Re: at91_can.c: Data transmission stops

[Wolfgang Grandegger <wg@grandegger.com>]

On 11/28/2012 03:22 PM, Henrik Bork Steffensen wrote:
> On 11/27/2012 05:31 PM, Wolfgang Grandegger wrote:
>> On 11/27/2012 03:11 PM, Henrik Bork Steffensen wrote:
>> Hm, could you show your diffs.
> Do You mean a diff on these 7 lines, or a diff to the original file?
> 
>>> I this case "at91_poll" is basicly the same as "c_can_poll", in both
>>> cases they call the function with the spinlock in the rx chain.
>> You don't need to protect against RX. Sorry, forgot that. On the c_can
>> this is necessary due to concurrent accesses to the same message RAM.
> Ok, I think that at91_can.c might have an issue in register access.
> I am not sure, but I will look into it.
> 
>>
>>> Looking at the patch Wolfgang sugested, I became uncertain of what this
>>> patch actually wants to protect.
>>> Is it the registers in the cpu can interface? (mailboxes and control
>>> regs, i don't know the hw)
>> 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.
> Ok, thanks a lot.
> 
> In my case i changed the driver to only use one mailbox for transmission.
> Which means that the "net_stop_queue" will be called every time a packet
> is tx'ed.
> And the "net_wake_queue" will be called after the packet is actually
> transmitted.
> 
> This is as far as i can see this is 100% safe, provided that no further
> "ndo_start_xmit"
> calls come before the wake_queue call.

Yes, then the race should be gone. Anyway, we don't want that solution.

> Anyway, after removing the spin_lock from rx, it loads fine and seems to
> work.

It would help use to know if a spin-lock protecting the whole
at91_start_xmit() and at91_start_xmit() functions does *really* fix your
"tx-does-not-work-any-more" problem. Then we are rather sure that there
is a race.

> I will do a test with the suggested changes to the tx chain and get to
> the list
> if anything interesting appears.

Not sure what changes you have in mind?

> Thank You very much for Your help so far :-)

You are welcome.

Wolfgang.