Re: mapping a PCI bus

[Mitch Bradley <wmb-D5eQfiDGL7eakBO8gow8eQ@public.gmane.org>]

The reason why the ranges property for PCI is so complicated is because 
PCI addressing is complicated, so it's tricky to fit the PCI address 
information into a generic framework.  I'll explain how it works 
in-line, below.

> Hy everybody!
>
> I’m working on a project:
>
> On a proprietary board, (with a PowerPC 7448), a minimal Linux has 
> been embedded. My first aim consist in using a graphic chipset on the 
> board, that is to say make work the PCI bus. For the address mapping, 
> I try to write a dts, using the dts of the mpc7448hpc2 board as a 
> model. My problem is that I don’t understand how to fill the field 
> “range” of the PCI. The “width” used and the number of range is 
> different from on board to an other where PCI is used. I read the PCI 
> specification, and don’t find anything about it.
>
> Here is an extract of my dts, where the field in question is indicated 
> thanks to commentaries:
>
> pci@70000000 { compatible = "abac-pci";
> device_type = "pci";
> #size-cells = <2>;
> #address-cells = <3>;
> #interrupt-cells = <1>;
> reg = <0x70000000 0x1000>;
> bus-range = <0 0>;
> /* how can I know how to chose the number of range and their width?*/
The "ranges" property describes how the bus bridge translates between 
the "parent" address space (typically the main system bus) and the 
"child" address space (the PCI bus address space).  For the full details 
of the generic "ranges" property, consult page 172 of 
ftp://playground.sun.com/pub/1275/coredoc/1275-1994/1275.ps.gz

The number of ranges entries is equal to the number of disjoint 
translation regions.  Typically you need at least two for PCI bus, one 
for the PCI memory space and another for the PCI I/O space.

>
> Then, I have a second question: why PCI’s addresses seem to be 
> implemented on 96 bits, whereas the addressing is done on 32 bits?

I have moved this question up, as the answer is helpful for later.

PCI bus addressing can be either 32 bits or 64 bits, depending on the 
hardware implementation.  The Open Firmware binding to PCI uses 64 bits 
so it can accommodate the full PCI spec.  In addition to the 64 address 
bits, you need bits to indicate other information, such as which address 
space (I/O or memory) you are talking about, which Base Address Register 
is involved, and other information as described on page 4 (pdf page 9) 
of ftp://playground.sun.com/pub/1275/bindings/pci/pci2_1.pdf .  Much of 
that other information is irrelevant in the context of the "ranges" 
property - but the address space identifier is relevant.  Numeric values 
in the device tree are represented in units of 32 bits, so you need a 
total of 96 bits for the basic 64-bit address plus the extra info.

>
> ranges = <0x2000000 0x0 0xe0000000      0xe0000000      0x0 0x1a000000

Okay, now we can start to parse this ranges property.  I added spaces 
above to show grouping.

    0x2000000 0x0 0xe0000000

is the base PCI address of one of the ranges entries.  0x2000000  is the 
"phys.hi" cell that contains the "extra" information - in particular the 
"2" is in the "ss" field that indicates the address space - 2 means "PCI 
memory space".  The rest of the bits in that word are irrelevant to the 
ranges property.   0x0 0xe0000000 is a 64-bit offset, indicating that 
the mapped-through range begins at e0000000 in PCI memory address space.

   0xe0000000

is the entry's base address in the parent (system bus) address space.  
Apparently this bus bridge is a very simple one - probably just wires 
and a decoder - because the parent address and child address are 
identical.  That's not always the case.

   0x0 0x1a000000

is the size of this range.  It's a 64-bit number because PCI has 
#size-cells = 2.  So the range goes from 0xe0000000 to 0xf9ffffff.


>
> 0x1000000 0x0 0x0 0xfa000000 0x0 0x10000>;

Similarly - 0x1000000 means I/O space, 0x0 0x0 means PCI I/O space 
beginning at 0, 0xfa000000 means that the PCI I/O space appears at 
0xfa000000 in the system bus address space, and 0x0 0x10000 means the 
mapping is 64Kbytes long.


>
> …
>
> };
>
> Thanks for Your attention !
>
> Nicolas Lavocat