40 lines
1.8 KiB
Markdown
40 lines
1.8 KiB
Markdown
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# sysfs
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## I2C
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### Byte order
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All common libraries (smbus, digispark, firmata, i2cget) read and write I2C data in the order LSByte, MSByte.
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Often the devices store its bytes in the reverse order and therefor needs to be swapped after reading.
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### Linux syscall implementation
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In general there are different ioctl features for I2C
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* IOCTL I2C_RDWR, needs "I2C_FUNC_I2C"
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* IOCTL SMBUS, needs "I2C_FUNC_SMBUS.."
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* SYSFS I/O
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* call of "i2c_smbus_* methods"
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>The possible functions should be checked before by "I2C_FUNCS".
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for further reading see:
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* https://www.kernel.org/doc/Documentation/i2c/dev-interface
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* https://elixir.bootlin.com/linux/latest/source/include/uapi/linux/i2c-dev.h#L42
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* https://stackoverflow.com/questions/9974592/i2c-slave-ioctl-purpose (good for understanding, but there are some small errors in the provided example)
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>Qotation from kernel.org: "If possible, use the provided i2c_smbus_* methods described below instead of issuing direct ioctls." We do not do this at the moment, instead we using the "IOCTL SMBUS".
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Because the syscall needs uintptr in Go, there are some known pitfalls with that. Following documents could be helpful:
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* https://go101.org/article/unsafe.html
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* https://stackoverflow.com/questions/51187973/how-to-create-an-array-or-a-slice-from-an-array-unsafe-pointer-in-golang
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* https://stackoverflow.com/questions/59042646/whats-the-difference-between-uint-and-uintptr-in-golang
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* https://go.dev/play/p/Wd7hWn9Zsu
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* for go vet false positives, see: https://github.com/golang/go/issues/41205
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Basically by convert to an uintptr, which is than just a number to an object existing at the moment of creation without
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any other reference, the garbage collector will possible destroy the original object. Therefor uintptr should be avoided
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as long as possible.
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