[Intel-gfx] DP Aux interfaces inquiry

[Mario.Limonciello at dell.com]

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?