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

[Nicolas Saenz Julienne]

On Sat, 2020-10-10 at 12:36 +0200, Ard Biesheuvel wrote:
> 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.

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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