Re: [PATCH v3] dma-mapping: move dma_map_resource() sanity check into debug code

[Leon Romanovsky]

On Wed, May 13, 2026 at 03:22:09PM +0800, Jianpeng Chang wrote:
> 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 <robin.murphy@xxxxxxx>
> Signed-off-by: Jianpeng Chang <jianpeng.chang.cn@xxxxxxxxxxxxx>
> ---
> 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@xxxxxxxxxxxxx/
>    - 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@xxxxxxxxxxxxx/
> 

Thanks,
Reviewed-by: Leon Romanovsky <leonro@xxxxxxxxxx>