[Intel-gfx] DP Aux interfaces inquiry

[Ville Syrjälä]

On Wed, Jan 25, 2017 at 09:13:44AM -0800, Rafael Antognolli wrote:
> 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.

Or if you already know the connector that you want to use, then you can
find the sysfs stuff for the correct aux device directly under the
connector somewhere in /sys/class/drm/

-- 
Ville Syrjälä
Intel OTC