How to use IO BAR in linux PCIe device driver

How to use IO BAR in linux PCIe device driver

-v0.1 2017.6.8 Sherlock init

1. IO window parse analysis

   In an ACPI based system, we parse the IO window configured in DSDT table, as
   showed in this link: blog.csdn.net/scarecrow_byr/article/details/53966460.

   We can see in pci_acpi_root_prepare_resources:

        pci_acpi_root_prepare_resources
            --> acpi_pci_probe_root_resources
                --> acpi_pci_root_remap_iospace

in acpi_pci_root_remap_iospace, CPU address of one PCIe IO window will be
mapped to PCI_IOBASE based system IO space, like below picture:

     PCI_IOBASE            PCI_IOBASE + PCIE_IO_SIZE - 1
           |<------------->|
   --------+---------------+---------------------------   <- CPU VA in kernel
       |   |      |\      \
       |   |      | \      \                  
       v   |      |  \      \                  maps supported by MMU
           |      |   \      \       
           |      |    \      \
           |<---->|     |<---->|   <- CPU PA
       |    \      \     \      \
       |     \      \     \      \             maps supported by ATU
       v      \      \     \      \
               \      \     \      \
                \      \     \      \
                 |<---->|     |<---->|  <- PCI address

                IO window 1   IO window 2
               host bridge 1  host bridge 2

and offset between CPU VA and PCI_IOBASE will be stored in resource_entry,
which will be passed to PCI enumeration. The offset in resource_entry will be
offset(above) - PCI address. In the process of the enumeration, IO BAR will
be allocated in related IO window.

the base of IO BAR will be stored in (pci_dev -> resource[IO BAR].start), which
is the offset to PCI_IOBASE in CPU VA.

1. How to use in PCIe device driver

   In one hardware arch, we use inb/outb, inw/outw … function to access IO
   space. In ARM64, these functions are defined in linux/include/asm-generic/io.h,
   like:

        #ifndef outb
        #define outb outb
        static inline void outb(u8 value, unsigned long addr)
        {
                writeb(value, PCI_IOBASE + addr);
        }
        #endif

So when we want to read/write IO BAR in PCIe device driver, we should:

1. get the base of one IO BAR by: addr = pci_resource_start(dev, bar)
2. use outb(value, addr) for an example to do port input by byte-width.