Re: [PATCH] can: mcp251xfd: fix infinite loop when xmit fails

[vitor <ivitro@gmail.com>]

Hi Marc,

On Thu, 2024-03-07 at 13:23 +0100, Marc Kleine-Budde wrote:
> Sorry, resend. I fsck up the linux-can ML's address.

Thanks for that.
> 
> On 07.03.2024 12:04:42, Vitor Soares wrote:
> > From: Vitor Soares <vitor.soares@toradex.com>
> > 
> > When the mcp251xfd_start_xmit() function fails, the driver stops
> > processing messages, and the interrupt routine does not return,
> > running indefinitely even after killing the running application.
> > 
> > Error messages:
> > [  441.298819] mcp251xfd spi2.0 can0: ERROR in
> > mcp251xfd_start_xmit: -16
> > [  441.306498] mcp251xfd spi2.0 can0: Transmit Event FIFO buffer
> > not empty. (seq=0x000017c7, tef_tail=0x000017cf,
> > tef_head=0x000017d0, tx_head=0x000017d3).
> > ... and repeat forever.
> > 
> > The issue can be triggered when multiple devices share the same
> > SPI interface. And there is concurrent access to the bus.
> > 
> > The problem occurs because tx_ring->head increments even if
> > mcp251xfd_start_xmit() fails. Consequently, the driver skips one
> > TX package while still expecting a response in
> > mcp251xfd_handle_tefif_one().
> > 
> > This patch resolves the issue by decreasing tx_ring->head if
> > mcp251xfd_start_xmit() fails. With the fix, if we attempt to
> > trigger
> > the issue again, the driver prints an error and discard the
> > message.
> 
> What about returning NETDEV_TX_BUSY, then the networking stack will
> retry.

Do you mean when err == -EBUSY?

> 
> > Fixes: 55e5b97f003e ("can: mcp25xxfd: add driver for Microchip
> > MCP25xxFD SPI CAN")
> > Cc: stable@vger.kernel.org
> > Signed-off-by: Vitor Soares <vitor.soares@toradex.com>
> > ---
> >  drivers/net/can/spi/mcp251xfd/mcp251xfd-tx.c | 27 ++++++++++------
> > ----
> >  1 file changed, 14 insertions(+), 13 deletions(-)
> > 
> > diff --git a/drivers/net/can/spi/mcp251xfd/mcp251xfd-tx.c
> > b/drivers/net/can/spi/mcp251xfd/mcp251xfd-tx.c
> > index 160528d3cc26..a8eb941c1b95 100644
> > --- a/drivers/net/can/spi/mcp251xfd/mcp251xfd-tx.c
> > +++ b/drivers/net/can/spi/mcp251xfd/mcp251xfd-tx.c
> > @@ -181,25 +181,26 @@ netdev_tx_t mcp251xfd_start_xmit(struct
> > sk_buff *skb,
> >         tx_obj = mcp251xfd_get_tx_obj_next(tx_ring);
> >         mcp251xfd_tx_obj_from_skb(priv, tx_obj, skb, tx_ring-
> > >head);
> >  
> > -       /* Stop queue if we occupy the complete TX FIFO */
> >         tx_head = mcp251xfd_get_tx_head(tx_ring);
> > -       tx_ring->head++;
> > -       if (mcp251xfd_get_tx_free(tx_ring) == 0)
> > -               netif_stop_queue(ndev);
> > -
> >         frame_len = can_skb_get_frame_len(skb);
> > -       err = can_put_echo_skb(skb, ndev, tx_head, frame_len);
> > -       if (!err)
> > -               netdev_sent_queue(priv->ndev, frame_len);
> > +       can_put_echo_skb(skb, ndev, tx_head, frame_len);
> > +
> > +       tx_ring->head++;
> >  
> >         err = mcp251xfd_tx_obj_write(priv, tx_obj);
> > -       if (err)
> > -               goto out_err;
> > +       if (err) {
> > +               can_free_echo_skb(ndev, tx_head, NULL);
> >  
> > -       return NETDEV_TX_OK;
> > +               tx_ring->head--;
> > +

I'm testing returning here when err = -EBUSY, but can_put_echo_skb()
should be invoked after mcp251xfd_tx_obj_write(). Otherwise, I get a
Kernel NULL pointer dereference error.


> 
> I'm not sure, if we want an error message for -EBUSY. We could add
> proper ethtool statistics.
> 
> > +               netdev_err(priv->ndev, "ERROR in %s: %d\n",
> > __func__, err);
> > +       } else {
> > +               /* Stop queue if we occupy the complete TX FIFO */
> > +               if (mcp251xfd_get_tx_free(tx_ring) == 0)
> > +                       netif_stop_queue(ndev);
> >  
> > - out_err:
> > -       netdev_err(priv->ndev, "ERROR in %s: %d\n", __func__, err);
> > +               netdev_sent_queue(priv->ndev, frame_len);
> > +       }
> >  
> >         return NETDEV_TX_OK;
> >  }
> 
> regards,
> Marc
> 

Best regards,
Vitor Soares