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

[Catalin Marinas]

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.

>  	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.

-- 
Catalin