Re: at91_can.c: Data transmission stops

[Marc Kleine-Budde <mkl@pengutronix.de>]

[-- Attachment #1: Type: text/plain, Size: 1817 bytes --]

On 11/27/2012 05:31 PM, Wolfgang Grandegger wrote:
[...]

> As mentioned above, on the c_can there is definitely a race with the
> message ram due to the busy wait after accessing it. See:
> 
>   http://lxr.linux.no/#linux+v3.6.8/drivers/net/can/c_can/c_can.c#L237
> 
>> Or is it the potential race between "c_can_start_xmit" and "c_can_do_tx" ?
>> Or even the access to the net api?
>>
>> Would someone care to explain?
> 
> I will try. In at91_start_xmit, if we get interrupted
> 
> 	if (!(at91_read(priv, AT91_MSR(get_tx_next_mb(priv))) &
>               AT91_MSR_MRDY) ||
>              (priv->tx_next & get_next_mask(priv)) == 0)
> 
> 		/* HERE */
> 
> 		netif_stop_queue(dev);
> 
> and then at91_irq_tx() is called executing netif_wake_queue() we may end
> up with a stopped tx queue. But I'm not yet 100% sure.

I don't think so, because after this follows[1]

	/* Enable interrupt for this mailbox */
	at91_write(priv, AT91_IER, 1 << mb);

[1] http://lxr.free-electrons.com/source/drivers/net/can/at91_can.c#L524

So first the queued ist stopped, then the interrupt activated.

Then in the interrupt handler[2], [3]:

	reg_sr = at91_read(priv, AT91_SR);
	reg_imr = at91_read(priv, AT91_IMR);

	reg_sr &= reg_imr;

...

	if (reg_sr & AT91_IRQ_MB_TX)
		at91_irq_tx(dev, reg_sr);

[2] http://lxr.free-electrons.com/source/drivers/net/can/at91_can.c#L1081
[3] http://lxr.free-electrons.com/source/drivers/net/can/at91_can.c#L1104

The at91_irq_tx() function works on the reg_sr.

Marc
-- 
Pengutronix e.K.                  | Marc Kleine-Budde           |
Industrial Linux Solutions        | Phone: +49-231-2826-924     |
Vertretung West/Dortmund          | Fax:   +49-5121-206917-5555 |
Amtsgericht Hildesheim, HRA 2686  | http://www.pengutronix.de   |


[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 261 bytes --]