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To avoid clock-stretching problems, using one of the small Arduinos that support USB serial to handle the I2C bus can help considerably, at the expense of a bit more programming complexity.

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I'm not at all familiar with this space, but I'm a bit surprised the kernel isn't wrapping the I2C transmission and helping to work around clock stretching issues anyway. Can someone who's more familiar with this implementation weigh in? It seems like that'd be the primary feature of writing to the /dev/i2c* device instead of manually bit-banging the GPIO pins from userland.

The Pi's hardware i2c has buggy silicon and does not handle clock stretching properly. There's also a kernel driver to bitbang i2c over the GPIO pins and clock stretching does work properly with that.

Does the PI 4 also have this problem?

There's a claim that it was fixed here:

https://www.raspberrypi.org/forums/viewtopic.php?p=1484482#p...

Apparent confirmation here (by mapping a different I2C bus?):

https://www.raspberrypi.org/forums/viewtopic.php?p=1544123&s...

I'm not sure about the 4, though it was present at least from the original up through the 3B+.

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