[RFC net-next 3/3] net: airoha: npu: use coherent memory for mailbox messages

[Chris Scott <gilroyscott1970@gmail.com>]

From: Daniel Pawlik <pawlik.dan@gmail.com>

airoha_npu_send_msg() maps the caller's kzalloc() buffer with
DMA_BIDIRECTIONAL. These buffers are small and not cacheline aligned,
so on arm64 the mapping bounces through swiotlb. On a 2 GiB EN7581
board the default swiotlb pool sits above 0xc0000000 (0xff6cf000 on
W1700K), outside the 0x80000000-0xbfffffff window the NPU can address;
higher addresses alias the lower 1 GiB. On W1700K + MT7996 this makes
WLAN_FUNC_GET_WAIT_NPU_VERSION read back 0.0 and mt76 does not bind
NPU offload.

Allocate one coherent buffer per NPU core at probe and reuse it under
the per-core mailbox lock. With the previous patch it comes from the
NPU DMA pool, inside the window. airoha_npu_send_msg() is also called
from PPE foe_commit under spin_lock_bh(), which rules out a per-message
GFP_ATOMIC coherent allocation.

Add __airoha_npu_send_msg() with an optional reply buffer so GET
callers copy only the trailing response payload; airoha_npu_send_msg()
keeps copying the whole buffer back, like the bidirectional mapping.

The original version of this patch was written with Cursor (composer-2);
it was rebased onto net-next and its commit message rewritten with
Claude (Anthropic).

Link: https://lore.kernel.org/r/20260809152813.585797-1-pawlik.dan@gmail.com
Link: https://lore.kernel.org/r/atCRsquzA1UxSpCc@mediatek.com
Signed-off-by: Daniel Pawlik <pawlik.dan@gmail.com>
Signed-off-by: Chris Scott <gilroyscott1970@gmail.com>
---
 drivers/net/ethernet/airoha/airoha_npu.c  | 65 +++++++++++++++++------
 include/linux/soc/airoha/airoha_offload.h |  3 ++
 2 files changed, 51 insertions(+), 17 deletions(-)

diff --git a/drivers/net/ethernet/airoha/airoha_npu.c b/drivers/net/ethernet/airoha/airoha_npu.c
index 069fd60680..3d1aa63dd5 100644
--- a/drivers/net/ethernet/airoha/airoha_npu.c
+++ b/drivers/net/ethernet/airoha/airoha_npu.c
@@ -24,6 +24,12 @@
 #define NPU_EN7581_FIRMWARE_RV32_MAX_SIZE	0x200000
 #define NPU_EN7581_FIRMWARE_DATA_MAX_SIZE	0x10000
 #define NPU_DUMP_SIZE				512
+/*
+ * Mailbox DMA payload size. Covers sizeof(struct ppe_mbox_data) (28) and
+ * WLAN TLV messages (header + payload); largest in-tree WLAN payload today
+ * is 16 bytes (INODE_TXRX_REG_ADDR). Keep headroom for future commands.
+ */
+#define AIROHA_NPU_MBOX_SIZE			256
 
 #define REG_NPU_LOCAL_SRAM		0x0
 
@@ -161,23 +167,32 @@ struct wlan_mbox_data {
 	DECLARE_FLEX_ARRAY(u8, d);
 };
 
-static int airoha_npu_send_msg(struct airoha_npu *npu, int func_id,
-			       void *p, int size)
+static int __airoha_npu_send_msg(struct airoha_npu *npu, int func_id,
+				 const void *data, int len, void *reply,
+				 u16 reply_len)
 {
-	u16 core = 0; /* FIXME */
-	u32 val, offset = core << 4;
-	dma_addr_t dma_addr;
+	struct airoha_npu_core *core = &npu->cores[0]; /* FIXME: core */
+	u32 val, offset = 0;
 	int ret;
 
-	dma_addr = dma_map_single(npu->dev, p, size, DMA_BIDIRECTIONAL);
-	ret = dma_mapping_error(npu->dev, dma_addr);
-	if (ret)
-		return ret;
+	if (len > AIROHA_NPU_MBOX_SIZE)
+		return -EINVAL;
+
+	if (reply && reply_len > len)
+		return -EINVAL;
+
+	/*
+	 * The NPU only reaches DRAM at 0x80000000-0xbfffffff; a streaming
+	 * mapping of the caller buffer may bounce through swiotlb outside
+	 * that window. Use the probe-time coherent buffer, which comes from
+	 * the NPU DMA pool when one is declared.
+	 */
+	spin_lock_bh(&core->lock);
 
