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

[Nathan Ciobanu]

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?

-Nathan
> 
> -DK
> 
> 
> > 
> > 
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