[RFC] arm64: mm: Do not use both DMA zones when 30-bit address space unavailable

[Nicolas Saenz Julienne]

Hi Catalin, thanks for taking the time.

On Tue, 2020-09-08 at 12:14 +0100, Catalin Marinas wrote:
> > Also note the usage of 'zone_dma_bits' in the DMA code, which assumes that
> > ZONE_DMA's physical address space is always smaller than (1 << zone_dma_bits) -
> > 1.
> 
> I think part of those uses are broken. dma_direct_supported() does the
> right thing and uses the DMA address instead of the physical one. Here
> __phys_to_dma() subtracts the dma_pfn_offset, which in my above example
> would be (0b10 << (30 - PAGE_SHIFT)).
> 
> dma_direct_optimal_gfp_mask(), OTOH, seems to start ok with a
> __dma_to_phys() on the dma_limit but it ends up comparing the physical
> address with the DMA mask. This gives the wrong result on arm64
> platforms where RAM starts above 4GB and still expect a ZONE_DMA32. It
> should compare *phys_limit with __dma_to_phys(DMA_BIT_MASK(...)). I
> guess it ends up bouncing via swiotlb more often.

I'll look into this.

> We assumed such offsets on arm64 since commit d50314a6b070 ("arm64:
> Create non-empty ZONE_DMA when DRAM starts above 4GB").
> 
> > > An alternative (and I think we had a patch at some point) is to make it
> > > generic and parse the dma-range in the DT to identify the minimum mask
> > > and set ZONE_DMA accordingly. But this doesn't solve ACPI, so if Linux
> > > can boot with ACPI on RPi4 it would still be broken.
> > 
> > ACPI is being worked on by, among others, Jeremy Linton (one of your colleagues
> > I believe).
> > 
> > We could always use sane defaults for ACPI and be smarter with DT. Yet,
> > implementing this entails translating nested dma-ranges with the only help of
> > libfdt, which isn't trivial (see devices/of/address.c). IIRC RobH said that it
> > wasn't worth the effort just for a board.
> 
> That would have been the ideal, more generic solution. But I agree that
> it's not worth the effort if the only SoC affected is RPi4.
> 
> To summarise, I'd like ZONE_DMA to overlap with ZONE_DMA32 (i.e. expand
> zone_dma_bits to 32 and drop ZONE_DMA32) for all SoCs other than RPi4.
> The solutions so far:
> 
> 1. Assume that, if RAM starts at 0, we need a zone_dma_bits == 30. This
>    also assumes that it's only RPi4 in this category or that any such
>    future SoC has a need for 30-bit DMA.
> 
> 2. Adjust zone_dma_bits at boot-time only if the SoC is RPi4.
> 
> 3. Use the more complex dma-ranges approach to calculate the correct
>    zone_dma_bits as a minimum of all dma masks in the DT.
> 
> We can discount (3) as not worth the effort. I'd go with (1) (this
> patch) if we can guarantee that no current or future SoC has RAM
> starting at 0 while not needing 30-bit DMA masks. If not, we can go with
> (2) unless others have a better suggestion.

After a quick check at the devices we have for testing at suse it's clear that
(1) is impossible. So I'll push for solution (2).

Regards,
Nicolas

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