netdev - Re: can: c_can: TX echo

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Date:	Wed, 23 Mar 2011 09:51:33 +0100
From:	Kurt Van Dijck <kurt.van.dijck@....be>
To:	Jan Altenberg <jan@...utronix.de>
Cc:	bhupesh.sharma@...com, wg@...ndegger.com, b.spranger@...utronix.de,
	netdev@...r.kernel.org
Subject: Re: can: c_can: TX echo

I'm not too familiar with the c_can chip.
So I have a few questions inside my reply.

On Tue, Mar 22, 2011 at 04:59:23PM +0100, Jan Altenberg wrote:
> Hi all,
> 
> I did some more testing on the SocketCAN driver for the Bosch c_can
> controller and I observed some strange behaviour for the TX handling.
> First of all the TX bytes are not accounted correctly. The reason for that
> seems to be quite obvious if we look into c_can_do_tx():
> 
> [...]
> 
> c_can_inval_msg_object(dev, 0, msg_obj_no);
> val = c_can_read_reg32(priv, &priv->regs->txrqst1);
> if (!(val & (1 << msg_obj_no))) {
>         can_get_echo_skb(dev,
>                         msg_obj_no - C_CAN_MSG_OBJ_TX_FIRST);
>         stats->tx_bytes += priv->read_reg(priv,
>                         &priv->regs->ifregs[0].msg_cntrl)
>                         & IF_MCONT_DLC_MASK;
> 
> So, we first invalidate the message object and afterwards we read the DLC
> value from the msg_cntrl (which is 0 after invalidating the
> message object) to account the TX bytes. So tx_bytes will always be 0. The
> fix should be easy, I think, we can just move
> c_can_inval_msg_object to the end of that loop.

IMO, it looks necessary to call c_can_inval_msg_object inside the if (),
after it has been transmitted. Otherwise, if for some other (TX) reason you
get in this loop, you may clear a pending transmission?
Again, I haven't read this one's datasheet. I was familiar with its predecessor.
> 
> Cheers,
>         Jan

Regards,
Kurt
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