linux-kernel - Questions about logic

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Message-ID: <1705dbe62ce.10ae800394772.9222265269135747883@flygoat.com>
Date:   Wed, 19 Feb 2020 21:58:43 +0800
From:   Jiaxun Yang <jiaxun.yang@...goat.com>
To:     "yuanzhichang" <yuanzhichang@...ilicon.com>,
        "johngarry" <john.garry@...wei.co>,
        "xuwei5" <xuwei5@...ilicon.com>,
        "gabrielepaoloni" <gabriele.paoloni@...wei.com>,
        "bhelgaas" <bhelgaas@...gle.com>,
        "andyshevchenko" <andy.shevchenko@...il.com>
Cc:     "linux-kernel" <linux-kernel@...r.kernel.org>
Subject: Questions about logic_pio

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?

Thanks. 

--
Jiaxun Yang