[PATCH v3] riscv: mm: fix SWIOTLB initialization for systems with DRAM above 4GB

[Troy Mitchell]

On RISC-V platforms where the entire physical memory (DRAM) resides
above the 32-bit address space (i.e., above dma32_phys_limit), the
current SWIOTLB initialization logic fails.

This patch addresses two interconnected issues on such platforms:

1. Incorrect 32-bit DMA bounce assumption:
The existing condition `max_pfn > PFN_DOWN(dma32_phys_limit)` assumes
that a 32-bit DMA bounce buffer is required simply because the maximum
PFN exceeds the 32-bit limit. However, if all DRAM starts above 4GB,
no memory exists below the limit to satisfy this allocation. Fix
this by adding a check to ensure `memblock_start_of_DRAM()` is actually
below the 32-bit limit before enforcing 32-bit SWIOTLB.

2. kmalloc() bounce buffer allocation failure on non-coherent systems:
For non-coherent DMA, kmalloc() buffers whose sizes are not
cache-line-aligned still require bouncing, even if 32-bit DMA bouncing
is skipped. Without the `SWIOTLB_ANY` flag, swiotlb_init() defaults to
allocating from low memory, which fails completely when DRAM only exists
in high memory. By appending `SWIOTLB_ANY` to swiotlb_flags, the allocator
is permitted to allocate this bounce buffer from high memory.

With this patch, systems with non-coherent DMA and DRAM entirely above
4GB can successfully map the software IO TLB in high memory and boot
normally.

Tested-by: Anirudh Srinivasan <asrinivasan at oss.tenstorrent.com>
Signed-off-by: Troy Mitchell <troy.mitchell at linux.dev>
---
Changes in v3:
- clarify when kmalloc() buffers require bouncing for non-coherent DMA
- Link to v2: https://patch.msgid.link/20260429-fix-riscv-swiotlb-v2-1-fa99dfdfc94d@linux.dev

Changes in v2:
- add Anirudh's TB tag
- Link to v1: https://lore.kernel.org/r/20260331-fix-riscv-swiotlb-v1-1-74dd5e6be0f1@linux.dev

To: Paul Walmsley <pjw at kernel.org>
To: Palmer Dabbelt <palmer at dabbelt.com>
To: Albert Ou <aou at eecs.berkeley.edu>
To: Alexandre Ghiti <alex at ghiti.fr>
Cc: linux-riscv at lists.infradead.org
Cc: linux-kernel at vger.kernel.org
---
 arch/riscv/mm/init.c | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
index 811e03786c56..7459e1fdb04a 100644
--- a/arch/riscv/mm/init.c
+++ b/arch/riscv/mm/init.c
@@ -168,7 +168,9 @@ static void print_vm_layout(void) { }
 
 void __init arch_mm_preinit(void)
 {
-	bool swiotlb = max_pfn > PFN_DOWN(dma32_phys_limit);
+	bool swiotlb = max_pfn > PFN_DOWN(dma32_phys_limit) &&
+		       memblock_start_of_DRAM() < dma32_phys_limit;
+	unsigned int swiotlb_flags = SWIOTLB_VERBOSE;
 #ifdef CONFIG_FLATMEM
 	BUG_ON(!mem_map);
 #endif /* CONFIG_FLATMEM */
@@ -176,17 +178,22 @@ void __init arch_mm_preinit(void)
 	if (IS_ENABLED(CONFIG_DMA_BOUNCE_UNALIGNED_KMALLOC) && !swiotlb &&
 	    dma_cache_alignment != 1) {
 		/*
-		 * If no bouncing needed for ZONE_DMA, allocate 1MB swiotlb
-		 * buffer per 1GB of RAM for kmalloc() bouncing on
-		 * non-coherent platforms.
+		 * No 32-bit DMA bouncing needed (either all DRAM is within
+		 * the 32-bit limit, or it all starts above it), but
+		 * kmalloc() buffers whose sizes are not cache-line-aligned
+		 * still require bouncing for non-coherent DMA.  Use
+		 * SWIOTLB_ANY so that the buffer can be allocated from high
+		 * memory when DRAM starts above dma32_phys_limit.  Allocate
+		 * ~1 MB per 1 GB of RAM.
 		 */
 		unsigned long size =
 			DIV_ROUND_UP(memblock_phys_mem_size(), 1024);
 		swiotlb_adjust_size(min(swiotlb_size_or_default(), size));
 		swiotlb = true;
+		swiotlb_flags |= SWIOTLB_ANY;
 	}
 
-	swiotlb_init(swiotlb, SWIOTLB_VERBOSE);
+	swiotlb_init(swiotlb, swiotlb_flags);
 
 	print_vm_layout();
 }

---
base-commit: 6de23f81a5e08be8fbf5e8d7e9febc72a5b5f27f
change-id: 20260331-fix-riscv-swiotlb-6f1c226071d1

Best regards,
--  
Troy Mitchell <troy.mitchell at linux.dev>