Aliases for DRI connectors?

Aliases for DRI connectors?

[Matt Hoosier]

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I'm searching for some sort of scheme that will let my DRI master query the
set of available connectors and select the one carrying a prearranged
designation. The problem I'm trying to solve is to allow deploying one
standardized userspace component across a fleet of devices that have
varying numbers of displays, with the use cases not always driven on the
same connector topologically.

I had a look around the properties available on my system's DRI connectors,
and nothing immediate jumped out at me. Is there a standard connector
property that (assuming I can find the right place in DeviceTree or similar
to) that would be a good fit for this?

-Matt

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Re: Aliases for DRI connectors?

[Simon Ser]

On Wednesday, April 1, 2020 9:38 PM, Matt Hoosier <matt.hoosier@gmail.com> wrote:

> I'm searching for some sort of scheme that will let my DRI master query the set of available connectors and select the one carrying a prearranged designation. The problem I'm trying to solve is to allow deploying one standardized userspace component across a fleet of devices that have varying numbers of displays, with the use cases not always driven on the same connector topologically.
>
> I had a look around the properties available on my system's DRI connectors, and nothing immediate jumped out at me. Is there a standard connector property that (assuming I can find the right place in DeviceTree or similar to) that would be a good fit for this?

Maybe information in the EDID would help? You'll find the manufacturer,
the model number and the serial number.
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Re: Aliases for DRI connectors?

[Matt Hoosier]

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On Wed, Apr 1, 2020 at 3:17 PM Simon Ser <contact@emersion.fr> wrote:

> On Wednesday, April 1, 2020 9:38 PM, Matt Hoosier <matt.hoosier@gmail.com>
> wrote:
>
> > I'm searching for some sort of scheme that will let my DRI master query
> the set of available connectors and select the one carrying a prearranged
> designation. The problem I'm trying to solve is to allow deploying one
> standardized userspace component across a fleet of devices that have
> varying numbers of displays, with the use cases not always driven on the
> same connector topologically.
> >
> > I had a look around the properties available on my system's DRI
> connectors, and nothing immediate jumped out at me. Is there a standard
> connector property that (assuming I can find the right place in DeviceTree
> or similar to) that would be a good fit for this?
>
> Maybe information in the EDID would help? You'll find the manufacturer,
> the model number and the serial number.
>

That might be a possibility. The trouble there is that there can be more
than one instance of the same display model connected to the system, so the
EDID blob wouldn't be particularly helpful to distinguish them. The
connectors in question are generally something like LVDS or DSI that don't
typically expose EDID properties, too.

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Re: Aliases for DRI connectors?

[Pekka Paalanen]

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On Wed, 1 Apr 2020 14:38:37 -0500
Matt Hoosier <matt.hoosier@gmail.com> wrote:

> I'm searching for some sort of scheme that will let my DRI master query the
> set of available connectors and select the one carrying a prearranged
> designation. The problem I'm trying to solve is to allow deploying one
> standardized userspace component across a fleet of devices that have
> varying numbers of displays, with the use cases not always driven on the
> same connector topologically.
> 
> I had a look around the properties available on my system's DRI connectors,
> and nothing immediate jumped out at me. Is there a standard connector
> property that (assuming I can find the right place in DeviceTree or similar
> to) that would be a good fit for this?

Hi,

I've never heard of a thing that could accomplish that. DRM connector
names are not even actually communicated to userspace. What userspace
sees is connector type (enum) and some counter numbers (which are not
persistent, so not reliable if you have e.g. multiple DRM drivers
racing to initialize), and then userspace manufactures a connector name
from those. This has been most painful with Xorg, where the different
video DDX drivers used to use different conventions in making up the
names, meaning that if you switched DDXes (e.g. between driver-specific
driver and modesetting driver), the connector names changed
invalidating your xorg.conf.

However, the problem of non-persistent connector names has been thought
of, see e.g.:
https://lists.freedesktop.org/archives/dri-devel/2019-June/221902.html
The thread has messages also in July and August.

If you had reliable, persistent connector names (and used e.g. device
path from udev to reliably identify DRM devices), maybe you could build
something on top of that?

