Subject: Re: [PATCH rfc v0 9/9] net: dsa: qca8k: Move inband mutex into DSA core

From: Christian Marangi <[email-masked]>
Date: Tue, 20 Sep 2022 05:19:07 +0200

On Tue, Sep 20, 2022 at 12:18:53AM +0200, Andrew Lunn wrote:
> The mutex serves two purposes:
> 
> It serialises operations on the switch, so that only one
> request/response can be happening at once.
> 
> It protects priv->mgmt_master, which itself has two purposes.  If the
> hardware is wrongly wires, the wrong switch port is connected to the
> cpu, inband cannot be used. In this case it has the value
> NULL. Additionally, if the master is down, it is set to
> NULL. Otherwise it points to the netdev used to send frames to the
> switch.
> 
> The protection of priv->mgmt_master is not required. It is a single
> pointer, which will be updated atomically. It is not expected that the
> interface disappears, it only goes down. Hence mgmt_master will always
> be valid, or NULL.
> 
> Move the check for the master device being NULL into the core.  Also,
> move the mutex for serialisation into the core.
> 
> The MIB operations don't follow request/response semantics, so its
> mutex is left untouched.
> 
> Signed-off-by: Andrew Lunn <[email-masked]>

BTW this patch makes the router crash with a kernel panic. Rest of the
patchset works correctly and seems to be no regression. (I had to fix
the clear_skb return value)

