Re: [PATCH] ARM/dma-mapping: invalidate caches on arch_dma_prep_coherent
From: Catalin Marinas <catalin.marinas@arm.com>
Date: 2025-06-17 13:17:15
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linux-media, lkml
On Tue, Jun 17, 2025 at 09:54:46AM +0200, Christian Meissl wrote:
since switching to dma-direct, memory using DMA_ATTR_NO_KERNEL_MAPPING
is no longer allocated using the arch specific handlers and instead
will use dma_direct_alloc_no_mapping. While the arm specific allocation
handlers implicitly clear the allocated dma buffers and will flush any caches
dma-direct relies on ARCH_HAS_DMA_PREP_COHERENT to flush the caches.
Without this flush video frame corruption can occur in drivers
like the coda v4l2 driver which explicitly sets the DMA_ATTR_NO_KERNEL_MAPPING flag.
Fixes: ae626eb97376 ("ARM/dma-mapping: use dma-direct unconditionally")
Signed-off-by: Christian Meissl <redacted>[...]
quoted hunk ↗ jump to hunk
diff --git a/arch/arm/mm/dma-mapping.c b/arch/arm/mm/dma-mapping.c index 88c2d68a69c9..bde7ae4ba31a 100644 --- a/arch/arm/mm/dma-mapping.c +++ b/arch/arm/mm/dma-mapping.c@@ -1821,3 +1821,11 @@ void arch_dma_free(struct device *dev, size_t size, void *cpu_addr, { __arm_dma_free(dev, size, cpu_addr, dma_handle, attrs, false); } + +void arch_dma_prep_coherent(struct page *page, size_t size) +{ + void *ptr = page_address(page); + + dmac_flush_range(ptr, ptr + size); + outer_flush_range(__pa(ptr), __pa(ptr) + size); +}
It probably doesn't make any difference in practice, FWIW arm64 only does a clean rather than flush (clean+invalidate) here. What I noticed is that arch_dma_prep_coherent() is only called for lowmem pages, so doing page_address() is safe. However, I don't think we have anything to flush the caches for highmem pages. -- Catalin