Revert "Revert "arm64: dma: Drop cache invalidation from arch_dma_prep_coherent()""

This reverts commit b7d9aae.

With the Qualcomm remoteproc driver now modified to use a carveout
memory region in 57f7217 ("remoteproc: qcom_q6v5_mss: Use a
carveout to authenticate modem headers"), we can reinstate c44094e
("arm64: dma: Drop cache invalidation from arch_dma_prep_coherent()")
which relaxes the arm64 implementation of arch_dma_prep_coherent() to
perform only a data cache clean operation, rather than a
clean-and-invalidate.

Signed-off-by: Will Deacon <[email protected]>

Author: willdeacon

arch/arm64/mm/dma-mapping.c

@@ -36,22 +36,7 @@ void arch_dma_prep_coherent(struct page *page, size_t size)
 {
 	unsigned long start = (unsigned long)page_address(page);
 
-	/*
-	 * The architecture only requires a clean to the PoC here in order to
-	 * meet the requirements of the DMA API. However, some vendors (i.e.
-	 * Qualcomm) abuse the DMA API for transferring buffers from the
-	 * non-secure to the secure world, resetting the system if a non-secure
-	 * access shows up after the buffer has been transferred:
-	 *
-	 * https://lore.kernel.org/r/[email protected]
-	 *
-	 * Using clean+invalidate appears to make this issue less likely, but
-	 * the drivers themselves still need fixing as the CPU could issue a
-	 * speculative read from the buffer via the linear mapping irrespective
-	 * of the cache maintenance we use. Once the drivers are fixed, we can
-	 * relax this to a clean operation.
-	 */
-	dcache_clean_inval_poc(start, start + size);
+	dcache_clean_poc(start, start + size);
 }
 
 #ifdef CONFIG_IOMMU_DMA