project: Add README

Author: smaeul

Diff for: README.md

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+# Overview
+
+The goal of this project is to understand the various blobs provided by
+Allwinner for their `sunxi` series of ARM SoCs. This includes understanding the
+interfaces the blobs provide and the bits of undocumented hardware they use.
+Currently, the tools in this repository are set up to process two types of
+blobs: the boot ROMs and the ARISC firmware.
+
+From the original, untouched blobs, the tools here can generate ELF objects with
+symbols, annotated assembly listings, and SVG graphs documenting the control
+flow of code within the blob.
+
+## Boot ROMs (BROM)
+
+The boot ROM, is flashed into the SoC hardware, and is generally the same for
+every chip of the same model. It comes in two flavors: normal (NBROM) and secure
+(SBROM). The secure boot ROM is much larger and more complicated because it must
+verify the next stage of firmware before running it. Unfortunately, only one of
+the ROMs is visible at a time, depending on the state of the device's "secure
+boot" eFuse.
+
+The BROM blobs in this repository were dumped from running devices using `md`
+from within u-boot.
+
+## ARISC firmware
+
+These files are closed-source firmware provided by Allwinner with their board
+support package (BSP), usually with the name `scp.bin`. Slightly different
+versions of the firmware are provided in the BSPs from different board
+manufacturers. By looking at the changes between versions (and knowing the order
+from the embedded version number), we can determine the history of changes made
+to the firmware.
+
+While the firmware is proprietary, source code is provided in an AES-encrypted
+tarball in [various released Linux kernel trees][tinalinux].
+
+[tinalinux]:
+https://github.com/tinalinux/linux-3.10/tree/r18-v0.9/drivers/arisc/binary
+
+# Prerequisites
+
+These tools require a cross `binutils` (or full toolchain) for each architecture
+of blobs you want to process. Note that even on SoCs that support AArch64, the
+BROM is still AArch32, so you will need ARM cross tools for them. Cross
+toolchains can be downloaded from Linaro for ARM, [GitHub][or1k-toolchains] for
+OpenRISC, or they can be built manually with [musl-cross-make][musl-cross-make].
+If your cross toolchain is named differently, edit the top of the `Makefile` to
+provide your path or prefix.
+
+[musl-cross-make]: https://github.com/smaeul/musl-cross-make
+[or1k-toolchains]: https://github.com/openrisc/or1k-gcc/releases
+
+Beyond that, the scripts only require `bash` and standard POSIX utilities.
+
+# Using the tools
+
+`make BLOB=<soc>/<blob_name>` will build a single blob all the way from the
+hexdump to the call graph. You can simply run `make` to build all files for all
+blobs. You can run `make <soc>/<blob_name>/<file>` for any file (`annotated.s`
+`blob.elf`, `blob.s`, `callgraph.dot`, and `callgraph.svg`) to only make that
+file and its dependencies.
+
+There are some additional scripts that are useful for processing newly-found
+blobs or forr other specific use cases. They are not hooked up to the
+`Makefile`, so they must be run manually. Documentation is at the top of each
+script.
+
+# Contributing
+
+After modifying the annotations in `annotated.s`, run `make save` to copy them
+back to the `comments` file. You can then commit your changes to `comments`. If
+you find a new, previously-unmarked function, create a symbol for it in the
+`symbols` file. *Add two blank lines before the first instruction in
+`annotated.s` and run `make save` before running `make` again*. This is
+important to keep the comments aligned, as `objdump` will place a blank line and
+a line with the symbol name before the function.
+
+Please run `make check` to check the consistency of your symbols and sections
+files.