Is it necessary to ensure the thread-safety when writing a device driver?

[PowerMan]

       I am writing a uart driver and this uart is used as
the console of my board.
      Is it necessary to do some protection in function like that

int uart_write(const char *buf, int len)
{
int i;
for (i = 0; i < len; i++)
   TX_Regsiter = buf[i];
}

I worried about that if one thread called printf("abc") and another
called printf("def") then I would got "adbecf".

I noticed that the uart driver of some arm board do nothing to it.
Does the protection is made by the new-lib?

If so, I should only use standard library to access the console,
never directly access the uart_write, right?
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