> ---
>  drivers/net/dsa/qca/qca8k-8xxx.c | 68 ++++++--------------------------
>  drivers/net/dsa/qca/qca8k.h      |  1 -
>  include/net/dsa.h                |  1 +
>  net/dsa/dsa.c                    |  7 ++++
>  4 files changed, 19 insertions(+), 58 deletions(-)
> 
> diff --git a/drivers/net/dsa/qca/qca8k-8xxx.c b/drivers/net/dsa/qca/qca8k-8xxx.c
> index 234d79a09e78..3e60bbe2570d 100644
> --- a/drivers/net/dsa/qca/qca8k-8xxx.c
> +++ b/drivers/net/dsa/qca/qca8k-8xxx.c
> @@ -238,15 +238,6 @@ static int qca8k_read_eth(struct qca8k_priv *priv, u32 reg, u32 *val, int len)
>  	if (!skb)
>  		return -ENOMEM;
>  
> -	mutex_lock(&mgmt_eth_data->mutex);
> -
> -	/* Check mgmt_master if is operational */
> -	if (!priv->mgmt_master) {
> -		kfree_skb(skb);
> -		mutex_unlock(&mgmt_eth_data->mutex);
> -		return -EINVAL;
> -	}
> -
>  	skb->dev = priv->mgmt_master;
>  
>  	ret = dsa_inband_request(&mgmt_eth_data->inband, skb,
> @@ -258,8 +249,6 @@ static int qca8k_read_eth(struct qca8k_priv *priv, u32 reg, u32 *val, int len)
>  	if (len > QCA_HDR_MGMT_DATA1_LEN)
>  		memcpy(val + 1, &data[1], len - QCA_HDR_MGMT_DATA1_LEN);
>  
> -	mutex_unlock(&mgmt_eth_data->mutex);
> -
>  	return ret;
>  }
>  
> @@ -267,32 +256,18 @@ static int qca8k_write_eth(struct qca8k_priv *priv, u32 reg, u32 *val, int len)
>  {
>  	struct qca8k_mgmt_eth_data *mgmt_eth_data = &priv->mgmt_eth_data;
>  	struct sk_buff *skb;
> -	int ret;
>  
>  	skb = qca8k_alloc_mdio_header(MDIO_WRITE, reg, val,
>  				      QCA8K_ETHERNET_MDIO_PRIORITY, len);
>  	if (!skb)
>  		return -ENOMEM;
>  
> -	mutex_lock(&mgmt_eth_data->mutex);
> -
> -	/* Check mgmt_master if is operational */
> -	if (!priv->mgmt_master) {
> -		kfree_skb(skb);
> -		mutex_unlock(&mgmt_eth_data->mutex);
> -		return -EINVAL;
> -	}
> -
>  	skb->dev = priv->mgmt_master;
>  
> -	ret = dsa_inband_request(&mgmt_eth_data->inband, skb,
> -				 qca8k_mdio_header_fill_seq_num,
> -				 NULL, 0,
> -				 QCA8K_ETHERNET_TIMEOUT);
> -
> -	mutex_unlock(&mgmt_eth_data->mutex);
> -
> -	return ret;
> +	return dsa_inband_request(&mgmt_eth_data->inband, skb,
> +				  qca8k_mdio_header_fill_seq_num,
> +				  NULL, 0,
> +				  QCA8K_ETHERNET_TIMEOUT);
>  }
>  
>  static int
> @@ -438,7 +413,6 @@ qca8k_phy_eth_command(struct qca8k_priv *priv, bool read, int phy,
>  	struct sk_buff *write_skb, *clear_skb, *read_skb;
>  	struct qca8k_mgmt_eth_data *mgmt_eth_data;
>  	u32 write_val, clear_val = 0, val;
> -	struct net_device *mgmt_master;
>  	u32 resp_data[4];
>  	int ret, ret1;
>  
> @@ -484,19 +458,9 @@ qca8k_phy_eth_command(struct qca8k_priv *priv, bool read, int phy,
>  	 * 3. Get the data if we are reading
>  	 * 4. Reset the mdio master (even with error)
>  	 */
> -	mutex_lock(&mgmt_eth_data->mutex);
> -
> -	/* Check if mgmt_master is operational */
> -	mgmt_master = priv->mgmt_master;
> -	if (!mgmt_master) {
> -		mutex_unlock(&mgmt_eth_data->mutex);
> -		ret = -EINVAL;
> -		goto err_mgmt_master;
> -	}
> -
> -	read_skb->dev = mgmt_master;
> -	clear_skb->dev = mgmt_master;
> -	write_skb->dev = mgmt_master;
> +	read_skb->dev = priv->mgmt_master;
> +	clear_skb->dev = priv->mgmt_master;
> +	write_skb->dev = priv->mgmt_master;
>  
>  	ret = dsa_inband_request(&mgmt_eth_data->inband, write_skb,
>  				 qca8k_mdio_header_fill_seq_num,
> @@ -533,18 +497,11 @@ qca8k_phy_eth_command(struct qca8k_priv *priv, bool read, int phy,
>  	}
>  exit:
>  
> -	ret = dsa_inband_request(&mgmt_eth_data->inband, clear_skb,
> -				 qca8k_mdio_header_fill_seq_num,
> -				 NULL, 0,
> -				 QCA8K_ETHERNET_TIMEOUT);
> -
> -	mutex_unlock(&mgmt_eth_data->mutex);
> -
> -	return ret;
> +	return dsa_inband_request(&mgmt_eth_data->inband, clear_skb,
> +				  qca8k_mdio_header_fill_seq_num,
> +				  NULL, 0,
> +				  QCA8K_ETHERNET_TIMEOUT);
>  
> -	/* Error handling before lock */
> -err_mgmt_master:
> -	kfree_skb(read_skb);
>  err_read_skb:
>  	kfree_skb(clear_skb);
>  err_clear_skb:
> @@ -1526,13 +1483,11 @@ qca8k_master_change(struct dsa_switch *ds, const struct net_device *master,
>  	if (dp->index != 0)
>  		return;
>  
> -	mutex_lock(&priv->mgmt_eth_data.mutex);
>  	mutex_lock(&priv->mib_eth_data.mutex);
>  
>  	priv->mgmt_master = operational ? (struct net_device *)master : NULL;
>  
>  	mutex_unlock(&priv->mib_eth_data.mutex);
> -	mutex_unlock(&priv->mgmt_eth_data.mutex);
>  }
>  
>  static int qca8k_connect_tag_protocol(struct dsa_switch *ds,
> @@ -1850,7 +1805,6 @@ qca8k_sw_probe(struct mdio_device *mdiodev)
>  	if (!priv->ds)
>  		return -ENOMEM;
>  
> -	mutex_init(&priv->mgmt_eth_data.mutex);
>  	dsa_inband_init(&priv->mgmt_eth_data.inband, U32_MAX);
>  
>  	mutex_init(&priv->mib_eth_data.mutex);
> diff --git a/drivers/net/dsa/qca/qca8k.h b/drivers/net/dsa/qca/qca8k.h
> index 70494096e251..6da36ed6486b 100644
> --- a/drivers/net/dsa/qca/qca8k.h
> +++ b/drivers/net/dsa/qca/qca8k.h
> @@ -347,7 +347,6 @@ enum {
>  
>  struct qca8k_mgmt_eth_data {
>  	struct dsa_inband inband;
> -	struct mutex mutex; /* Enforce one mdio read/write at time */
>  };
>  
>  struct qca8k_mib_eth_data {
> diff --git a/include/net/dsa.h b/include/net/dsa.h
> index dad9e31d36ce..7a545b781e7d 100644
> --- a/include/net/dsa.h
> +++ b/include/net/dsa.h
> @@ -1281,6 +1281,7 @@ bool dsa_mdb_present_in_other_db(struct dsa_switch *ds, int port,
>   * frames and expecting a response in a frame.
>  */
>  struct dsa_inband {
> +	struct mutex lock; /* Serialise operations */
>  	struct completion completion;
>  	u32 seqno;
>  	u32 seqno_mask;
> diff --git a/net/dsa/dsa.c b/net/dsa/dsa.c
> index 4fa0ab4ae58e..82c729d631eb 100644
> --- a/net/dsa/dsa.c
> +++ b/net/dsa/dsa.c
> @@ -521,6 +521,7 @@ EXPORT_SYMBOL_GPL(dsa_mdb_present_in_other_db);
>  void dsa_inband_init(struct dsa_inband *inband, u32 seqno_mask)
>  {
>  	init_completion(&inband->completion);
> +	mutex_init(&inband->lock);
>  	mutex_init(&inband->resp_lock);
>  	inband->seqno_mask = seqno_mask;
>  	inband->seqno = 0;
> @@ -567,6 +568,11 @@ int dsa_inband_request(struct dsa_inband *inband, struct sk_buff *skb,
>  	reinit_completion(&inband->completion);
>  	inband->err = 0;
>  
> +	if (!skb->dev)
> +		return -EOPNOTSUPP;
> +
> +	mutex_lock(&inband->lock);
> +
>  	mutex_lock(&inband->resp_lock);
>  	inband->resp = resp;
>  	inband->resp_len = resp_len;
> @@ -585,6 +591,7 @@ int dsa_inband_request(struct dsa_inband *inband, struct sk_buff *skb,
>  	inband->resp = NULL;
>  	inband->resp_len = 0;
>  	mutex_unlock(&inband->resp_lock);
> +	mutex_unlock(&inband->lock);
>  
>  	if (ret < 0)
>  		return ret;
> -- 
> 2.37.2
> 

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