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

[Will Deacon]

This reverts commit c44094eee32f32f175aadc0efcac449d99b1bbf7.

Although the semantics of the DMA API require only a clean operation
here, it turns out that the Qualcomm 'qcom_q6v5_mss' remoteproc driver
(ab)uses the DMA API for transferring the modem firmware to the secure
world via calls to Trustzone [1].

Once the firmware buffer has changed hands, _any_ access from the
non-secure side (i.e. Linux) will be detected on the bus and result in a
full system reset [2]. Although this is possible even with this revert
in place (due to speculative reads via the cacheable linear alias of
memory), anecdotally the problem occurs considerably more frequently
when the lines have not been invalidated, assumedly due to some
micro-architectural interactions with the cache hierarchy.

Revert the offending change for now, along with a comment, so that the
Qualcomm developers have time to fix the driver [3] to use a firmware
buffer which does not have a cacheable alias in the linear map.

Link: https://lore.kernel.org/r/20221114110329.68413-1-manivannan.sadhasivam@linaro.org [1]
Link: https://lore.kernel.org/r/CAMi1Hd3H2k1J8hJ6e-Miy5+nVDNzv6qQ3nN-9929B0GbHJkXEg@mail.gmail.com/ [2]
Link: https://lore.kernel.org/r/20221206092152.GD15486@thinkpad [2]
Reported-by: Amit Pundir <amit.pundir at linaro.org>
Reported-by: Manivannan Sadhasivam <manivannan.sadhasivam at linaro.org>
Cc: Catalin Marinas <catalin.marinas at arm.com>
Cc: Thorsten Leemhuis <regressions at leemhuis.info>
Cc: Sibi Sankar <quic_sibis at quicinc.com>
Signed-off-by: Will Deacon <will at kernel.org>
---
 arch/arm64/mm/dma-mapping.c | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/mm/dma-mapping.c b/arch/arm64/mm/dma-mapping.c
index 3cb101e8cb29..5240f6acad64 100644
--- a/arch/arm64/mm/dma-mapping.c
+++ b/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/20221114110329.68413-1-manivannan.sadhasivam@linaro.org
+	 *
+	 * 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
-- 
2.39.0.rc0.267.gcb52ba06e7-goog