xf86-video-dummy: resize to exact resolution

Aaron Plattner aplattner at nvidia.com
Wed Jun 15 20:22:41 UTC 2016

On 06/14/2016 03:53 PM, Erik Jensen wrote:
> When using Xorg with the dummy driver to host a virtual session for
> remote access, it is often desirable to resize the session to exactly
> match the resolution of the client. While the dummy driver supports
> switching between predefined resolutions, it provides no facility for
> switching to a resolution not defined in xorg.conf. The common
> workaround used by tools such as Xpra seems to be to define a long list
> of resolution in xorg.conf and hope the client matches or is close to
> one of them.
> Chrome Remote Desktop is currently exploring support for using Xorg with
> the dummy driver instead of Xvfb, but exact resize would be needed for
> feature parity. As such, I would like to work to get this functionality
> included in the dummy driver. I am looking for feedback regarding the
> preferred approach for implementation (see below), and the next steps to
> move the process along.
> I have found two proposed patches to add this functionality. The first
> is https://lists.x.org/archives/xorg-devel/2014-November/044580.html,
> which keeps the current virtual monitor model, but updates RandR support
> to 1.2, allowing custom modes to be added on the fly using
> --newmode/--addmode. The second patch is
> https://lists.x.org/archives/xorg-devel/2015-January/045395.html, which
> introduces a more fundamental change: it does away with providing a fake
> output and fake monitor all together, instead caring only about the
> virtual resolution, which can be set using --fb.
> The first patch works the same way as the RandR 1.2 support added to
> Xvfb, last year, and has the advantage of allowing existing software
> such as Xpra to continue to work as is until updated to the new
> functionality. It has the disadvantage of requiring one to calculate a
> mode for the desired resolution that meets the timing requirements of an
> imaginary monitor, create the new mode, add it to the virtual output,
> and finally switch to it. Additionally, the maximum resolution that can
> be specified is limited by VideoRam, which still can only be specified
> in the config file.
> The second patch has the advantage of making dummy simpler and cleaner:
> it is no longer necessary to calculate modelines to work with an
> imaginary monitor (and indeed no virtual output is presented), and one
> only needs to make a single RandR call to update the virtual size. It
> also allocates the needed memory on the fly, so one can pick any size up
> to 32767x32767. The main disadvantage is that programs expecting to be
> able to switch between predefined sizes using RandR 1.0 would stop
> working. E.g., Xpra wouldn't be able to resize the display at all until
> it was updated. It is also different from the approach taken with Xvfb,
> which means that code wanting to support both would need to handle them
> differently. (On the other hand, aside from Chrome Remote Desktop and
> Xpra, I'm not sure how many tools are trying to do something like this.)
> Which approach seems more likely to gain traction?

Without a champion for either one, neither. :)

I'd obviously prefer to make -dummy simpler by removing the fake
outputs, but I don't have time at the moment to push for review or look
into the crash reported in


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