[Intel-gfx] DP Aux interfaces inquiry

[Mario.Limonciello at dell.com]

> -----Original Message-----
> From: Ville Syrjälä [mailto:ville.syrjala at linux.intel.com]
> Sent: Wednesday, January 25, 2017 12:41 PM
> To: Limonciello, Mario <Mario_Limonciello at Dell.com>
> Cc: intel-gfx at lists.freedesktop.org
> Subject: Re: [Intel-gfx] DP Aux interfaces inquiry
> 
> On Wed, Jan 25, 2017 at 04:43:58PM +0000, Mario.Limonciello at dell.com
> wrote:
> > Thanks for your comments.  Some nested below.
> >
> > > -----Original Message-----
> > > From: Ville Syrjälä [mailto:ville.syrjala at linux.intel.com]
> > > Sent: Wednesday, January 25, 2017 9:57 AM
> > > To: Limonciello, Mario <Mario_Limonciello at Dell.com>
> > > Cc: intel-gfx at lists.freedesktop.org
> > > Subject: Re: [Intel-gfx] DP Aux interfaces inquiry
> > >
> > > On Tue, Jan 24, 2017 at 07:51:28PM +0000, Mario.Limonciello at dell.com
> > > wrote:
> > > > Hi,
> > > >
> > > > Recently Synaptics collaborated with Dell on a plugin [1] for
> > > > fwupd that
> > > allows flashing Synaptics MST hubs using the DP aux interface to
> > > manipulate the DPCD [2].
> > > > Currently the plugin hardcodes the max number of DP aux devices to
> > > > look for
> > > to 3 (as that's what we've seen so far on HW), but we were wondering:
> > > >
> > > > 	1) If there is a way to query the number of devices that the
> > > > kernel is
> > > going to be creating?
> > > > 	2) Are there any instances of more than 3 devices in the wild
> > > > today that
> > > anyone is aware of?
> > >
> > > These depend on the board you're dealing with, and on the number of
> > > gpus you have in the system (and whether they actually have drivers loaded
> for them).
> >
> > OK, that's what I was suspecting.
> >
> > >
> > > We should also perhaps expose the aux device node for MST devices.
> > > At which point the number of aux nodes could change dynamically when
> > > you plug MST devices in/out.
> > >
> >
> > Hmm, for the devices themselves?  The way Synaptics handles cascaded
> > MST devices today is a remote control mechanism.  They're able to turn
> > on remote control for one MST hub, and it will forward control
> > commands (and payloads) to the next cascaded hub.
> 
> I'm not really any kind of MST expert, so I don't really know what people get up
> to with these things. But there are definitely remote DPCD read/write
> messages in the spec, so I think we should be able to expose the DPCD of any
> downstream device via a device node.
> 
> I took a quick stab at it:
> git://github.com/vsyrjala/linux.git dp_mst_port_aux_dev
> 
> Doesn't quite seem to work though, so I probably made a mistake somewhere.
> 

Nodes for remote devices aren't too important really for this fwupd support
since the plugin knows how to use the remote control messages.  Actually if
extra nodes are added it would probably confuse the existing userspace stuff
now.

> >
> > If/when you do this can you send a uevent up to userspace?  It would
> > be good for fwupd to be able to listen to it and refresh devices based
> > upon what happened from nodes coming and going.
> 
> Hotplug uevents should be fired off whenever a new display
> appears/disappears.
> 

Ah great, I'll add in udev support to the plugin then.