Is it safe to pass a uint32_t to a function which defines the input type as uint64_t?

Pass a uint32_t to a function that requires uint64_t: You can do this, and all compilers I know accept it, even in strict modes.

• Is it explicitly allowed by the standard?
• Is it safe?
• What does MISRA-C say about this?

void MyFunction(uint64_t arg) {}

int main(void) {
    uint32_t xyz = 1u;
    MyFunction(xyz);
}

• uintN_t types are in the C standard library C99 and later.

• Optional types uint32_t and/or uint64_t might not exist, yet that is only on very unusual machines. (E. g. CHAR_BIT == 9)

• Code is missing #include <stdint.h>

Is it safe to pass a uint32_t to a function which defines the input type as uint64_t?

Yes. Yet see above notes.

Is it explicitly allowed by the standard?

Yes.

Is it safe?

Yes.

What does MISRA-C say about this?

Type sign-ness and value changes are things MISRA and well enabled compilers whine about.

A sign-ness/value change does not occur in OP's code.