Merge tag 'arm64-fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux

Pull arm64 fix from Catalin Marinas:
 "Revert the dropping of the cache invalidation from the arm64
  arch_dma_prep_coherent() as it caused a regression in the
  qcom_q6v5_mss remoteproc driver.

  The driver is already buggy but the original arm64 change made
  the problem obvious. The change will be re-introduced once the
  driver is fixed"

* tag 'arm64-fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux:
  Revert "arm64: dma: Drop cache invalidation from arch_dma_prep_coherent()"

Author: torvalds

arch/arm64/mm/dma-mapping.c

@@ -36,7 +36,22 @@ void arch_dma_prep_coherent(struct page *page, size_t size)
 {
 	unsigned long start = (unsigned long)page_address(page);
 
-	dcache_clean_poc(start, start + size);
+	/*
+	 * 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);
 }
 
 #ifdef CONFIG_IOMMU_DMA