From: Mikhail Gavrilov Date: Fri, 27 Mar 2026 12:41:56 +0000 (+0500) Subject: dma-debug: suppress cacheline overlap warning when arch has no DMA alignment requirement X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=3d48c9fd78dd0b1809669ec49c4d0997b8127512;p=thirdparty%2Fkernel%2Flinux.git dma-debug: suppress cacheline overlap warning when arch has no DMA alignment requirement When CONFIG_DMA_API_DEBUG is enabled, the DMA debug infrastructure tracks active mappings per cacheline and warns if two different DMA mappings share the same cacheline ("cacheline tracking EEXIST, overlapping mappings aren't supported"). On x86_64, ARCH_KMALLOC_MINALIGN defaults to 8, so small kmalloc allocations (e.g. the 8-byte hub->buffer and hub->status in the USB hub driver) frequently land in the same 64-byte cacheline. When both are DMA-mapped, this triggers a false positive warning. This has been reported repeatedly since v5.14 (when the EEXIST check was added) across various USB host controllers and devices including xhci_hcd with USB hubs, USB audio devices, and USB ethernet adapters. The cacheline overlap is only a real concern on architectures that require DMA buffer alignment to cacheline boundaries (i.e. where ARCH_DMA_MINALIGN >= L1_CACHE_BYTES). On architectures like x86_64 where dma_get_cache_alignment() returns 1, the hardware is cache-coherent and overlapping cacheline mappings are harmless. Suppress the EEXIST warning when dma_get_cache_alignment() is less than L1_CACHE_BYTES, indicating the architecture does not require cacheline-aligned DMA buffers. Verified with a kernel module reproducer that performs two kmalloc(8) allocations back-to-back and DMA-maps both: Before: allocations share a cacheline, EEXIST fires within ~50 pairs After: same cacheline pair found, but no warning emitted Fixes: 2b4bbc6231d7 ("dma-debug: report -EEXIST errors in add_dma_entry") Link: https://bugzilla.kernel.org/show_bug.cgi?id=215740 Suggested-by: Harry Yoo Tested-by: Mikhail Gavrilov Signed-off-by: Mikhail Gavrilov Signed-off-by: Marek Szyprowski Link: https://lore.kernel.org/r/20260327124156.24820-1-mikhail.v.gavrilov@gmail.com --- diff --git a/kernel/dma/debug.c b/kernel/dma/debug.c index 0677918f06a8..1a725edbbbf6 100644 --- a/kernel/dma/debug.c +++ b/kernel/dma/debug.c @@ -615,6 +615,7 @@ static void add_dma_entry(struct dma_debug_entry *entry, unsigned long attrs) } else if (rc == -EEXIST && !(attrs & DMA_ATTR_SKIP_CPU_SYNC) && !(entry->is_cache_clean && overlap_cache_clean) && + dma_get_cache_alignment() >= L1_CACHE_BYTES && !(IS_ENABLED(CONFIG_DMA_BOUNCE_UNALIGNED_KMALLOC) && is_swiotlb_active(entry->dev))) { err_printk(entry->dev, entry,