[mythtv-users] Problem with Nvidia Prime on laptop

[Stephen Worthington]

Mythbuntu 14.04.1
MythTV 0.27-fixes

I have an MSI GT70 laptop I use for MythTV when I am away from home.
It has an Intel i7 processor with builtin HD graphics, plus an Nvidia
GTX 670M card.  The Nvidia card needs to send its data via the Intel
GPU, so I am using the Nvidia Prime packages to do that.  I recently
upgraded it from Mythbuntu 12.04 LTS + hardware enablement stack to
Mythbuntu 14.04.1.  With 12.04, it was working correctly with Nvidia
Prime.  If I switched to the Intel GPU and changed to using VAAPI, it
would play recordings, and if I switched to the Nvidia GPU and changed
to using VDPAU, it would play them.  But now with 14.04, I have a
problem when switched to the Nvidia GPU.  The screen is 1920x1080, so
that is what the desktop is displayed as.  As soon as I start
mythfrontend, the desktop switches to 640x480 mode instead of the
correct 1920x1080 mode (I used xrandr to see what the current settings
were).  Only the top left corner of mythfrontend or the 1920x1080 size
desktop is visible in the 640x480 window.  If I then alt-tab to an
xterm I have open and do an "xrandr --auto" or "xrandr --output LVDS1
--mode 1920x1080" command, the screen switches back to the normal
1920x1080 mode and I can play recordings as normal.

So does anyone have any suggestions as to why this might be happening?
Is mythfrontend sending a 640x480 mode command somehow, or do I have
some problem with my Nvidia driver settings so that the 1920x1080 mode
command being sent is being misinterpreted as 640x480?  What is a good
way of debugging this?