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Add a QFbScreen implementation that uses DRM dumb buffers and page
flipping, thus implementing a vsynced, double (triple) buffered plain
software platform plugin. Besides better supporting systems that by
now only do fbdev via (possibly buggy) emulation, this can also be an
enabler for implementing certain types of 2D accelerated compositors
e.g. for Wayland.
Enable with QT_QPA_FB_DRM=1. Right now there is no good way to
autodetect the support for dumb buffers early enough so the default is
always the traditional fbdev path.
Long-term this will also enable proper multi-display support like with
eglfs' DRM backends. Multiple QScreens are not yet reported currently
due to the single-screen limitations of fbconvenience.
The generic DRM code is shared with eglfs, meaning configuration
happens via the JSON file specified in QT_QPA_(EGLFS_)KMS_CONFIG. This
allows changing the screen resolution, disabling outputs, etc.
[ChangeLog][Platform Specific Changes][linuxfb] Added experimental
support for DRM dumb buffers. Enable by setting the environment
variable QT_QPA_FB_DRM to 1.
Task-number: QTBUG-56306
Change-Id: I4bdcf16fff67412183220dffa0c4ea235c5cad3d
Reviewed-by: Andy Nichols <andy.nichols@qt.io>
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