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    SubjectRe: [PATCH 1/4] of/fdt: Update zone_dma_bits when running in bcm2711
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    On Sat, 2020-10-10 at 12:36 +0200, Ard Biesheuvel wrote:
    > On Fri, 9 Oct 2020 at 19:10, Catalin Marinas <catalin.marinas@arm.com> 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@arm.com> 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.

    Yes I agree it's weird, and IMO plain useless. I sent a series this summer to
    address this[1], which ultimately triggered the discussion we're having right
    now.

    Although with with your latest patch and the DT counterpart, we should be OK.
    It would be weird for a HW description to define DMA constraints that are
    impossible to reach on that system.

    > 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.

    From my experience, you can't have empty ZONE_DMA when enabled. Empty
    ZONE_DMA32 is OK though. Although I'm sure it's something that can be changed.

    Regards,
    Nicolas

    [1] https://lkml.org/lkml/2020/8/19/1022


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