[PATCH 1/6] Add weston randr protocol

Jason Ekstrand jason at jlekstrand.net
Tue Mar 4 19:56:14 PST 2014

I think what Jasper is getting at is the difference between a configuration
interface and a client-facing interface.  Unfortunately, in X the RandR
interface was used in both capacities.  Allow me to try and clerify this

A "client-facing interface" is one that is detected and used by various
clients.  The intention of most client-facing interfaces is to make them
general enough for all compositors and eventually get them mainlined into
the core wayland protocol.  If you are writing this kind of interface, you
need to very specifically justify why clients need this kind of interface
and not another.  In particular, most clients have no buiseness changing
the screen resolution.  Some clients may cause a screen resolution change
due to, for example, making their surface fullscreen.  However, that is a
very different thing from making arbitrary resolution changes.  If you have
a good reason for a client to make arbitrary RandR type changes other than
dynamic configuration, you need to be very clear about why and we need to
analize if there is a better way to do that than simply giving them access
to RandR.

When I say "configuration interface" I mean something that is intended for
the express purpose of changing Weston's confguration dynamically.  This
could be a command-line or graphical utility that allows the user to select
a new screen resolution, orientation, or whatever.  If this is your
intention, then you should probably use the weston_ prefix rather than the
wl_ prefix and it should be considered weston-specific.  Also, if possible,
clients that use this interface should be launched from weston to ensure
that they can only be used with the user's permission.  I'm not sure how to
make this work properly for a command-line client, but a graphical one
could be special-cased inside weston to be allowed the interface.

What I really don't think we need (and where I'm afraid this is headed) is
a priviledged client-facing interface.  I can see very little use for
general modesetting interface that all compositors support.  Output
configuration GUI's aren't that hard to write and each compositor can have
their own.  I highly doubt someone will write a particularly spectacular
third-party output configuration GUI that someone will want to use with
GNOME or KDE.  What about a standard command-line utility?  Frankly, I
can't see that ending well.  The primary use for it would be by scripts and
other clients completely breaking whatever security procedures we try to
set up.

So far, every use case you have given indicates that this is entirely a
configuration interface.  I'm not saying that you don't have a good reason
for wanting to be able to change the output configuration.  If what you're
doing is something other than just on-the-fly weston configuration, I have
a feeling that you have a more specific use in mind that would be better
served by a more specific interface.

