I noticed that both “mem=” and “tegrafb_mem=” entries differ. By what means did you figure the amount to reduce mem, and add to tegra_fbmem?
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I stumbled onto the difference when I was looking for a fix to switch on the USB 3.0 functionality. I found this link http://jetsonhacks.com/2014/11/08/jetson-tk1-linux-tegra-l4t-21-1-enable-usb-3-0/ . In the video I noticed the difference from his/her extlinux file and mine. After looking closer at the differences I hoped that maybe this might actually fix my main problem as it seemed to assign memory to what I believe is the video frame buffer and my main problem is related to video memory. I waited a couple weeks to see if I had a recurrence and then updated this sight. Nothing more then just blind luck.
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I have not used these options, but it would make sense that when system ram is used for both video and cpu and ram goes up high enough video trying to use what it assumed was there could cause X11/graphics to die. This looks like a good thing to test for anyone getting broken logout of the GUI.