Re: [PATCH v2 1/7] arm64: Unconditionally call unflatten_device_tree()

[Stephen Boyd <sboyd@kernel.org>]

Quoting Rob Herring (2024-01-31 12:54:05)
> On Mon, Jan 29, 2024 at 04:45:00PM -0800, Stephen Boyd wrote:
> > Call this function unconditionally so that we can populate an empty DTB
> > on platforms that don't boot with a firmware provided or builtin DTB.
> > Override 'initial_boot_params' to NULL when ACPI is in use but the
> > bootloader has loaded a DTB so that we don't allow both ACPI and DT to
> > be used during boot. If there isn't a valid initial_boot_params dtb then
> > unflatten_device_tree() returns early so this is fine.
> > 
> > Cc: Rob Herring <robh+dt@kernel.org>
> > Cc: Frank Rowand <frowand.list@gmail.com>
> > Cc: Catalin Marinas <catalin.marinas@arm.com>
> > Cc: Will Deacon <will@kernel.org>
> > Cc: Mark Rutland <mark.rutland@arm.com>
> > Cc: <linux-arm-kernel@lists.infradead.org>
> > Signed-off-by: Stephen Boyd <sboyd@kernel.org>
> > ---
> >  arch/arm64/kernel/setup.c | 7 +++++--
> >  1 file changed, 5 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/arm64/kernel/setup.c b/arch/arm64/kernel/setup.c
> > index 417a8a86b2db..ffb1942724ae 100644
> > --- a/arch/arm64/kernel/setup.c
> > +++ b/arch/arm64/kernel/setup.c
> > @@ -351,8 +351,11 @@ void __init __no_sanitize_address setup_arch(char **cmdline_p)
> >       /* Parse the ACPI tables for possible boot-time configuration */
> >       acpi_boot_table_init();
> >  
> > -     if (acpi_disabled)
> > -             unflatten_device_tree();
> > +     /* Don't use the FDT from boot if ACPI is in use */
> > +     if (!acpi_disabled)
> > +             initial_boot_params = NULL;
> 
> I still think this is a problem for kexec. See 
> of_kexec_alloc_and_setup_fdt(). You see it uses initial_boot_params. At 
> first glance it looks like it would just write out everything we need. 
> But for UEFI boot, I think we need all the chosen properties like 
> linux,uefi-mmap-start preserved from the current boot for the next 
> kernel we kexec.

Ok, got it.

> 
> I think you'll have to check acpi_disabled in unflatten_device_tree() 
> and unflatten the empty tree leaving initial_boot_params alone. That 
> means our FDT and unflattened tree will be different DTs, but I think 
> that's fine.

It's sort of scary given that 'initial_boot_params' is an exported
global. Maybe that should be hidden away and accessed with a function
instead so that this mismatch doesn't break something later on?