Re: [PATCH v3] drm/i915/dp: Give up link training clock recovery after 10 failed attempts

[Nathan Ciobanu <nathan.d.ciobanu@linux.intel.com>]

On Thu, Jul 19, 2018 at 10:01:41AM -0700, Rodrigo Vivi wrote:
> On Tue, Jul 17, 2018 at 06:05:51PM -0700, Nathan Ciobanu wrote:
> > On Tue, Jul 17, 2018 at 03:21:17PM -0700, Dhinakaran Pandiyan wrote:
> > > On Mon, 2018-07-16 at 16:51 -0700, Marc Herbert wrote:
> > > > > 
> > > > > > 
> > > > > > I think the bug is with this infinite loop which is at the mercy
> > > > > > of an external device
> > > > > > and in my case I have this MST hub which appears to be DP 1.2
> > > > > > that triggers the
> > > > > > infinite loop case. We have to limit the number of iterations and
> > > > > > DP 1.4 spec happened to fix this issue. I'm a newbie in this area
> > > > > > but in this case
> > > > > > shouldn't we apply the same counter to all <= "DP 1.4" devices?
> > > > > ok, the infinite loop situation is really bad... but I don't
> > > > > believe
> > > > > we are going with the right fix...
> > > > > and a good indication is that your fix is for DP-1.4 while your bug
> > > > > is seeing on DP-1.2
> > > > I thought the only reason the infinite loop fix isn't in 1.2 is just
> > > > because there's
> > > > no 1.2.1 spec... (plus the naive assumption that buggy sinks don't
> > > > exist)
> > > > 
> > > Irrespective of whether the sink is DP1.2 or 1.4, if there are sinks
> > > out there that keep toggling between two values there should be an
> > > overall limit to how many times this loop gets executed. Even if this
> > > isn't right fix for the issue at hand, the loop has to break.
> > > 
> > > Then there's the question of what the limit should be. We could use the
> > > DP1.4 limit as a reference and apply it widely. But there's a problem
> > > here, we have 4 vswing values and 4 pre-emphasis values. If the sink
> > > progressively changes one variable at a time, we'll need at least 16
> > > iterations. Nathan's patch here will prematurely error out and that
> > > doesn't sound reasonable to impose on non DP1.4 sinks.
> > 
> > I think it would be a max of 13 iterations since one of the vswing
> > values will be max_vswing and the loop will terminate at that point
> > without trying the other 3 preemph values for that voltage, correct?
> 
> I was talking to DK yesterday and he suggested that we should go with
> a huge number for DP_1.2 and with the spec for DP_1.4. According to DK
> 80 was covering all combinations few times.
> 
> So you get your patch and create some max_cr_tries = dp_1.4 ? spec : 80;
> 
> or something like that.
> 
> I think I like that because infinite loop situation here is so bad
> and we should avoid even if it was something else that got really wrong.
I just sent v4 to do that. Thanks!
-Nathan
> 
> Thanks,
> Rodrigo.
> 
> > 
> > -Nathan
> > > 
> > > -DK
> > > 
> > > 
> > > > 
> > > > 
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> > > 
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