[Intel-gfx] DP Aux interfaces inquiry

[Rafael Antognolli]

Hi Mario, please see below...

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.
> 
> 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.
> 
> > So I don't think you should be making any assumptions on the number/order of
> > these device nodes.
> > 
> 
> From userspace would it be better to just scan /dev for /dev/drm_dp_aux# nodes?
> No assumptions about the order of them, just look for all the ones with that prefix?
> 
> If not, do you have a better recommendation on how to do this?

Alternatively, you can also scan /sys/class/drm_dp_aux_dev/. The number
of the files that you have there should match what you have inside /dev.
It's not necessarily better though.

Regards,
Rafael