[RFC] drm + i915 DP MST code preview

Daniel Vetter daniel at ffwll.ch
Wed May 7 01:07:34 PDT 2014

On Wed, May 07, 2014 at 12:16:37AM -0700, Aaron Plattner wrote:
> On 05/03/2014 02:00 AM, Chris Wilson wrote:
> >On Sat, May 03, 2014 at 07:08:02AM +1000, Dave Airlie wrote:
> >>On 2 May 2014 18:52, Chris Wilson <chris at chris-wilson.co.uk> wrote:
> >>>On Fri, May 02, 2014 at 02:39:37PM +1000, Dave Airlie wrote:
> >>the GUID is only on DP 1.2 devices, so you don't get one for ever
> >>port, also GUIDs are wiped on powerdown on most devices, default GUID
> >>is 0 except where devices have USB hubs as well, so it probably
> >>doesn't make much sense to bother exposing them directly.
> >
> >Ok. It looks like if we do attempt to maintain persistent naming, we need
> >to do it in the kernel anyway. That is to make sure that a downstream
> >device always has the same type-id upon reconnection - at least for the
> >lifetime of module. Or maybe the output name is irrelevant for
> >preserving extended desktop configurations?
> Dunno if it helps, but for roughly similar reasons we ended up naming the
> outputs based on their topology paths in the NVIDIA driver.  So for example
> a port named DP-3 that has a Dell UP2414Q attached will show up as two
> outputs named DP-3.1 and DP-3.8 since its internal bridge uses downstream
> ports 1 and 8.  This has worked out fairly well in practice.
> Here's how I described it in the README:
>     When DisplayPort 1.2 branch devices are present, display
>     devices will be created with type- and connector-based names
>     that are based on how they are connected to the branch device
>     tree. For example, if a connector named DP-2 has a branch
>     device attached and a DisplayPort device is connected to the
>     branch device's first downstream port, a display device named
>     DP-2.1 might be created. If another branch device is
>     connected between the first branch device and the display
>     device, the name might be DP-2.1.1.
>     To avoid cluttering the output list, DisplayPort 1.2 devices
>     can be deleted when they are no longer connected and are not
>     named in any MetaModes. This behavior can be enabled with the
>     DeleteUnusedDP12Displays option.
> http://us.download.nvidia.com/XFree86/Linux-x86/337.19/README/displaydevicenames.html

I'm unclear how you name non-DP branch devices. Do they show up as
DP-2.1-HDMI or something similar? Or do you just keep the DP-2.1 name but
set the connector type to HDMI?

In general I think your prosoal of adding branch downstream ports to name
MST connectors sounds really good. The kms object ids will still be
random, but at least if users connect the same topology to the same ports
we should have stable names.

And of course userspace can still check the EDID serial.
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch

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