Questions about logic_pio

From: Jiaxun Yang
Date: Wed Feb 19 2020 - 09:15:17 EST

Hi there,

Logic PIO gives us a way to make indirect PIO access, however,
the way it handles direct (MMIO) I/O access confused me.

I was trying to create a PCI controller Driver and noticed that I/O range parsed
from DeviceTree will be added to the Logic PIO range by logic_pio_register_range.
And than PCI subsystem will use the ioport obtained from `logic_pio_trans_cpuaddr`
to allocate resources for the host bridge. In my case, the range added to the logic pio
was set as hw_start 0x4000, size 0x4000. Later, `logic_pio_trans_cpuaddr` called
by `pci_address_to_pio` gives a ioport of 0x0, which is totally wrong.

After dig into logic pio logic, I found that logic pio is trying to "allocate" an io_start
for MMIO ranges, the allocation starts from 0x0. And later the io_start is used to calculate
cpu_address. In my opinion, for direct MMIO access, logic_pio address should always
equal to hw address, because there is no way to translate address from logic pio address
to actual hw address in {in,out}{b,sb,w,sb,l,sl} operations.

How this mechanism intends to work? What is the reason that we are trying to
allocate a io_start for MMIO rather than take their hw_start ioport directly?


Jiaxun Yang