doc: Add build instructions

Signed-off-by: Sylvain Munaut <tnt@246tNt.com>

doc/build.md

@@ -0,0 +1,76 @@
+Building Image
+==============
+
+Building the image is pretty well documented on ADI's wiki about the pluto :
+
+* [Obtaining the sources](https://wiki.analog.com/university/tools/pluto/obtaining_the_sources)
+* [Building the image](https://wiki.analog.com/university/tools/pluto/building_the_image)
+
+So this document will only describe the broad lines and deviations from the process described in those links.
+
+
+Environment
+-----------
+
+Xilinx Vivado 2017.4.1 is required to build the image. We will also use the Vivado hardfloat toolchain rather than the default one (documentation for this can be found in the `README.md` of the `plutosdr-fw` repository).
+
+Required environment variables :
+
+```
+export CROSS_COMPILE=arm-linux-gnueabihf-
+export PATH=$PATH:/opt/Xilinx/SDK/2017.4/gnu/aarch32/lin/gcc-arm-linux-gnueabi/bin/
+export VIVADO_SETTINGS=/opt/Xilinx/Vivado/2017.4/settings64.sh
+```
+
+**DO NOT** source the vivado settings ( `settings64.sh` ) directly into your shell. This will break the build ! The build process will use the environment variable `VIVADO_SETTINGS` to manually setup the required envirtonment whenever it's needed.
+
+The build process also needs X running (because Xilinx's SDK needs X). But no interaction is needed, so running Xvfb in the background and manually setting up `DISPLAY` works fine to build on headless servers.
+
+```
+Xvfb &
+export DISPLAY=:0
+```
+
+
+Prepare Sources
+---------------
+
+First step is to clone them from ADI :
+
+```
+git clone --recursive https://github.com/analogdevicesinc/plutosdr-fw.git
+cd plutosdr-fw
+```
+
+Then apply the patches in this repository to the appropriate submodules :
+
+* `patch-hdl-add-signal-delay-datapath.diff` : This is the main patch add custom logic in the datapath of the FPGA
+* `patch-buildroot-vivado-crosscompiler.diff`: This updates the buildroot configuration to be compatible with the _arm-linux-gnueabihf_ toolchain.
+* `patch-buildroot-use-ecm-instead-of-rndis.diff` : This replaces the propriatary RNDIS mode with the standard CDC ECM mode for network gadget.
+* `patch-buildroot-add-ssh-key-example.diff` : Example patch to add SSH key directly into the image for easy access
+
+
+Finally you can copy the latest version of the `sig_combine.v` and `sig_delay.v` files from this repository (in `gw/`) to `plutosdr-fw/hdl/library/common/`.
+
+
+Build
+-----
+
+If everything above went well, just type `make` in the `plutosdr-fw` directory and it will build everything ...
+
+
+
+Building Control Software
+=========================
+
+To build the control software, you need the 'SDK' / 'SYSROOT' that will be built as part of building the image above. It can be found under `plutosdr-fw/buildroot/output/host/arm-buildroot-linux-gnueabihf/sysroot/`.
+
+Then in the `sw` directory of this repository, simply type `make` while specifying the sysroot to use:
+
+```
+make SYSROOT=xxx/plutosdr-fw/buildroot/output/host/arm-buildroot-linux-gnueabihf/sysroot/
+```
+
+This should result in a `osmo-rfds` binary.
+
+To include it directly on the image, you can copy it to `plutosdr-fw/buildroot/output/target/usr/sbin/` and run the `make` in `plutosdr-fw` again to update the image.