linux-kernel - Re: [RESEND PATCH 4/4] can: c_can: cache frames to operate as a true FIFO

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Message-ID: <1485600069.218377.1628194566441@mail1.libero.it>
Date:   Thu, 5 Aug 2021 22:16:06 +0200 (CEST)
From:   Dario Binacchi <dariobin@...ero.it>
To:     Marc Kleine-Budde <mkl@...gutronix.de>
Cc:     linux-kernel@...r.kernel.org,
        Gianluca Falavigna <gianluca.falavigna@...ind.it>,
        Andrew Lunn <andrew@...n.ch>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Oliver Hartkopp <socketcan@...tkopp.net>,
        Vincent Mailhol <mailhol.vincent@...adoo.fr>,
        Wolfgang Grandegger <wg@...ndegger.com>,
        linux-can@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [RESEND PATCH 4/4] can: c_can: cache frames to operate as a
 true FIFO

Hi Marc,

> Il 04/08/2021 11:45 Marc Kleine-Budde <mkl@...gutronix.de> ha scritto:
> 
>  
> On 25.07.2021 18:11:50, Dario Binacchi wrote:
> > diff --git a/drivers/net/can/c_can/c_can.h b/drivers/net/can/c_can/c_can.h
> > index 8fe7e2138620..fc499a70b797 100644
> > --- a/drivers/net/can/c_can/c_can.h
> > +++ b/drivers/net/can/c_can/c_can.h
> > @@ -200,6 +200,7 @@ struct c_can_priv {
> >  	atomic_t sie_pending;
> >  	unsigned long tx_dir;
> >  	int last_status;
> > +	spinlock_t tx_lock;
> 
> What does the spin lock protect?
> 
> >  	struct c_can_tx_ring tx;
> >  	u16 (*read_reg)(const struct c_can_priv *priv, enum reg index);
> >  	void (*write_reg)(const struct c_can_priv *priv, enum reg index, u16 val);
> > @@ -236,4 +237,9 @@ static inline u8 c_can_get_tx_tail(const struct c_can_tx_ring *ring)
> >  	return ring->tail & (ring->obj_num - 1);
> >  }
> >  
> > +static inline u8 c_can_get_tx_free(const struct c_can_tx_ring *ring)
> > +{
> > +	return ring->obj_num - (ring->head - ring->tail);
> > +}
> > +
> >  #endif /* C_CAN_H */
> > diff --git a/drivers/net/can/c_can/c_can_main.c b/drivers/net/can/c_can/c_can_main.c
> > index 451ac9a9586a..4c061fef002c 100644
> > --- a/drivers/net/can/c_can/c_can_main.c
> > +++ b/drivers/net/can/c_can/c_can_main.c
> > @@ -427,20 +427,6 @@ static void c_can_setup_receive_object(struct net_device *dev, int iface,
> >  	c_can_object_put(dev, iface, obj, IF_COMM_RCV_SETUP);
> >  }
> >  
> > -static u8 c_can_get_tx_free(const struct c_can_tx_ring *ring)
> > -{
> > -	u8 head = c_can_get_tx_head(ring);
> > -	u8 tail = c_can_get_tx_tail(ring);
> > -
> > -	/* This is not a FIFO. C/D_CAN sends out the buffers
> > -	 * prioritized. The lowest buffer number wins.
> > -	 */
> > -	if (head < tail)
> > -		return 0;
> > -
> > -	return ring->obj_num - head;
> > -}
> > -
> >  static bool c_can_tx_busy(const struct c_can_priv *priv,
> >  			  const struct c_can_tx_ring *tx_ring)
> >  {
> > @@ -470,7 +456,7 @@ static netdev_tx_t c_can_start_xmit(struct sk_buff *skb,
> >  	struct can_frame *frame = (struct can_frame *)skb->data;
> >  	struct c_can_priv *priv = netdev_priv(dev);
> >  	struct c_can_tx_ring *tx_ring = &priv->tx;
> > -	u32 idx, obj;
> > +	u32 idx, obj, cmd = IF_COMM_TX;
> >  
> >  	if (can_dropped_invalid_skb(dev, skb))
> >  		return NETDEV_TX_OK;
> > @@ -483,7 +469,11 @@ static netdev_tx_t c_can_start_xmit(struct sk_buff *skb,
> >  	if (c_can_get_tx_free(tx_ring) == 0)
> >  		netif_stop_queue(dev);
> >  
> > -	obj = idx + priv->msg_obj_tx_first;
> > +	spin_lock_bh(&priv->tx_lock);
> 
> What does the spin_lock protect? The ndo_start_xmit function is properly
> serialized by the networking core.
> 

The spin_lock protects the access to the IF_TX interface. Enabling the transmission 
of cached messages occur inside interrupt and the use of the IF_RX interface,
which would avoid the use of the spinlock, has not been validated by
the tests.

Thanks and regards,
Dario

> Otherwise the patch looks good!
> 
> Marc
> 
> -- 
> Pengutronix e.K.                 | Marc Kleine-Budde           |
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