Re: [PATCH 2/3] riscv: Remove non-standard linux,elfcorehdr handling

[Geert Uytterhoeven]

Hi Nick,

On Wed, Jun 16, 2021 at 1:19 AM Nick Kossifidis <mick@xxxxxxxxxxxx> wrote:
> Στις 2021-06-15 22:54, Rob Herring έγραψε:
> > On Tue, Jun 15, 2021 at 12:40 PM Nick Kossifidis <mick@xxxxxxxxxxxx>
> > wrote:
> >> Στις 2021-06-15 21:17, Geert Uytterhoeven έγραψε:
> >> > RISC-V uses platform-specific code to locate the elf core header in
> >> > memory.  However, this does not conform to the standard
> >> > "linux,elfcorehdr" DT bindings, as it relies on a reserved memory node
> >> > with the "linux,elfcorehdr" compatible value, instead of on a
> >> > "linux,elfcorehdr" property under the "/chosen" node.
> >> >
> >> > The non-compliant code can just be removed, as the standard behavior is
> >> > already implemented by platform-agnostic handling in the FDT core code.
> >> >
> >> > Fixes: 5640975003d0234d ("RISC-V: Add crash kernel support")
> >> > Signed-off-by: Geert Uytterhoeven <geert+renesas@xxxxxxxxx>
> >>
> >> NACK
> >>
> >> There is nothing standard about "linux,elfcorehdr", it's an
> >
> > It is and it is documented which is more than we can say for
> > "linux,elfcorehdr" as a node.
> >
>
> Standard stuff goes on /drivers/of, not on /arch/arm64. The

... which is what I'm fixing ;-)

Once in a while, new code is added where it's used, and moved to
common code later.

> reserved-memory binding I use is on /drivers/of, is definitely a
> standard / documented binding and the only issue here is that the
> compatible string I used matched that property from arm64.

It's always a good idea to document your compatible strings, and run
your patches through the devicetree list, exactly to avoid issues
like this.

> >> arm64-specific property on /chosen and it's suboptimal, it gets the
> >> addr/length of ELF core of the previous kernel through that property
> >> and
> >> then goes on to reserve that region at:
> >> https://elixir.bootlin.com/linux/v5.13-rc6/source/arch/arm64/mm/init.c#L155
> >>
> >> Why on earth is this cleaner than just defining a reserved-region in
> >> the
> >> first place (a standard binding) with and hook up a callback with
> >> RESERVEDMEM_OF_DECLARE for it to also initialize elfcorehdr_addr/size
> >> ?
> >> If you don't like the compatible string I'm ok to change it, but this
> >> patch breaks kdump on riscv since that region won't be reserved any
> >> more
> >> and kernel will corrupt it.
> >
> > I might agree if we were designing this all from scratch, but we're
> > not. We've got powerpc doing /memreserve/ + kernel cmdline, arm64
> > using chosen, and RiscV a 3rd way.
> >
>
> I get it and I'd also like to consolidate things, but forcing riscv to
> use a suboptimal approach just because arm64 uses it doesn't make sense
> either, the goal should be for all to use the best possible approach
> (disclaimer: I'm not saying my approach is the best possible, I'm saying
> it's cleaner than arm64's).
>
> > What happens when/if RiscV wants to add an IMA buffer? That's no
> > different than this case. The 2 architectures supporting it both use
> > /chosen. Specifying an initrd is no different either.
>
> Those two are already on drivers/of/fdt.c and drivers/of/kexec.c, it's
> also interesting to note that for both of them, including
> "linux,elfcorehdr", the newly added drivers/of/kexec.c adds an entry to
> the fdt's memory reservation map when creating the fdt for the next
> kernel, so they are all basically reserved regions. Why this was chosen
> (a property on /chosen + an entry on the reservation map), effectively
> adding each region twice on the fdt, instead of just adding a
> reserved-memory node for each one beats me. Note that in case of arm64
> this is not what happens on kexec-tools, which is probably the reason
> why arm64 still reserves them in any case.

I can't comment on the duplication on arm64, but to me, /chosen
sounds like the natural place for both "linux,elfcorehdr" and
"linux,usable-memory-range".  First rule of DT is "DT describes
hardware, not software policy", with /chosen describing some software
configuration.

> Anyway I guess switching arm64 to reserved-memory is too much to ask
> since they would have to also change kexec-tools, handle different
> versions etc, and although I don't like it consolidation is more
> important than a duplicate region on the fdt, so let's go with
> "linux,elfcorehdr" on /chosen + entry on the reservation map. I'll
> update my kexec-tools patch instead.

OK, thanks!
But do you need the entry on the reservation map?

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds