64 lines
3.0 KiB
Markdown
64 lines
3.0 KiB
Markdown
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# Flashing via HID (CMSIS-DAP)
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When the web app has access to a HID connection to the board, it can flash
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the board via the hardware debugger interface.
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The PXT localhost server can proxy HID connections (over a WebSocket),
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and native apps can access HID via various custom APIs (which are
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likely to have lower latency than the HID proxy).
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This is generally done via
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writing a little flashing program to the RAM, then writing the page to be
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flashed to the RAM, and then running the program. For next page, one keeps
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the flashing program, but replaces the data. Internally, the DAPLink
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software on the @boardname@ does the same.
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The flashing via DAP over HID is quite a bit slower than the regular
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drag&drop kind. This is because of overheads of the DAP protocol
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and the limited throughput of HID (1 packet, of maximum 64 bytes, per millisecond).
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Additionally, the DAP protocol requires every HID packet to be acknowledged
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effectively halving the bandwidth.
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Thus, typical flashing speeds (using HID proxy) are around 14k/s, with a typical
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full flash taking 15s. Theoretical maximum is around 25k/s.
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A custom flashing protocol, like [HF2](https://github.com/Microsoft/uf2/blob/master/hf2.md),
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can achieve around 60k/s, however this would require updates of DAPLink software,
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and is still not very fast.
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## Partial flashing
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Instead, we take care to only flash the pages that have changed.
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In typical software development only a very small fragment of the program
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changes on every re-deployment. Additionally, most of the program is pretty
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much constant (two bootloaders, the softdevice, and the compiled C++ runtime).
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This is achieved by first deploying a small program which computes
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checksums of every page. Then, these checksums are read from the device
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and compared with checksums of pages of the `.hex` file to be deployed.
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Only pages which checksums that do not match are flashed.
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The particular checksum algorithm used is [Murmur3](https://en.wikipedia.org/wiki/MurmurHash#MurmurHash3).
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The algorithm is simplified by removing checks for unaligned data, or hashing
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the data length, since all blocks hashed are of the same, aligned length.
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The Murmur3 hash was chosen since it's very fast (around 4x faster than CRC32 and around
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15x faster than SHA256). Hashing the entire flash takes about 200ms.
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In fact, two 32 bit Murmur3 hashes (using different starting seeds) are computed in
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parallel, to produce a 64 bit checksum.
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## Hash length analysis
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Let's compute the probability of some people running into trouble because
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of hash collisions on pages. Assume:
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* uniform distribution of hashes
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* 10M users
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* each user programming for 50h, and flashing every 2 minutes, i.e., 1500 flashes
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* each flashing changing 10 pages
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With 64 bit hashes, the probability that a collision occurs
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`1 - ((2^64 - 1) / 2^64) ^ (1e7 * 1500 * 10)` which is `0.000016`
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(Bing says so; Google wrongly said `0`).
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With 32 bit, even with only 1M users we get `97%` probability of some collisions.
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The uniformity of hashes is questionable, but the `0.000016`
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gives us some wiggle room.
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