Though I doubt if maintainers would like your config to be in DT or
kernel, maybe it needs to be handled in userspace?


Thanks,
pq

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Re: Aliases for DRI connectors?

[Guillermo Rodriguez]

Hi Pekka,

El vie., 3 abr. 2020 a las 10:14, Pekka Paalanen
(<ppaalanen@gmail.com>) escribió:
>
> On Wed, 1 Apr 2020 14:38:37 -0500
> Matt Hoosier <matt.hoosier@gmail.com> wrote:
>
> > I'm searching for some sort of scheme that will let my DRI master query the
> > set of available connectors and select the one carrying a prearranged
> > designation. The problem I'm trying to solve is to allow deploying one
> > standardized userspace component across a fleet of devices that have
> > varying numbers of displays, with the use cases not always driven on the
> > same connector topologically.
> >
> > I had a look around the properties available on my system's DRI connectors,
> > and nothing immediate jumped out at me. Is there a standard connector
> > property that (assuming I can find the right place in DeviceTree or similar
> > to) that would be a good fit for this?
>
> Hi,
>
> I've never heard of a thing that could accomplish that. DRM connector
> names are not even actually communicated to userspace. What userspace
> sees is connector type (enum) and some counter numbers (which are not
> persistent, so not reliable if you have e.g. multiple DRM drivers
> racing to initialize),

I may be misreading you, but does this mean that the connector names
used in the [output] section of the weston.ini configuration file are
not reliable?
Then what is the proper way to configure one specific (physical)
output in Weston?

BR,

Guillermo

> and then userspace manufactures a connector name
> from those. This has been most painful with Xorg, where the different
> video DDX drivers used to use different conventions in making up the
> names, meaning that if you switched DDXes (e.g. between driver-specific
> driver and modesetting driver), the connector names changed
> invalidating your xorg.conf.
>
> However, the problem of non-persistent connector names has been thought
> of, see e.g.:
> https://lists.freedesktop.org/archives/dri-devel/2019-June/221902.html
> The thread has messages also in July and August.
>
> If you had reliable, persistent connector names (and used e.g. device
> path from udev to reliably identify DRM devices), maybe you could build
> something on top of that?
>
> Though I doubt if maintainers would like your config to be in DT or
> kernel, maybe it needs to be handled in userspace?
>
>
> Thanks,
> pq
> _______________________________________________
> wayland-devel mailing list
> wayland-devel@lists.freedesktop.org
> https://lists.freedesktop.org/mailman/listinfo/wayland-devel
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Re: Aliases for DRI connectors?

[Pekka Paalanen]

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On Fri, 3 Apr 2020 10:28:52 +0200
Guillermo Rodriguez <guillerodriguez.dev@gmail.com> wrote:

> Hi Pekka,
> 
> El vie., 3 abr. 2020 a las 10:14, Pekka Paalanen
> (<ppaalanen@gmail.com>) escribió:
> >
> > On Wed, 1 Apr 2020 14:38:37 -0500
> > Matt Hoosier <matt.hoosier@gmail.com> wrote:
> >  
> > > I'm searching for some sort of scheme that will let my DRI master query the
> > > set of available connectors and select the one carrying a prearranged
> > > designation. The problem I'm trying to solve is to allow deploying one
> > > standardized userspace component across a fleet of devices that have
> > > varying numbers of displays, with the use cases not always driven on the
> > > same connector topologically.
> > >
> > > I had a look around the properties available on my system's DRI connectors,
> > > and nothing immediate jumped out at me. Is there a standard connector
> > > property that (assuming I can find the right place in DeviceTree or similar
> > > to) that would be a good fit for this?  
> >
> > Hi,
> >
> > I've never heard of a thing that could accomplish that. DRM connector
> > names are not even actually communicated to userspace. What userspace
> > sees is connector type (enum) and some counter numbers (which are not
> > persistent, so not reliable if you have e.g. multiple DRM drivers
> > racing to initialize),  
> 
> I may be misreading you, but does this mean that the connector names
> used in the [output] section of the weston.ini configuration file are
> not reliable?

