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This blog post provides a big picture of the Linux Tegra OSS world. The Wikipedia article about Tegra serves well to get an overview of the Tegra SoC in general, especially from the hardware side.
Linux/Android software provided from NVidia for the Tegra processors are generally known as Linux for Tegra (L4T). Beside the Kernel and its sources, this includes proprietary driver binaries and utilities.
On the other side, today, open source projects such as Linux or U-Boot have a fairly well upstream support for Tegra too. Also thanks to the effort of NVidia providing open source code and documentation as well as pushing things upstream.
This article should provide a big picture about the boot process, available boot software and the state of upstream implementation. Its not a complete view of things, but it would have helped me learning about the Tegra world, so it might help others too :-).