Re: [PATCH 1/4] of/fdt: Update zone_dma_bits when running in bcm2711

[Ard Biesheuvel]

On Fri, 9 Oct 2020 at 19:10, Catalin Marinas <catalin.marinas@xxxxxxx> wrote:
>
> On Fri, Oct 09, 2020 at 06:23:06PM +0200, Ard Biesheuvel wrote:
> > On Fri, 9 Oct 2020 at 17:24, Lorenzo Pieralisi
> > <lorenzo.pieralisi@xxxxxxx> wrote:
> > > We can move this check to IORT code and call it from arm64 if it
> > > can be made to work.
> >
> > Finding the smallest value in the IORT, and assigning it to
> > zone_dma_bits if it is < 32 should be easy. But as I understand it,
> > having these separate DMA and DMA32 zones is what breaks kdump, no? So
> > how is this going to fix the underlying issue?
>
> If zone_dma_bits is 32, ZONE_DMA32 disappears into ZONE_DMA (GFP_DMA32
> allocations fall back to ZONE_DMA).
>
> kdump wants DMA-able memory and, without a 30-bit ZONE_DMA, that would
> be the bottom 32-bit. With the introduction of ZONE_DMA, this suddenly
> became 1GB. We could change kdump to allocate ZONE_DMA32 but this one
> may also be small as it lost 1GB to ZONE_DMA. However, the kdump kernel
> would need to be rebuilt without ZONE_DMA since it won't have any. IIRC
> (it's been a while since I looked), the kdump allocation couldn't span
> multiple zones.
>
> In a separate thread, we try to fix kdump to use allocations above 4G as
> a fallback but this only fixes platforms with enough RAM (and maybe it's
> only those platforms that care about kdump).
>

One thing that strikes me as odd is that we are applying the same
shifting logic to ZONE_DMA as we are applying to ZONE_DMA32, i.e., if
DRAM starts outside of the zone, it is shifted upwards.

On a typical ARM box, this gives me

[    0.000000] Zone ranges:
[    0.000000]   DMA      [mem 0x0000000080000000-0x00000000bfffffff]
[    0.000000]   DMA32    [mem 0x00000000c0000000-0x00000000ffffffff]
[    0.000000]   Normal   [mem 0x0000000100000000-0x0000000fffffffff]

i.e., the 30-bit addressable range has bit 31 set, which is weird.

I wonder if it wouldn't be better (and less problematic in the general
case) to drop this logic for ZONE_DMA, and simply let it remain empty
unless there is really some memory there.