Re: [PATCH v4 5/7] arm64: mm: Set ZONE_DMA size based on devicetree's dma-ranges

[Nicolas Saenz Julienne]

Hi Catalin,

On Thu, 2020-10-22 at 19:06 +0100, Catalin Marinas wrote:
> On Wed, Oct 21, 2020 at 02:34:35PM +0200, Nicolas Saenz Julienne wrote:
> > @@ -188,9 +186,11 @@ static phys_addr_t __init max_zone_phys(unsigned int zone_bits)
> >  static void __init zone_sizes_init(unsigned long min, unsigned long max)
> >  {
> >  	unsigned long max_zone_pfns[MAX_NR_ZONES]  = {0};
> > +	unsigned int __maybe_unused dt_zone_dma_bits;
> >  
> >  #ifdef CONFIG_ZONE_DMA
> > -	zone_dma_bits = ARM64_ZONE_DMA_BITS;
> > +	dt_zone_dma_bits = ilog2(of_dma_get_max_cpu_address(NULL));
> > +	zone_dma_bits = min(32U, dt_zone_dma_bits);
> 
> A thought: can we remove the min here and expand ZONE_DMA to whatever
> dt_zone_dma_bits says? More on this below.

On most platforms we'd get PHYS_ADDR_MAX, or something bigger than the actual
amount of RAM. Which would ultimately create a system wide ZONE_DMA. At first
sight, I don't see it breaking dma-direct in any way.

On the other hand, there is a big amount of MMIO devices out there that can
only handle 32-bit addressing. Be it PCI cards or actual IP cores. To make
things worse, this limitation is often expressed in the driver, not FW (with
dma_set_mask() and friends). If those devices aren't behind an IOMMU we have be
able to provide at least 32-bit addressable memory. See this comment from
dma_direct_supported():

/*
 * Because 32-bit DMA masks are so common we expect every architecture
 * to be able to satisfy them - either by not supporting more physical
 * memory, or by providing a ZONE_DMA32.  If neither is the case, the
 * architecture needs to use an IOMMU instead of the direct mapping.
 */

I think, for the common case, we're stuck with at least one zone spanning the
32-bit address space.

> >  	arm64_dma_phys_limit = max_zone_phys(zone_dma_bits);
> >  	max_zone_pfns[ZONE_DMA] = PFN_DOWN(arm64_dma_phys_limit);
> >  #endif
> 
> I was talking earlier to Ard and Robin on the ZONE_DMA32 history and the
> need for max_zone_phys(). This was rather theoretical, the Seattle
> platform has all RAM starting above 4GB and that led to an empty
> ZONE_DMA32 originally. The max_zone_phys() hack was meant to lift
> ZONE_DMA32 into the bottom of the RAM, on the assumption that such
> 32-bit devices would have a DMA offset hardwired. We are not aware of
> any such case on arm64 systems and even on Seattle, IIUC 32-bit devices
> only work if they are behind an SMMU (so no hardwired offset).
> 
> In hindsight, it would have made more sense on platforms with RAM above
> 4GB to expand ZONE_DMA32 to cover the whole memory (so empty
> ZONE_NORMAL). Something like:
> 
> diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
> index a53c1e0fb017..7d5e3dd85617 100644
> --- a/arch/arm64/mm/init.c
> +++ b/arch/arm64/mm/init.c
> @@ -187,8 +187,12 @@ static void __init reserve_elfcorehdr(void)
>   */
>  static phys_addr_t __init max_zone_phys(unsigned int zone_bits)
>  {
> -	phys_addr_t offset = memblock_start_of_DRAM() & GENMASK_ULL(63, zone_bits);
> -	return min(offset + (1ULL << zone_bits), memblock_end_of_DRAM());
> +	phys_addr_t zone_mask = 1ULL << zone_bits;
> +
> +	if (!(memblock_start_of_DRAM() & zone_mask))
> +		zone_mask = PHYS_ADDR_MAX;
> +
> +	return min(zone_mask, memblock_end_of_DRAM());
>  }
>  
>  static void __init zone_sizes_init(unsigned long min, unsigned long max)
> 
> I don't think this makes any difference for ZONE_DMA unless a
> broken DT or IORT reports the max CPU address below the start of DRAM.
> 
> There's a minor issue if of_dma_get_max_cpu_address() matches
> memblock_end_of_DRAM() but they are not a power of 2. We'd be left with
> a bit of RAM at the end in ZONE_NORMAL due to ilog2 truncation.

I agree it makes no sense to create more than one zone when the beginning of
RAM is located above the 32-bit address space. I'm all for disregarding the
possibility of hardwired offsets. As a bonus, as we already discussed some time
ago, this is something that never played well with current dma-direct code[1].

Regards,
Nicolas

[1] https://lkml.org/lkml/2020/9/8/377

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