Access files from the space code of the kernel?

Question

I am looking at the source code for drivers / staging / comedi / drivers / serial2002.c "and the implementation represents access to the file from the kernel code.

System calls (read, write) provide a standard interface that prevents incorrect access to h / w and other system resources. In short, device file access should be done from user space. Therefore, I confuse why we need to access the file from the kernel space , because this type of implementation does not provide security functions.

Please provide me a pointer to understand the reason.