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

[Anirudh Srinivasan <asrinivasan@oss.tenstorrent.com>]

On Tue, Mar 31, 2026 at 03:37:22PM +0800, Troy Mitchell wrote:
> 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 hardware, a bounce buffer is still mandatory for
> cache-line-aligned kmalloc(), 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 alignment 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.
> 
> Signed-off-by: Troy Mitchell <troy.mitchell@linux.dev>
> ---
>  arch/riscv/mm/init.c | 16 +++++++++++-----
>  1 file changed, 11 insertions(+), 5 deletions(-)

Tested on Tenstorrent Blackhole, which doesn't have any memory < 4GB.
With this patch, these lines no longer appear during boot

[    0.000000] software IO TLB: area num 4.
[    0.000000] software IO TLB: swiotlb_memblock_alloc: Failed to allocate 67108864 bytes tlb structure
[    0.000000] software IO TLB: swiotlb_memblock_alloc: Failed to allocate 33554432 bytes tlb structure
[    0.000000] software IO TLB: swiotlb_memblock_alloc: Failed to allocate 16777216 bytes tlb structure
[    0.000000] software IO TLB: swiotlb_memblock_alloc: Failed to allocate 8388608 bytes tlb structure
[    0.000000] software IO TLB: swiotlb_memblock_alloc: Failed to allocate 4194304 bytes tlb structure
[    0.000000] software IO TLB: swiotlb_memblock_alloc: Failed to allocate 2097152 bytes tlb structure
[    0.000000] software IO TLB: swiotlb_memblock_alloc: Failed to allocate 1048576 bytes tlb structure

Tested-by: Anirudh Srinivasan <asrinivasan@oss.tenstorrent.com>

Regards
Anirudh Srinivasan