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charset="utf-8" dma_map_resource() uses pfn_valid() to ensure the range is not RAM. However, pfn_valid() only checks for availability of the memory map for a PFN but it does not ensure that the PFN is actually backed by RAM. On ARM64 with SPARSEMEM (128MB section granularity), MMIO addresses that share a section with RAM will falsely trigger the WARN_ON_ONCE and cause dma_map_resource() to return DMA_MAPPING_ERROR. This causes a WARNING on Raspberry Pi 4 during spi_bcm2835 probe because the SPI FIFO register (0xfe204004) falls in the same sparsemem section as the end of RAM (0xf8000000-0xfbffffff), both in section 31 (0xf8000000-0xffffffff). Move the sanity check from dma_map_resource() into debug_dma_map_phys() and replace the unreliable pfn_valid() with pfn_valid() && !PageReserved(), which correctly identifies actual usable RAM without false positives for MMIO regions that happen to have struct pages. Since dma_map_resource() is dma_map_phys(DMA_ATTR_MMIO), the check applies equally to both APIs. Any non-reserved page represents kernel memory to a sufficient degree that using DMA_ATTR_MMIO on it is almost certainly wrong and risks breaking coherency on non-coherent platforms. ZONE_DEVICE pages used for PCI P2P DMA (MEMORY_DEVICE_PCI_P2PDMA) have PageReserved set, so they will not trigger a false positive. The check no longer blocks the mapping and uses err_printk() to integrate with dma-debug filtering. Fixes: f7326196a781 ("dma-mapping: export new dma_*map_phys() interface") Reviewed-by: Robin Murphy Signed-off-by: Jianpeng Chang Reviewed-by: Leon Romanovsky --- v3: - WARN_ONCE -> err_printk() - move the MMIO check down, and delete the return v2: https://lore.kernel.org/all/20260511083133.1096171-1-jianpeng.chang.cn@= windriver.com/ - move check to debug_dma_map_phys and replace pfn_valid() with pfn_valid() && !PageReserved() as Robin suggested. - update commit message to explain why PageReserved is safe for ZONE_DEVICE PCI_P2PDMA pages v1: https://lore.kernel.org/all/20260507032120.4072283-1-jianpeng.chang.cn@= windriver.com/ kernel/dma/debug.c | 9 ++++++++- kernel/dma/mapping.c | 4 ---- 2 files changed, 8 insertions(+), 5 deletions(-) diff --git a/kernel/dma/debug.c b/kernel/dma/debug.c index 1a725edbbbf6..3248f8b4d096 100644 --- a/kernel/dma/debug.c +++ b/kernel/dma/debug.c @@ -1251,7 +1251,14 @@ void debug_dma_map_phys(struct device *dev, phys_add= r_t phys, size_t size, entry->direction =3D direction; entry->map_err_type =3D MAP_ERR_NOT_CHECKED; =20 - if (!(attrs & DMA_ATTR_MMIO)) { + if (attrs & DMA_ATTR_MMIO) { + unsigned long pfn =3D PHYS_PFN(phys); + + if (pfn_valid(pfn) && !PageReserved(pfn_to_page(pfn))) + err_printk(dev, entry, + "dma_map_resource called for RAM address %pa\n", + &phys); + } else { check_for_stack(dev, phys); =20 if (!PhysHighMem(phys)) diff --git a/kernel/dma/mapping.c b/kernel/dma/mapping.c index 23ed8eb9233e..e6b07f160d20 100644 --- a/kernel/dma/mapping.c +++ b/kernel/dma/mapping.c @@ -365,10 +365,6 @@ EXPORT_SYMBOL(dma_unmap_sg_attrs); dma_addr_t dma_map_resource(struct device *dev, phys_addr_t phys_addr, size_t size, enum dma_data_direction dir, unsigned long attrs) { - if (IS_ENABLED(CONFIG_DMA_API_DEBUG) && - WARN_ON_ONCE(pfn_valid(PHYS_PFN(phys_addr)))) - return DMA_MAPPING_ERROR; - return dma_map_phys(dev, phys_addr, size, dir, attrs | DMA_ATTR_MMIO); } EXPORT_SYMBOL(dma_map_resource); --=20 2.51.2