what are mapbase and membase members of UART 8250 port structure?

问题

I am trying to understand the 8250 serial port on pci express bus of our board by going through the driver code and I am having hard time understanding the below two members of struct uart_8250_port

struct uart_8250_port x;
memset(&x, 0, sizeof(x));
....
....
x.port.regshift = 0;
x.port.iotype = UPIO_MEM;
x.port.flags = UPF_SHARE_IRQ | UPF_LOW_LATENCY;
x.port.membase = raw_address;
x.port.mapbase = ioremap(raw_address);

What are the members membase and mapbase? Also, why does membase take physical pci bar address whereas mapbase takes address returned from ioremap ?

回答1:

mapbase is basically a bus address of the port register space in the question. membase is the same address in CPU virtual address space for memory mapped IO (MMIO) and iobase is the similar for IO.

Yes, you may already notice that in your example you have a mistake, i.e. membase and mapbase should be exchanged.

mapbase address is needed to do, for example, DMA operations on top of UART hardware since DMA operates with bus addresses.

来源：https://stackoverflow.com/questions/38602052/what-are-mapbase-and-membase-members-of-uart-8250-port-structure