Linux does not have the concept of IO ports in ARM. When registers and memory are located in the memory space, they are called IO memory.
Keywords：ARM memory
Reference address：ARM IO ports/memory
Here are the steps in brief:
1. Application
2. Mapping
3. Access
4. Release
The specific operation steps are as follows:
1. Application - request_mem_region
After the application, the system is equivalent to marking the existing resources, that is, the resources have been applied for and occupied. If the same resources are applied for again, the kernel will report an error.
2. Mapping - ioremap Only
through ioremap can the physical register address be mapped into a virtual address so that it can be applied in the kernel. The advantage here is that the register can be accessed by adding the base address to the offset.
3. Access
There are specific read and write operation functions such as writel etc.
4. Release
After mapping, the function used to release resources is iounmap+release_mem_region
Note: Sometimes access is directly mapped without applying, but this is a bit irregular, but there are still many applications in the kernel, and they are easy to affect each other.
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