From mboxrd@z Thu Jan 1 00:00:00 1970 From: Wolfgang Grandegger Subject: Re: at91_can.c: Data transmission stops Date: Wed, 28 Nov 2012 15:38:23 +0100 Message-ID: <50B621DF.3050505@grandegger.com> References: <50B37C90.3040904@rosetechnology.dk> <50B389D6.4050308@grandegger.com> <50B398E6.2070101@rosetechnology.dk> <50B4CA2D.5080309@rosetechnology.dk> <50B4EAE1.6070400@grandegger.com> <50B61E1B.8040904@rosetechnology.dk> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Return-path: Received: from ngcobalt02.manitu.net ([217.11.48.102]:43947 "EHLO ngcobalt02.manitu.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754974Ab2K1Oi3 (ORCPT ); Wed, 28 Nov 2012 09:38:29 -0500 In-Reply-To: <50B61E1B.8040904@rosetechnology.dk> Sender: linux-can-owner@vger.kernel.org List-ID: To: Henrik Bork Steffensen Cc: linux-can@vger.kernel.org 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.