[PATCH net] net: airoha: Fix DMA direction for NPU mailbox buffer

[Wayen Yan]

airoha_npu_send_msg() always maps the mailbox buffer with DMA_TO_DEVICE,
but some callers expect the NPU to write response data back into the
same buffer:

- airoha_npu_wlan_msg_get() (NPU_OP_GET): NPU writes response into
  the buffer, then the caller reads it via memcpy()
- airoha_npu_ppe_stats_setup() (NPU_OP_SET): NPU writes back
  npu_stats_addr field in the response

On non-cache-coherent architectures like EN7581 (Cortex-A53 without
hardware cache coherency for NPU DMA), DMA_TO_DEVICE unmap is a no-op
— it does not invalidate the CPU cache. If the NPU-written cache line
is still present in the CPU cache when the caller reads the buffer,
the CPU observes stale data instead of the NPU response.

This is a timing-sensitive bug: small mailbox buffers (~24 bytes)
typically fit in a single cache line and may survive in the cache
until the caller reads them, producing silent data corruption rather
than a crash. The bug is more likely to trigger when the caller reads
the response immediately after dma_unmap_single() without intervening
cache-evicting operations.

Fix by using DMA_BIDIRECTIONAL for both map and unmap, which ensures
dma_unmap_single() invalidates the CPU cache on non-coherent systems.
The mailbox buffers are small so there is no performance concern.

Fixes: c52918744ee1e49cea86622a2633b9782446428f ("net: airoha: npu: Move memory allocation in airoha_npu_send_msg() caller")
Signed-off-by: Wayen Yan <win847 at gmail.com>
---
 drivers/net/ethernet/airoha/airoha_npu.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/net/ethernet/airoha/airoha_npu.c b/drivers/net/ethernet/airoha/airoha_npu.c
index 870d61fdd9c6..b679bed952de 100644
--- a/drivers/net/ethernet/airoha/airoha_npu.c
+++ b/drivers/net/ethernet/airoha/airoha_npu.c
@@ -168,7 +168,7 @@ static int airoha_npu_send_msg(struct airoha_npu *npu, int func_id,
 	dma_addr_t dma_addr;
 	int ret;
 
-	dma_addr = dma_map_single(npu->dev, p, size, DMA_TO_DEVICE);
+	dma_addr = dma_map_single(npu->dev, p, size, DMA_BIDIRECTIONAL);
 	ret = dma_mapping_error(npu->dev, dma_addr);
 	if (ret)
 		return ret;
@@ -191,7 +191,7 @@ static int airoha_npu_send_msg(struct airoha_npu *npu, int func_id,
 
 	spin_unlock_bh(&npu->cores[core].lock);
 
-	dma_unmap_single(npu->dev, dma_addr, size, DMA_TO_DEVICE);
+	dma_unmap_single(npu->dev, dma_addr, size, DMA_BIDIRECTIONAL);
 
 	return ret;
 }
-- 
2.51.0