Subject: Re: [PATCH net-next 2/2] dpaa2-eth: fix return codes used in ndo_setup_tc

From: Jesper Dangaard Brouer <[email-masked]>
Date: Fri, 24 Apr 2020 09:04:26 +0200

On Thu, 23 Apr 2020 12:56:00 -0700
Jakub Kicinski <[email-masked]> wrote:

> On Thu, 23 Apr 2020 12:33:56 -0700 Stephen Hemminger wrote:
> > On Thu, 23 Apr 2020 17:38:04 +0200
> > Jesper Dangaard Brouer <[email-masked]> wrote:
> >   
> > > On Thu, 23 Apr 2020 08:28:58 -0700
> > > Stephen Hemminger <[email-masked]> wrote:
> > >     
> > > > On Thu, 23 Apr 2020 16:57:50 +0200
> > > > Jesper Dangaard Brouer <[email-masked]> wrote:
> > > >       
> > > > > Drivers ndo_setup_tc call should return -EOPNOTSUPP, when it cannot
> > > > > support the qdisc type. Other return values will result in failing the
> > > > > qdisc setup.  This lead to qdisc noop getting assigned, which will
> > > > > drop all TX packets on the interface.
> > > > > 
> > > > > Fixes: ab1e6de2bd49 ("dpaa2-eth: Add mqprio support")
> > > > > Signed-off-by: Jesper Dangaard Brouer <[email-masked]>        
> > > > 
> > > > Would it be possible to use extack as well?      
> > > 
> > > That is what patch 1/2 already does.
> > >     
> > > > Putting errors in dmesg is unhelpful      
> > > 
> > > This patchset does not introduce any dmesg printk.
> > >     
> > 
> > I was thinking that this  
> > 	if (num_tc  > dpaa2_eth_tc_count(priv)) {
> >  		netdev_err(net_dev, "Max %d traffic classes supported\n",
> >  			   dpaa2_eth_tc_count(priv));
> > -		return -EINVAL;
> > +		return -EOPNOTSUPP;
> >  	}
> > 
> > could be an extack message  

First of all, this is a fix, and we need to keep it simple, as it needs
to be backported to v5.3.

Talking about converting this warning message this into a extack, I'm
actually not convinced that is a good idea, or will even work.  First
the extack cannot contain the %d number.  Second returning -EOPNOTSUPP
this is actually not an error, and I don't think tc will print the
extack in that case?
 
> That's a good question, actually. In this case Jesper was seeing a
> failure when creating the default qdisc. The extack would go nowhere,
> we'd have to print it to the logs, no? Which we should probably do,
> anyway.

Good point. We probably need a separate dmesg error when we cannot
configure the default qdisc.  As there is not end-user to receive the
extack.   But I would place that at a higher level in qdisc_create_dflt().
It would definitely have helped me to identify what net-subsystem was
dropping packets, and after my patch[1/2] adding the extack, an
end-user would get a meaning full message to ease the troubleshooting.

(Side-note: First I placed an extack in qdisc_create_dflt() but I
realized it was wrong, because it could potentially override messages
from the lower layers.)

(For a separate patch:)

We should discuss, that when creating the default qdisc, we should IMHO
not allow that to fail.  As you can see in [1], this step happens
during the qdisc init function e.g. it could also fail due to low
memory. IMHO we should have a fallback, for when the default qdisc init
fails, e.g. assign pfifo_fast instead or even noqueue.

> > but doing that would require a change
> > to the ndo_setup_tc hook to allow driver to return its own error message
> > as to why the setup failed.  
> 
> Yeah :S The block offload command contains extack, but this driver
> doesn't understand block offload, so it won't interpret it...
> 
> That brings me to an important point - doesn't the extack in patch 1
> override any extack driver may have set?

Nope, see above side-note.  I set the extack at the "lowest level",
e.g. closest to the error that cause the err back-propagation, when I
detect that this will cause a failure at higher level.

> I remember we discussed this when adding extacks to the TC core, but 
> I don't remember the conclusion now, ugh.

When adding the extack code, I as puzzled that during debugging I
managed to override other extack messages.  Have anyone though about a
better way to handle if extack messages gets overridden?

[1] https://github.com/xdp-project/xdp-project/blob/master/areas/arm64/board_nxp_ls1088/nxp-board04-troubleshoot-qdisc.org
-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer