[Intel-gfx] [RFC][PATCH 0/2] drm: PATH prop for all connectors?

[Lyude Paul]

(adding sunpeng.li at amd.com to the thread here, since this is relevant to the DP
aux device work)

I mentioned this in IRC, but figured I should mention it on the ML as well so it
can be discussed further. Honestly: I don't like the way we implement the path
prop for MST. Mainly because

 * It looks ugly: mst:<obj-id>-<port#1>-<port#2> is ambiguous looking. I didn't
   even realize the first number was actually supposed to be the object ID until
   I looked at the code
 * I strongly doubt object IDs are consistent enough for the path prop to
   actually be as meaningful as it looks
 * 
Obviously we can't just remove the path property, since it's being used in
userspace. This has me somewhat convinced that I think it might be a better idea
to just make a whole new path_v2 prop, and document that the path prop was a bad
idea and is now deprecated (but still functional). If we did this, we could come
up with a much nicer MST naming scheme as well! Consider:

For a connector with the RAD 0.1 living on the topology on DP-1 on card0:

mst:DP-1:0.1

I see multiple benefits to this:
 * Look how easy it is to read!
 * DP-1 isn't guaranteed to be consistent, but it is certainly far more likely
   to be consistent than an object ID.
 * This seems a lot easier to write udev rules for, imho

The only thing I'm not sure about whether or not we should also prepend the
connector name with the device (e.g. card0, card1, etc.). I thought this might
be necessary at first, but thinking about it - it shouldn't be hard to figure
out the device in question from looking at sysfs since any userspace application
will know which DRM fd the connector comes from.

Does this sound like a good idea? If so, I'd be happy to write up some patches
for this

On Thu, 2019-06-13 at 21:43 +0300, Ville Syrjala wrote:
> From: Ville Syrjälä <ville.syrjala at linux.intel.com>
> 
> Here's a possible apporoach for providing userspace with
> some stable connector identifiers. Combine with the bus
> name of the GPU and you should have some kind of real
> physical path description. Unfortunately the ship has
> sailed for MST connectors because userspace is already
> parsing the property and expects to find certain things
> there. So if we want stable names for those we'd probably
> have introduce another PATH prop (PHYS_PATH?).
> 
> I suppose one alternative would to make the connector 
> type_id stable. Currently that is being populated by drm 
> core and it's just a global counter. Not sure how badly
> things would turn out if we'd allow each driver to set
> that. It could result in conflicting xrandr connector
> names between different GPUs which I suppose would
> confuse existing userspace?
> 
> Cc: Daniel Vetter <daniel at ffwll.ch>
> Cc: Pekka Paalanen <ppaalanen at gmail.com>
> Cc: Ilia Mirkin <imirkin at alum.mit.edu>
> 
> Ville Syrjälä (2):
>   drm: Improve PATH prop docs
>   drm/i915: Populate PATH prop for every connector
> 
>  drivers/gpu/drm/drm_connector.c        | 13 ++++++++--
>  drivers/gpu/drm/i915/icl_dsi.c         |  3 +++
>  drivers/gpu/drm/i915/intel_connector.c | 20 +++++++++++++++
>  drivers/gpu/drm/i915/intel_connector.h |  3 +++
>  drivers/gpu/drm/i915/intel_crt.c       |  2 ++
>  drivers/gpu/drm/i915/intel_dp.c        |  6 ++++-
>  drivers/gpu/drm/i915/intel_dp_mst.c    |  3 +--
>  drivers/gpu/drm/i915/intel_dvo.c       |  3 +++
>  drivers/gpu/drm/i915/intel_hdmi.c      |  4 +++
>  drivers/gpu/drm/i915/intel_lvds.c      |  2 ++
>  drivers/gpu/drm/i915/intel_sdvo.c      | 35 ++++++++++++++++++--------
>  drivers/gpu/drm/i915/intel_tv.c        |  2 ++
>  drivers/gpu/drm/i915/vlv_dsi.c         |  3 +++
>  13 files changed, 83 insertions(+), 16 deletions(-)
>