--Jason Ekstrand

On Mar 4, 2014 9:22 PM, "Wang, Quanxian" <quanxian.wang at intel.com> wrote:
> From: wayland-devel-bounces at lists.freedesktop.org [mailto:
wayland-devel-bounces at lists.freedesktop.org] On Behalf Of Jasper St. Pierre
> Sent: Wednesday, March 05, 2014 10:53 AM
> To: Wang, Quanxian
> Cc: Hardening; Matthias Clasen; wayland-devel at lists.freedesktop.org;
Pekka Paalanen; Jason Ekstrand; Zhang, Xiong Y
> Subject: Re: [PATCH 1/6] Add weston randr protocol
> So, before we go further on this, I need to ask three basic questions:
what are the goals of this interface? Who is supposed to use this
interface? Why would they use this interface?
> [Wang, Quanxian]
> What are the goals of this interface?
> The goal of this interface is to provide dynamic mode setting for output.
Make wayland desktop configuration more flexible for scale, transform or
mode setting.
> Who is supposed to use this interface?
> Any customer or user want to rotate, scale or mode the output when they
needed. For example, Automotive, there are 2 screens, one is left another
is on back, want to rotate the screen. You can sleep, stand, and more body
postures, you want to rotate the screen to make you comfortable. For
Automotive, mobile, TV or even if desktop. Just like xrandr for xserver.
> You want to get another resolution (mode), for example 1440x900, or
1920x1080. Monitor producer provide such things in order to let user use
this. This interface provides such function dynamically to meet the
> Why would they use this interface? Dynamic mode setting for output will
be provided to developer.
> Here is a general case:
> In window system or linux system, you want to change the resolution of
your desktop at will. For example 1440x900 to 1920x1080, do you want to
kill desktop server and then configure it and then restart desktop server.
You will not, you just open the display configuration, and set it to
1920x1080 and then save it. The resolution of desktop will be changed. If
you want another, same thing.
> Dynamic mode setting for output is found everywhere in matured window or
linux desktop system. Providing this interface to embedded system, multi
screen supported system will be fine.
> On Tue, Mar 4, 2014 at 9:07 PM, Wang, Quanxian <quanxian.wang at intel.com>
> Hi, All
> With the first version of randr protol, I got many useful idea and
comments. Thanks.
> Before I send the email, I have make it happen in real world but need
more deep testing.
> Here are new changes and idea for that protocol based on the idea and
comments, welcome your input.
> 1) Unique operation issue
> Given that one client has two more threads to do mode setting on the same
output at the same time, how to identify what response (done event) is
belong to one or another request when they want to get response?
> This is a design flaw in the first version of randr protocol.
> The solution is to use the wayland/weston mechanism, every request will
generate a resource which be used to send done event to the owner who start
it. Owner could set the listener on that and keep tuning on the response
> For example
> In client:
> randr_transform =
> /* Here will will add listen callback to get the response for this unique
request */
> wl_randr_add_listener(randr_transform, &randr_transform_listener, &randr);
> In compositor:
> randr_resource = wl_resource_create(client,&wl_randr_interface,1, action);
> wl_randr_send_action_done(randr_resource, 1<<WL_RANDR_ACTION_TRANSFORM,
ret, action);
> wl_resource_destroy(randr_resource);
> 2) Security and mess up issue
> Will take randr protocol implementation as a module or plugin. This will
keep compositor clear and avoid messing up in compositor.c. Also it is will
be fine for compositor when this could be public.
> The security should be a general issue for wayland instead of for randr
protocol only. Take randr protocol as a module for user to determine if
they want to public randr interface for their clients. Once security
mechanism is built up in wayland, randr could adopt the mechanism to
enhance the security of randr protocol.
> weston --tty=1 --modules=wrandr.so
> or set it in weston.ini like that
> [core]
> modules=wrandr.so
> 3) Group randr operations
> After talking with PQ, in order to avoid glitches, group operation is
needed. However, if operate on two outputs more at one time, it will bring
more issues which could not control. In this randr design, will not provide
group operation on multiple outputs. We provide atomic operation on one
output, multi outputs operation logic are left to designer/developers.
Group operation on one output will be designed. For example, you can set
mode, scale, and transform at one time with one randr request.
> 4) Configuration interface
> Weston randr protocol will be taken as configuration interface for output
mode setting in wayland. So the permission for that will be strictly under
the control. Before security mechanism is ready, randr module will be
designed for compositor as a choice. It is just a loadable module or plugin
for special compositors. User will take the decision if start it when boot
up compositor as a module.
> 5) mode setting parameters control
> Mode and output will be under the control. User could not randomly to set
their mode. They have to select the available modes and outputs provided by
compositor. Don't allow random mode setting. The mode and output
information could be provided by weston-randr apps or wl_output interface.
> 6) Disconnected outputs
> When user query output information, showing connected and disconnected
output as a whole will be fine for user and QA people. Sometime, QA people
or user like that information. It will be helpful for them to identify how
many outputs are provided by their machine. Which is connected and which is
