On Fri, 16 Aug 2024 12:52:47 +0100
Will Deacon [off-list ref] wrote:

On Sun, Aug 11, 2024 at 10:09:35AM +0300, Baruch Siach wrote:

quoted

From: Catalin Marinas <catalin.marinas@arm.com>

Hardware DMA limit might not be power of 2. When RAM range starts above
0, say 4GB, DMA limit of 30 bits should end at 5GB. A single high bit
can not encode this limit.

Use plain address for DMA zone limit.

Since DMA zone can now potentially span beyond 4GB physical limit of
DMA32, make sure to use DMA zone for GFP_DMA32 allocations in that case.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Co-developed-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
---
 arch/arm64/mm/init.c       | 30 +++++++++++++++---------------
 arch/powerpc/mm/mem.c      |  5 ++++-
 arch/s390/mm/init.c        |  2 +-
 include/linux/dma-direct.h |  2 +-
 kernel/dma/direct.c        |  6 +++---
 kernel/dma/pool.c          |  4 ++--
 kernel/dma/swiotlb.c       |  6 +++---
 7 files changed, 29 insertions(+), 26 deletions(-)

diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
index 9b5ab6818f7f..c45e2152ca9e 100644
--- a/arch/arm64/mm/init.c
+++ b/arch/arm64/mm/init.c

@@ -115,35 +115,35 @@ static void __init arch_reserve_crashkernel(void)
 }
 
 /*
- * Return the maximum physical address for a zone accessible by the given bits
- * limit. If DRAM starts above 32-bit, expand the zone to the maximum
+ * Return the maximum physical address for a zone given its limit.
+ * If DRAM starts above 32-bit, expand the zone to the maximum
  * available memory, otherwise cap it at 32-bit.
  */
-static phys_addr_t __init max_zone_phys(unsigned int zone_bits)
+static phys_addr_t __init max_zone_phys(phys_addr_t zone_limit)
 {
-	phys_addr_t zone_mask = DMA_BIT_MASK(zone_bits);
 	phys_addr_t phys_start = memblock_start_of_DRAM();
 
 	if (phys_start > U32_MAX)
-		zone_mask = PHYS_ADDR_MAX;
-	else if (phys_start > zone_mask)
-		zone_mask = U32_MAX;
+		zone_limit = PHYS_ADDR_MAX;
+	else if (phys_start > zone_limit)
+		zone_limit = U32_MAX;
 
-	return min(zone_mask, memblock_end_of_DRAM() - 1) + 1;
+	return min(zone_limit, memblock_end_of_DRAM() - 1) + 1;  

Why do we need to adjust +-1 now that we're no longer using a mask?

Subtracting 1 is needed to get the highest valid DRAM address so it can
be compared to the highest address in the zone (zone_limit). Adding 1
is necessary to get the lowest address beyond the zone.

AFAICT this is the right thing here:

	arm64_dma_phys_limit = max_zone_phys(zone_dma_bits);
	max_zone_pfns[ZONE_DMA] = PFN_DOWN(arm64_dma_phys_limit);

max_zone_pfns[] max values are exclusive, i.e. the lowest PFN which is
_not_ within the zone.

It is also the right thing when arm64_dma_phys_limit is passed to
dma_contiguous_reserve().

It would be subtly broken if phys_addr_t could be a 32-bit integer, but
that's not possible on arm64.

In short, LGTM.

Petr T