Yes, they are not generally reliable. They are reliable enough in most
cases, particularly if you have only one DRM device with connectors.
DRM drivers themselves often manage to not race internally, so they are
fairly consistent in what order they create their connectors per device.

Xorg has the exact same problem IIRC, as has probably any display
server on Linux. This means that to find a case where the reliability
actually breaks down you have to try quite hard or be very unlucky,
since it has gone unfixed for so long.

Likewise you cannot rely that /dev/dri/card0 is always the same device
if you have multiple. That issue is showing up a little more than the
connector naming issue. Hence /dev/dri/by-path/ aliases exist. Network
devices have hit the same naming problem a long time ago already, hence
we nowadays have "Predictable Network Interface Names".

> Then what is the proper way to configure one specific (physical)
> output in Weston?

There is no better way yet. I've had some ideas, but no time to look
into them.

MST connectors have a KMS property for the hardware path, but Weston
does not look at that, and the other connectors don't even have it.


Thanks,
pq

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Re: Aliases for DRI connectors?

[Guillermo Rodriguez]

Hi Pekka,

El vie., 3 abr. 2020 a las 12:03, Pekka Paalanen
(<ppaalanen@gmail.com>) escribió:
>
> On Fri, 3 Apr 2020 10:28:52 +0200
> Guillermo Rodriguez <guillerodriguez.dev@gmail.com> wrote:
>
> > Hi Pekka,
> >
> > El vie., 3 abr. 2020 a las 10:14, Pekka Paalanen
> > (<ppaalanen@gmail.com>) escribió:
> > >
> > > On Wed, 1 Apr 2020 14:38:37 -0500
> > > Matt Hoosier <matt.hoosier@gmail.com> wrote:
> > >
> > > > I'm searching for some sort of scheme that will let my DRI master query the
> > > > set of available connectors and select the one carrying a prearranged
> > > > designation. The problem I'm trying to solve is to allow deploying one
> > > > standardized userspace component across a fleet of devices that have
> > > > varying numbers of displays, with the use cases not always driven on the
> > > > same connector topologically.
> > > >
> > > > I had a look around the properties available on my system's DRI connectors,
> > > > and nothing immediate jumped out at me. Is there a standard connector
> > > > property that (assuming I can find the right place in DeviceTree or similar
> > > > to) that would be a good fit for this?
> > >
> > > Hi,
> > >
> > > I've never heard of a thing that could accomplish that. DRM connector
> > > names are not even actually communicated to userspace. What userspace
> > > sees is connector type (enum) and some counter numbers (which are not
> > > persistent, so not reliable if you have e.g. multiple DRM drivers
> > > racing to initialize),
> >
> > I may be misreading you, but does this mean that the connector names
> > used in the [output] section of the weston.ini configuration file are
> > not reliable?
>
> Yes, they are not generally reliable. They are reliable enough in most
> cases, particularly if you have only one DRM device with connectors.
> DRM drivers themselves often manage to not race internally, so they are
> fairly consistent in what order they create their connectors per device.
>
> Xorg has the exact same problem IIRC, as has probably any display
> server on Linux. This means that to find a case where the reliability
> actually breaks down you have to try quite hard or be very unlucky,
> since it has gone unfixed for so long.
>
> Likewise you cannot rely that /dev/dri/card0 is always the same device
> if you have multiple. That issue is showing up a little more than the
> connector naming issue. Hence /dev/dri/by-path/ aliases exist. Network
> devices have hit the same naming problem a long time ago already, hence
> we nowadays have "Predictable Network Interface Names".
>
> > Then what is the proper way to configure one specific (physical)
> > output in Weston?
>
> There is no better way yet. I've had some ideas, but no time to look
> into them.
>
> MST connectors have a KMS property for the hardware path, but Weston
> does not look at that, and the other connectors don't even have it.

Ok so this is a known "problem" which has never been fixed, probably
because in most cases (single DRM device) the current solution is good
enough.

Thank you for the clarification.

BR,

Guillermo Rodriguez Garcia
guille.rodriguez@gmail.com
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