-	spin_lock_bh(&npu->cores[core].lock);
+	memcpy(core->buf, data, len);
 
-	regmap_write(npu->regmap, REG_CR_MBQ0_CTRL(0) + offset, dma_addr);
-	regmap_write(npu->regmap, REG_CR_MBQ0_CTRL(1) + offset, size);
+	regmap_write(npu->regmap, REG_CR_MBQ0_CTRL(0) + offset, core->addr);
+	regmap_write(npu->regmap, REG_CR_MBQ0_CTRL(1) + offset, len);
 	regmap_read(npu->regmap, REG_CR_MBQ0_CTRL(2) + offset, &val);
 	regmap_write(npu->regmap, REG_CR_MBQ0_CTRL(2) + offset, val + 1);
 	val = FIELD_PREP(MBOX_MSG_FUNC_ID, func_id) | MBOX_MSG_WAIT_RSP;
@@ -190,13 +205,21 @@ static int airoha_npu_send_msg(struct airoha_npu *npu, int func_id,
 	if (!ret && FIELD_GET(MBOX_MSG_STATUS, val) != NPU_MBOX_SUCCESS)
 		ret = -EINVAL;
 
-	spin_unlock_bh(&npu->cores[core].lock);
+	/* Copy the trailing reply_len bytes of the response. */
+	if (!ret && reply)
+		memcpy(reply, core->buf + len - reply_len, reply_len);
 
-	dma_unmap_single(npu->dev, dma_addr, size, DMA_BIDIRECTIONAL);
+	spin_unlock_bh(&core->lock);
 
 	return ret;
 }
 
+static int airoha_npu_send_msg(struct airoha_npu *npu, int func_id,
+			       void *data, int len)
+{
+	return __airoha_npu_send_msg(npu, func_id, data, len, data, len);
+}
+
 static int airoha_npu_load_firmware(struct device *dev, void __iomem *addr,
 				    const char *fw_name, int fw_max_size)
 {
@@ -500,9 +523,8 @@ static int airoha_npu_wlan_msg_get(struct airoha_npu *npu, int ifindex,
 	wlan_data->func_type = NPU_OP_GET;
 	wlan_data->func_id = func_id;
 
-	err = airoha_npu_send_msg(npu, NPU_FUNC_WIFI, wlan_data, len);
-	if (!err)
-		memcpy(data, wlan_data->d, data_len);
+	err = __airoha_npu_send_msg(npu, NPU_FUNC_WIFI, wlan_data, len,
+				    data, data_len);
 	kfree(wlan_data);
 
 	return err;
@@ -794,6 +816,15 @@ static int airoha_npu_probe(struct platform_device *pdev)
 			return err;
 	}
 
+	for (i = 0; i < ARRAY_SIZE(npu->cores); i++) {
+		struct airoha_npu_core *core = &npu->cores[i];
+
+		core->buf = dmam_alloc_coherent(dev, AIROHA_NPU_MBOX_SIZE,
+						&core->addr, GFP_KERNEL);
+		if (!core->buf)
+			return -ENOMEM;
+	}
+
 	err = airoha_npu_run_firmware(dev, base, &res);
 	if (err)
 		return err;
diff --git a/include/linux/soc/airoha/airoha_offload.h b/include/linux/soc/airoha/airoha_offload.h
index 7589fccfee..16a988ad74 100644
--- a/include/linux/soc/airoha/airoha_offload.h
+++ b/include/linux/soc/airoha/airoha_offload.h
@@ -173,6 +173,9 @@ struct airoha_npu {
 		/* protect concurrent npu memory accesses */
 		spinlock_t lock;
 		struct work_struct wdt_work;
+		/* Coherent bounce buffer for mailbox cmd/rsp */
+		void *buf;
+		dma_addr_t addr;
 	} cores[NPU_NUM_CORES];
 
 	int irqs[NPU_NUM_IRQ];
-- 
2.53.0