Re: [PATCH] drm/doc: describe PATH format for DP MST

[Pekka Paalanen <ppaalanen@gmail.com>]

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On Tue, 24 Oct 2023 16:03:27 +0300
Ville Syrjälä <ville.syrjala@linux.intel.com> wrote:

> On Tue, Oct 24, 2023 at 09:03:22AM +0000, Simon Ser wrote:
> > On Tuesday, October 24th, 2023 at 09:36, Pekka Paalanen <ppaalanen@gmail.com> wrote:
> >   
> > > Are DP MST port numbers guaranteed to be tied to the physical hardware
> > > configuration (e.g. how cables are connected) and therefore stable
> > > across reboots? What about stable across kernel upgrades?
> > > 
> > > If I knew that, I could perhaps manufacture a stable identifier in
> > > userspace by replacing the parent connector ID with a stable connector
> > > designator.  
> > 
> > Hm, my assumption is that these are stable, but maybe that's also wrong?
> > Ville, Dmitry, do you know whether the DP MST port numbers are
> > guaranteed stable across reboots when retaining the exact same hardware
> > configuration (not the software, maybe the user upgraded the kernel)?  
> 
> I suspect in practice those should remain the same as long as the
> topology didn't change, but I don't think there's anything in the
> DP spec that actually guarantees that (eg. some branch device
> could in theory reshuffle its port numbers on a whim).
> 
> But there is no stable identifier for the corresponding SST
> connector anyway so I don't know what you would even do with
> stable port numbers.

You mean the index in the array of connectors exposed to userspace is
not stable either for the root DP connector?

KMS device bus path + connector array index for hardwired connectors is
what I've been imagining as stable enough for all end users.


Thanks,
pq

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