not connected.
> 7) wl_output property event
> Delete get_output_name request and event in protocol. wl_output name
event will be added into wl_output protocol. This event will send the
output name to user when they bind wl_output. For width and height of
wl_output, I am not sure if it is should be sent out at the same time. My
idea is it should be the same event to send name, width, height after
output is changed. But currently I will not add that.
> 8) adding set_scale request
> Mode, scale, transform is the basic property of output, I will add them
as a whole.
> Thanks
> Regards
> Quanxian Wang
> -----Original Message-----
> From: Pekka Paalanen [mailto:ppaalanen at gmail.com]
> Sent: Thursday, February 27, 2014 5:45 PM
> To: Wang, Quanxian
> Cc: wayland-devel at lists.freedesktop.org; Zhang, Xiong Y
> Subject: Re: [PATCH 1/6] Add weston randr protocol
> On Thu, 27 Feb 2014 09:15:55 +0000
> "Wang, Quanxian" <quanxian.wang at intel.com> wrote:
> >
> >
> > -----Original Message-----
> > From: Pekka Paalanen [mailto:ppaalanen at gmail.com]
> > Sent: Thursday, February 27, 2014 4:36 PM
> > To: Wang, Quanxian
> > Cc: wayland-devel at lists.freedesktop.org; Zhang, Xiong Y
> > Subject: Re: [PATCH 1/6] Add weston randr protocol
> >
> > On Thu, 27 Feb 2014 08:06:23 +0000
> > "Wang, Quanxian" <quanxian.wang at intel.com> wrote:
> >
> > >
> > >
> > > -----Original Message-----
> > > From: Pekka Paalanen [mailto:ppaalanen at gmail.com]
> > > Sent: Thursday, February 27, 2014 3:28 PM
> > > To: Wang, Quanxian
> > > Cc: wayland-devel at lists.freedesktop.org; Zhang, Xiong Y
> > > Subject: Re: [PATCH 1/6] Add weston randr protocol
> > >
> > > On Thu, 27 Feb 2014 11:28:00 +0800
> > > Quanxian Wang <quanxian.wang at intel.com> wrote:
> > >
> > > > Weston protocol wrandr will provide interface to
> > > > 1) set output mode
> > > > 2) set output transform
> > > > 3) move output to relative position
> > > > 4) provide disconnected display port information
> > > >
> > > > *Dynamic* mode setting is the main objective of this protocol.
> > > > Remember, it is one shot operation. For example, if setting the
> > > > mode, just call one request wl_randr_set_mode without any other
> > > > operation.
> > >
> > > Hi,
> > >
> > > like I said in my other reply, this is a configuration interface,
> > > not something for all applications to use. Protocol comments below
> > > assuming, that this will be a configuration interface and that the
> > > generic idea is acceptable.
> ...
> > > If I change the mode on two different outputs, how do I know which
> > > action_done corresponds to which request? [Wang, Quanxian] right,
> > > add wl_output parameter.
> >
> > No. If I do two mode set operations on the same wl_output, then again
> > I would not know which answer was for which. [Wang, Quanxian] I know
> > you mean. Yes, if the same client has more threads which send mode
> > change at the same time, we have to use unique number to stand for
> > every operation.
> That unique number could be a serial, but here more appropriate is a
unique number for each request. You can let Wayland do all the unique
number management for you by using the feedback object design I referred to
below. After all, a Wayland protocol object is essentially just a unique
number. They are very cheap.
> That is why you almost never need to manually fiddle with "unique
numbers" in the protocol.
> > Instead, a generic pattern for this kind of "return data" is to let
> > the original request also create a feedback protocol object. This
> > object is unique to the request that was sent, and can deliver any
> > return data without any ambiguity. An example of a feedback object is
> > wl_callback, except it can only deliver "done", not "yes"/"no"; not
> > delivering anything will cause problems. [Wang, Quanxian] Good,
> > thanks.
> >
> > >
> > > What if move succeeds but mode change fails? Wouldn't that leave the
> > > output in an unwanted state which is neither the original nor the
> > > wanted setting? [Wang, Quanxian] one by one. Not support complex. If
> > > you have such case, have to turn back. Call another move back. But
> > > firstly make sure the previous is successful.
> >
> > That will require a lot of roundtrips, and it essentially forces the
> > compositor to show all the intermediate steps on the monitors. IOW,
> > that is designed to be both slow and glitchy. That's not how you
> > should do dynamic mode setting. I think you are going to need atomic
> > group operations. [Wang, Quanxian] Yes, we could provide a request
> > with more parameters setting(group operations). Just like many
> > parameters in weston_output_switch_mode.
> Except you would need to let it cover an arbitrary number of outputs in
one go. That means that you will need something like what wl_surface.commit
does. Having a request with a huge number of arguments is not only ugly but
inflexible, and cannot be extended in the future.
> > > The solution to this would tie in with the solution to take changes
> > > atomic. For instance, to prepare for a configuration change, one
> > > might create a change object in the protocol, store all changes in
> > > that object, and then commit that set of changes atomically. Then
> > > have one return value: the whole set either succeeds or fails.
> > >
> > > I guess you could look for inspiration in the DRM atomic mode
> > > setting API. I don't know how the RandR X11 protocol works, if that
> > > would be a good example also.
> Thanks,
> pq
> _______________________________________________
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> http://lists.freedesktop.org/mailman/listinfo/wayland-devel
> --
>   Jasper
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