When the number or size of an object can vary, but can be determined at initialization time, the driver can allocate memory in the pfxinit(), pfxedtinit(), or pfxstart() entry point.
You can allocate memory dynamically in an upper-half entry point. When this is necessary, it should be done in an entry point that is called infrequently, such as pfxopen(). The reason is that memory allocation is subject to unpredictable delays.
Memory allocation should never be attempted in an interrupt routine. The resources that might be needed at interrupt time should be obtained and set aside by an upper-half entry point before the interrupt is made possible.