Re: Does my understanding correct?

[Richard Yang <weiyang@linux.vnet.ibm.com>]

On Thu, May 03, 2012 at 10:39:42AM -0600, Bjorn Helgaas wrote:
>On Thu, May 3, 2012 at 12:21 AM, Richard Yang
><weiyang@linux.vnet.ibm.com> wrote:
>> On Wed, May 02, 2012 at 08:59:40AM -0600, Bjorn Helgaas wrote:
>>>On Wed, May 2, 2012 at 12:24 AM, Richard Yang
>>><weiyang@linux.vnet.ibm.com> wrote:
>>>> On Mon, Apr 30, 2012 at 09:56:13AM -0600, Bjorn Helgaas wrote:
>>>>>On Fri, Apr 27, 2012 at 11:01 PM, Richard Yang
>>>>><weiyang@linux.vnet.ibm.com> wrote:
>>>>>
>>>> Thanks for your nice chart.
>>>>>I think assignments shown for the PCIe-to-PCI bridge are OK, although
>>>>>I would draw it like this because the bridge originates a single bus
>>>>>02 that may have multiple devices attached to it (this side is PCI,
>>>>>not PCIe, so it really is a shared bus):
>>>>>
>>>>>                                 ^
>>>>>                                 |
>>>>>                        +--------+--------+
>>>>>                        |     00:02.0     |
>>>>>                        | PCIe-PCI bridge |
>>>>>                        |                 |
>>>>>                        +--------+--------+
>>>>>                                 |
>>>>>                                 |
>>>>>                      +---------------------+    Bus 02
>>>>>                      |                     |
>>>>>                      |                     |
>>>>>                      |                     |
>>>>>                 +----v----+           +----v----+
>>>>>                 | 02:00.0 |           | 02:01.0 |
>>>>>                 +---------+           +---------+
>>>>>
>>>> So for this case, there is not internal bus, while this is really a
>>>> physical shared bus, not a logical one.
>>>
>>>Yes.  The downstream side of the PCIe-PCI bridge is PCI.
>>>
>>>>>I think the PCIe switch part is incorrect.  Here's Figure 1-3 from sec
>>>>>1.3.3 of the PCIe r3 spec:
>>>>
>>>>>A PCIe switch appears as two or more PCI-PCI bridges.  One is
>>>>>associated with the upstream port; the others with the downstream
>>>>>ports.
>>>>>
>>>>>A bridge always has a primary side and a secondary side.  In your
>>>>>diagram, the bridge associated with the upstream port would be 00:01.0
>>>>>(primary bus 00) and could have a secondary bus of 03 (since 02 is
>>>>>already consumed by the PCIe-PCI bridge).
>>>> Hmm... I am confused why is 03. 02 is used but 01 is not used.
>>>> Switch should be configured after PCIe2PCI bridge?
>>>
>>>It's likely that the PCIe switch would be configured first, since its
>>>device number is lower, but that is not a requirement.  The
>>>requirement is that the bus number ranges consumed by bridges be
>>>non-overlapping.  In this case (using your original topology plus my
>>>PCIe switch diagram), we'd have:
>>>
>>>  1. an endpoint at 00:00.0 -- consumes no additional buses
>>>  2. a PCIe switch at 00:01.0 -- consumes at least [bus 03-06]
>>>  3. a PCIe-PCI bridge at 00:02.0 -- consumes least [bus 02] (its secondary bus)
>
>> So this is a sequence issue. Below is also a valid configuration.
>>  2. a PCIe switch at 00:01.0 -- consumes at least [bus 02-05]
>>  3. a PCIe-PCI bridge at 00:02.0 -- consumes least [bus 06] (its secondary bus)
>
>Yes, this would also be valid.
>
>>>Bus 03 is the internal PCIe switch bus that connects the upstream port
>>>to the downstream ports.  Buses 04, 05, and 06 are the links
>>>originating from the downstream ports.
>>>
>>>>>The bridges associated with the downstream ports are all logically on
>>>>>bus 03.  Their primary bus number would be 03; they might be 03:00.0,
>>>>>03:01.0, 03:02.0, etc.  Each would have its own secondary bus number,
>>>>>for example 04, 05, 06.  That secondary bus number is for the
>>>>>downstream link from the corresponding downstream port.
>>>> Hmm, as you mentioned in previous letter, PCIe is an point-to-point
>>>> protocol, then the secondary bus should reside in the Switch?
>>>> Do you think my Bus#4 is correct?
>>>
>>>No.  Bus 04 is a PCIe link that connects the downstream port (03:00.0)
>>>to a single PCIe device.  That device (04:00) could be a PCIe
>>>endpoint, or it could be the upstream port of another PCIe switch.
>>>(If it is a switch, more bus numbers would be required.)
>>
>>
>>                                      ^
>>                                      |
>>      +-------------------------------|------------------------------+
>>      |                               |                              |
>>      |                          +----+----+                         |
>>      |                          | virtual |                         |
>>      |                          | PCI-PCI |                         |
>>      |                          | bridge  |                         |
>>      |                          +----+----+                         |
>>      |                               |                              |
>>      |                               |Bus#3                         |
>>      |                               |                              |
>>      |          +----------------------------------------+          |
>>      |          |                    |                   |          |
>>      |          |                    |                   |          |
>>      |          |03:00.0             |03:01.0            |03:02.0   |
>>      |     +----+----+          +----+----+         +----++---+     |
>>      |     | virtual |          | virtual |         | virtual |     |
>>      |     | PCI-PCI |          | PCI-PCI |         | PCI-PCI |     |
>>      |     | bridge  |          | bridge  |         | bridge  |     |
>>      |     +----+----+          +----+----+         +----+----+     |
>>      |          |                    |                   |          |
>>      |          |                    |                   |          |
>>      +----------|--------------------|-------------------|----------+
>>                 | Bus#4?             |                   |
>>                 v                    v                   v
>>
>> So the link itself is Bus#4?
>
>Exactly.
>
>> If there is no device under this link, will Bus#4 appear in kernel?
>
>Yes, I'm pretty sure we build the bus 04 pci_bus structure, then scan
>bus 04 for devices.  Even if we find none, the pci_bus struct for bus
>04 remains.
>
>> And then the PCIe switch is represented by 4 pci_dev structure and each
>> is bridge type?
>
>Yes.  And each has an associated pci_bus struct for its secondary bus.
Bjorn

Thanks a lot :)
Your patient reply answer my question clearly. 
Those questions confused me a lot. 
Now I think I get a more clear view about how pci system is represented
in the kernel. 

Thanks again.

>
>Bjorn

-- 
Richard Yang
Help you, Help me