2012-10-21 01:14:50 +00:00
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<title>Paying It Forward: Documenting your Hardware</title>
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<div class="footer">
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<a class="url" href="https://p.xobs.io/lca20-pif/">p.xobs.io/lca20-pif</a>
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<span class="theme">The Linux of Things</span><span class="hashtag"> | #LCA2020</span><span class="twitter"> |
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@linuxconfau</span>
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2011-06-07 19:10:59 +00:00
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</div>
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2020-01-02 03:01:08 +00:00
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<div class="slides">
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<section data-background-image="css/theme/lca2019-title-bg-transparent.svg">
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<h1>Paying it Forward: Documenting Your Hardware Project</h1>
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2020-01-02 03:55:53 +00:00
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<h4>Approaches to documenting a hardware description language using lxsocdoc</h4>
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2020-01-02 03:01:08 +00:00
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<p align="right">
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<small>Sean Cross - <a href="https://xobs.io/">https://xobs.io/</a> - @xobs</small>
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</p>
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</section>
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<section>
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2020-01-02 03:55:53 +00:00
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<h1>Hardware is different</h1>
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2020-01-02 03:01:08 +00:00
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<ol>
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2020-01-02 03:55:53 +00:00
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<li>Massively Parallel</li>
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2020-01-02 03:01:08 +00:00
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</ol>
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</section>
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2020-01-02 03:55:53 +00:00
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This is the Open ISA miniconf, which today tends to mean FPGAs. This means that
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hardware and software are both extensible, and developers will be able to extend
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the hardware in addition to making modifications to your software package.
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Undocumented hardware is bad. There are all sorts of quirks, and even if you have
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the source code, it can be very difficult to read. I'm the primary developer for
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the Fomu project, and this talk will cover some of the issues I've run into with
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respect to documentation. It is most directly related to the LiteX and Migen
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projects, but the concepts will carry over into any other Hardware Description
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Language you may use.
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Common approaches today involve comments in the HDL and/or C header files. This
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works, but we can do better. We just need to describe the hardware better.
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```//Hardware definitions of the SoC. Also is the main repo of documentation for the
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//programmer-centric view of the hardware.```
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Fomu uses LiteX, which is related to Migen. This is a hardware description language
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written in Python. You write Python code and run the program, and it generates
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a design file -- either Verilog code, or a Yosys netlist. There are many other
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alternatives such as SpinalHDL or Chisel. By writing in Python as opposed to
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direct Verilog, we get a lot of nice primitives.
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CSRStorage and CSRStatus are two such primitives. These enable trivial access to
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a hardware device from a CPU softcore. Instead of manually wiring up a crossbar
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and decoding the addresses ourselves, we just need to write `self.regname = CSRStatus(8)`,
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and the build system will wire up 8 bits of read-only memory to the target CPU.
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Similarly, `self.othername = CSRStorage(8)` will give 8-bits of write-only memory.
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This works well, but exposes a new problem: Documentation. As an example, I was
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working with the SPI Flash block in litex, and wanted to know how the bitbang
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driver worked. There wasn't any documentation except the source, which looked
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like this:
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self.bitbang = CSRStorage(4)
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If(self.bitbang.storage[3],
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dq.oe.eq(0)
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).Else(
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dq.oe.eq(1)
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),
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If(self.bitbang.storage[1], # CPOL=0/CPHA=0 or CPOL=1/CPHA=1 only.
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self.miso.status.eq(dq.i[1])
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),
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dq.o.eq(Cat(self.bitbang.storage[0], Replicate(1, spi_width-1)))
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You can kind of understand it, but it does take a lot of mental power to
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work through it. I started by creating aliases for the various elements
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in the storage array, but then I thought: There has to be a better way!
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This is when I hit upon the idea of `lxsocdoc`. The basic idea is that
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Python is really good at introspecting Python, so let's add a little bit
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more information to the CSR objects to make our life easier. So after
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working with the LiteX creator Florent, we refactored the bitbang
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definition to this:
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self.bitbang = CSRStorage(4, fields=[
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CSRField("mosi", description="Output value for MOSI pin, valid whenever ``dir`` is ``0``."),
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CSRField("clk", description="Output value for SPI CLK pin."),
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CSRField("cs_n", description="Output value for SPI CSn pin."),
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CSRField("dir", description="Sets the direction for *ALL* SPI data pins except CLK and CSn.", values=[
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("0", "OUT", "SPI pins are all output"),
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("1", "IN", "SPI pins are all input"),
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])
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], description="""
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Bitbang controls for SPI output. Only standard 1x SPI is supported, and as
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a result all four wires are ganged together. This means that it is only possible
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to perform half-duplex operations, using this SPI core.
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""")
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Now the actual bitbang logic looks like:
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If(self.bitbang.fields.dir,
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dq.oe.eq(0)
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).Else(
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dq.oe.eq(1)
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),
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If(self.bitbang.fields.clk, # CPOL=0/CPHA=0 or CPOL=1/CPHA=1 only.
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self.miso.status.eq(dq.i[1])
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),
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dq.o.eq(Cat(self.bitbang.fields.mosi, Replicate(1, spi_width-1)))
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This is a little bit easier to understand -- no longer are we looking at indices
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in an array to determine what field does what. Instead we get actual named fields.
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But because Python can introspect Python very easily, this is just the beginning.
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After the design is elaborated and the output file is generated, we can iterate
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through the resulting tree and pick out any CSR objects and using any additional
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information. We can actually generate a full reference manual, just like one you
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would receive from a SoC Vendor.
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For example, this is what the start of the Fomu SPI Flash documentation looks like:
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[Register Listing for LXSPI]
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This is already pretty useful. You can hand this file to someone and show them
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how your design works. But the title of this talk is called "Paying it Forward",
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and I can tell you from experience that having such a reference manual for yourself
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while developing software for your own hardware is still invaluable. Hardware
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designs are complex things, and not having to decode bitfield offsets in your
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head or constantly referring to various sections of code to see how it's implemented
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saves valuable time.
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So now we have register documentation. Can we do better? Of course we can.
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SoC reference manuals are more than just register definitions. They also include
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background information on protocols, as well as more elaboration on how the block
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works.
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lxsocdoc
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intro to litex/migen
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concept of mixins
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concept of documentation sections
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what the output can look like
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what's coming in the future
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documenting interrupts
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introspecting classes
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other approaches
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how you can help
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why this helps you
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Benefits:
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* Generating reference manuals
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* SVD
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* SVD2Rust
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2020-01-02 03:01:08 +00:00
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</div>
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multiplex: {
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url: 'https://p.xobs.io/',
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secret: Reveal.getQueryHash().s || null
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// More info https://github.com/hakimel/reveal.js#dependencies
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dependencies: [
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{ src: 'lib/js/classList.js', condition: function () { return !document.body.classList; } },
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{ src: 'plugin/markdown/markdown.js', condition: function () { return !!document.querySelector('[data-markdown]'); } },
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{ src: 'lib/js/socket.io.js', async: true },
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{
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src: presenter ?
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'plugin/multiplex/client.js', async: true
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});
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</script>
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</body>
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2012-03-24 16:48:16 +00:00
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2012-08-04 04:33:27 +00:00
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</html>
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