[PATCH] arm64: mm: set ZONE_DMA size based on early IORT scan

[Ard Biesheuvel]

On Mon, 12 Oct 2020 at 11:30, Ard Biesheuvel <ardb at kernel.org> wrote:
>
> On Mon, 12 Oct 2020 at 11:28, Catalin Marinas <catalin.marinas at arm.com> wrote:
> >
> > On Sat, Oct 10, 2020 at 11:31:53AM +0200, Ard Biesheuvel wrote:
> > > diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
> > > index f0599ae73b8d..829fa63c3d72 100644
> > > --- a/arch/arm64/mm/init.c
> > > +++ b/arch/arm64/mm/init.c
> > > @@ -191,6 +191,14 @@ static void __init zone_sizes_init(unsigned long min, unsigned long max)
> > >       unsigned long max_zone_pfns[MAX_NR_ZONES]  = {0};
> > >
> > >  #ifdef CONFIG_ZONE_DMA
> > > +     if (IS_ENABLED(CONFIG_ACPI)) {
> > > +             extern unsigned int acpi_iort_get_zone_dma_size(void);
> >
> > Nitpick: can we add this prototype to include/linux/acpi_iort.h?
> >
> > > +
> > > +             zone_dma_bits = min(zone_dma_bits,
> > > +                                 acpi_iort_get_zone_dma_size());
> > > +             arm64_dma_phys_limit = max_zone_phys(zone_dma_bits);
> > > +     }
> > > +
> > >       max_zone_pfns[ZONE_DMA] = PFN_DOWN(arm64_dma_phys_limit);
> >
> > I think we should initialise zone_dma_bits slightly earlier via
> > arm64_memblock_init(). We'll eventually have reserve_crashkernel()
> > called before this and it will make use of arm64_dma_phys_limit for
> > "low" reservations:
> >
> > https://lore.kernel.org/linux-arm-kernel/20200907134745.25732-7-chenzhou10@huawei.com/
> >
>
> We don't have access to the ACPI tables yet at that point.

Also, could someone give an executive summary of why it matters where
the crashkernel is loaded? As far as I can tell, reserve_crashkernel()
only allocates memory for the kernel's executable image itself, which
can usually be loaded anywhere in memory. I could see how a
crashkernel might need some DMA'able memory if it needs to use the
hardware, but I don't think that is what is going on here.