193f6000 (physical). The kernel is expected to run at 0x18008000, but
> >> > > > it is decompressed to 0x00008000 (see r4 reported before jumping from
> >> > > > within __enter_kernel). If I were to suggest something, there need to be
> >> > > > one more bit of information passed in the DTB telling the decompressor
> >> > > > to use the old masking technique to determain kernel address. It would
> >> > > > be set in the DTB loaded along with the crashdump kernel.
> >> > >
> >> > > Shouldn't the DTB passed to the crashkernel describe which region of
> >> > > memory is to be used instead?
> >> >
> >> > Definitely not.  The crashkernel needs to know where the RAM in the
> >> > machine is, so that it can create a coredump of the crashed kernel.
> >> 
> >> So the DTB should describe both ;-)
> >> 
> >> > > Describing "to use the old masking technique" sounds a bit hackish to me.
> >> > > I guess it cannot just restrict the /memory node to the reserved region,
> >> > > as the crashkernel needs to be able to dump the remains of the crashed
> >> > > kernel, which lie outside this region.
> >> >
> >> > Correct.
> >> >
> >> > > However, something under /chosen should work.
> >> >
> >> > Yet another sticky plaster...
> >> 
> >> IMHO the old masking technique is the hacky solution covered by
> >> plasters.
> >
> > One line of code is not "covered by plasters".  There are no plasters.
> > It's a solution that works for 99.99% of people, unlike your approach
> > that has had a stream of issues over the last four months, and has
> > required many reworks of the code to fix each one.  That in itself
> > speaks volumes about the suitability of the approach.
> 
> As I have been working with kexec code (patches soon) I would like to
> defend the DT approach a bit. It allows to avoid zImage relocation when
> a decompressed kernel is larger than ~128MiB. In such case zImage isn't
> small either and moving it around takes some time.

... which is something that has been supported for a very long time,
before the days of DT.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
FTTC for 0.8m (est. 1762m) line in suburbia: sync at 13.1Mbps down 424kbps up

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