> On EN7581 + MT7996 (Gemtek W1700K), mapping small caller buffers with
> DMA_BIDIRECTIONAL regresses NPU version probe: the mailbox completes
> successfully but WLAN_FUNC_GET_WAIT_NPU_VERSION reads as 0.0 instead of
> 0.1111.

IIUC you are suggesting the issue is if we allocate a small buffer it could fit
in the same cacheline of a neighbour object so if the DMA reads/writes the same
cacheline, it could invalidate data, right? (I am not sure if this device is
DMA-coherent or not).
If so, in the airoha_npu_send_msg() callers, I guess it is enough to allocate
something like:

	kzalloc(max(len, SMP_CACHE_BYTES))

and avoid the extra copy in __airoha_npu_send_msg(). What do you think?
IIRC kzalloc() aligns to SMP_CACHE_BYTES is len >= SMP_CACHE_BYTES.

Regards,
Lorenzo

> 
> Bounce mailbox traffic through a per-core cacheline-aligned buffer
> allocated at probe. Map ALIGN(len, SMP_CACHE_BYTES) with
> DMA_BIDIRECTIONAL while programming the original payload length into the
> mailbox length register. Copy the reply only after dma_unmap_single() so
> the CPU sees the NPU-written payload on non-coherent DMA.
> 
> Introduce __airoha_npu_send_msg() with an optional reply pointer so GET
> callers can copy the trailing response payload once. Keep
> airoha_npu_send_msg() as a wrapper that copies the full response back
> into the caller buffer, matching the old bidirectional mapping.
> 
> Tested on Quantum Fiber / Gemtek W1700K (EN7581 + MT7996), kernel
> 6.18.44, including two cold reboots and sustained WiFi use.
> 
> Fixes: 6f884eb87a79 ("net: airoha: Fix DMA direction for NPU mailbox buffer")
> Link: https://patchwork.kernel.org/project/linux-mediatek/patch/20260805070851.2885888-1-pawlik.dan@gmail.com/
> Assisted-by: Cursor:composer-2
> Signed-off-by: Daniel Pawlik <pawlik.dan@gmail.com>
> ---
>  drivers/net/ethernet/airoha/airoha_npu.c  | 62 ++++++++++++++++++-----
>  include/linux/soc/airoha/airoha_offload.h |  1 +
>  2 files changed, 49 insertions(+), 14 deletions(-)
> 
> diff --git a/drivers/net/ethernet/airoha/airoha_npu.c b/drivers/net/ethernet/airoha/airoha_npu.c
> index b679bed952de..2ba78bf0e97b 100644
> --- a/drivers/net/ethernet/airoha/airoha_npu.c
> +++ b/drivers/net/ethernet/airoha/airoha_npu.c
> @@ -5,6 +5,7 @@
>   */
>  
>  #include <linux/devcoredump.h>
> +#include <linux/dma-mapping.h>
>  #include <linux/firmware.h>
>  #include <linux/platform_device.h>
>  #include <linux/of_net.h>
> @@ -23,6 +24,8 @@
>  #define NPU_EN7581_FIRMWARE_RV32_MAX_SIZE	0x200000
>  #define NPU_EN7581_FIRMWARE_DATA_MAX_SIZE	0x10000
>  #define NPU_DUMP_SIZE				512
> +/* Maximum mailbox DMA payload (PPE ~28 bytes, WLAN TLV up to 24). */
> +#define AIROHA_NPU_MBOX_SIZE			256
>  
>  #define REG_NPU_LOCAL_SRAM		0x0
>  
> @@ -160,23 +163,33 @@ 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;
> +	struct airoha_npu_core *core = &npu->cores[0]; /* FIXME: core */
>  	dma_addr_t dma_addr;
> +	unsigned int map_len = ALIGN(len, SMP_CACHE_BYTES);
> +	u32 val, offset = 0;
>  	int ret;
>  
> -	dma_addr = dma_map_single(npu->dev, p, size, DMA_BIDIRECTIONAL);
> +	if (len <= 0 || len > AIROHA_NPU_MBOX_SIZE)
> +		return -EINVAL;
> +
> +	if (reply && reply_len > len)
> +		return -EINVAL;
> +
> +	spin_lock_bh(&core->lock);
> +
> +	memcpy(core->buf, data, len);
> +
> +	dma_addr = dma_map_single(npu->dev, core->buf, map_len, DMA_BIDIRECTIONAL);
>  	ret = dma_mapping_error(npu->dev, dma_addr);
>  	if (ret)
> -		return ret;
> -
> -	spin_lock_bh(&npu->cores[core].lock);
> +		goto unlock;
>  
>  	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(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;
> @@ -189,13 +202,23 @@ 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);
> +	dma_unmap_single(npu->dev, dma_addr, map_len, DMA_BIDIRECTIONAL);
>  
> -	dma_unmap_single(npu->dev, dma_addr, size, DMA_BIDIRECTIONAL);
> +	/* Copy the trailing reply_len bytes of the response. */
> +	if (!ret && reply)
> +		memcpy(reply, core->buf + len - reply_len, reply_len);
> +unlock:
> +	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)
>  {
> @@ -497,9 +520,8 @@ 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;
> @@ -770,6 +792,18 @@ static int airoha_npu_probe(struct platform_device *pdev)
>  	if (err)
>  		return err;
>  
> +	for (i = 0; i < ARRAY_SIZE(npu->cores); i++) {
> +		struct airoha_npu_core *core = &npu->cores[i];
> +		void *raw;
> +
> +		raw = devm_kmalloc(dev, AIROHA_NPU_MBOX_SIZE + SMP_CACHE_BYTES,
> +				   GFP_KERNEL);
> +		if (!raw)
> +			return -ENOMEM;
> +
> +		core->buf = PTR_ALIGN(raw, SMP_CACHE_BYTES);
> +	}
> +
>  	err = airoha_npu_run_firmware(dev, base, &res);
>  	if (err)
>  		return dev_err_probe(dev, err, "failed to run npu firmware\n");
> diff --git a/include/linux/soc/airoha/airoha_offload.h b/include/linux/soc/airoha/airoha_offload.h
> index 7589fccfeef6..f635766e3dba 100644
> --- a/include/linux/soc/airoha/airoha_offload.h
> +++ b/include/linux/soc/airoha/airoha_offload.h
> @@ -173,6 +173,7 @@ struct airoha_npu {
>  		/* protect concurrent npu memory accesses */
>  		spinlock_t lock;
>  		struct work_struct wdt_work;
> +		void *buf; /* mailbox DMA bounce buffer */
>  	} cores[NPU_NUM_CORES];
>  
>  	int irqs[NPU_NUM_IRQ];
> -- 
> 2.55.0
>