Re: [Qemu-devel] [PATCH v3 5/5] docs: update documentation considering PCIE-PCI bridge

[Laszlo Ersek]

On 08/01/17 23:39, Michael S. Tsirkin wrote:
> On Wed, Aug 02, 2017 at 12:33:12AM +0300, Alexander Bezzubikov wrote:
>> 2017-08-01 23:31 GMT+03:00 Laszlo Ersek <address@hidden>:
>>> (Whenever my comments conflict with Michael's or Marcel's, I defer to them.)
>>>
>>> On 07/29/17 01:37, Aleksandr Bezzubikov wrote:
>>>> Signed-off-by: Aleksandr Bezzubikov <address@hidden>
>>>> ---
>>>>  docs/pcie.txt            |  46 ++++++++++--------
>>>>  docs/pcie_pci_bridge.txt | 121 
>>>> +++++++++++++++++++++++++++++++++++++++++++++++
>>>>  2 files changed, 147 insertions(+), 20 deletions(-)
>>>>  create mode 100644 docs/pcie_pci_bridge.txt
>>>>
>>>> diff --git a/docs/pcie.txt b/docs/pcie.txt
>>>> index 5bada24..338b50e 100644
>>>> --- a/docs/pcie.txt
>>>> +++ b/docs/pcie.txt
>>>> @@ -46,7 +46,7 @@ Place only the following kinds of devices directly on 
>>>> the Root Complex:
>>>>      (2) PCI Express Root Ports (ioh3420), for starting exclusively PCI 
>>>> Express
>>>>          hierarchies.
>>>>
>>>> -    (3) DMI-PCI Bridges (i82801b11-bridge), for starting legacy PCI
>>>> +    (3) PCIE-PCI Bridge (pcie-pci-bridge), for starting legacy PCI
>>>>          hierarchies.
>>>>
>>>>      (4) Extra Root Complexes (pxb-pcie), if multiple PCI Express Root 
>>>> Buses
>>>
>>> When reviewing previous patches modifying / adding this file, I
>>> requested that we spell out "PCI Express" every single time. I'd like to
>>> see the same in this patch, if possible.
>>
>> OK, I didn't know it.
>>
>>>
>>>> @@ -55,18 +55,18 @@ Place only the following kinds of devices directly on 
>>>> the Root Complex:
>>>>     pcie.0 bus
>>>>     
>>>> ----------------------------------------------------------------------------
>>>>          |                |                    |                  |
>>>> -   -----------   ------------------   ------------------   --------------
>>>> -   | PCI Dev |   | PCIe Root Port |   | DMI-PCI Bridge |   |  pxb-pcie  |
>>>> -   -----------   ------------------   ------------------   --------------
>>>> +   -----------   ------------------   -------------------   --------------
>>>> +   | PCI Dev |   | PCIe Root Port |   | PCIE-PCI Bridge |   |  pxb-pcie  |
>>>> +   -----------   ------------------   -------------------   --------------
>>>>
>>>>  2.1.1 To plug a device into pcie.0 as a Root Complex Integrated Endpoint 
>>>> use:
>>>>            -device <dev>[,bus=pcie.0]
>>>>  2.1.2 To expose a new PCI Express Root Bus use:
>>>>            -device pxb-pcie,id=pcie.1,bus_nr=x[,numa_node=y][,addr=z]
>>>> -      Only PCI Express Root Ports and DMI-PCI bridges can be connected
>>>> +      Only PCI Express Root Ports, PCIE-PCI bridges and DMI-PCI bridges 
>>>> can be connected
>>>
>>> It would be nice if we could keep the flowing text wrapped to 80 chars.
>>>
>>> Also, here you add the "PCI Express-PCI" bridge to the list of allowed
>>> controllers (and you keep DMI-PCI as permitted), but above DMI was
>>> replaced. I think these should be made consistent -- we should make up
>>> our minds if we continue to recommend the DMI-PCI bridge or not. If not,
>>> then we should eradicate all traces of it. If we want to keep it at
>>> least for compatibility, then it should remain as fully documented as it
>>> is now.
>>
>> Now I'm beginning to think that we shouldn't keep the DMI-PCI bridge
>> even for compatibility and may want to use a new PCIE-PCI bridge
>> everywhere (of course, except some cases when users are
>> sure they need exactly DMI-PCI bridge for some reason)
> 
> Can dmi-pci support shpc? why doesn't it? For compatibility?

I don't know why, but the fact that it doesn't is the reason libvirt
settled on auto-creating a dmi-pci bridge and a pci-pci bridge under
that for Q35. The reasoning was (IIRC Laine's words correctly) that the
dmi-pci bridge cannot receive hotplugged devices, while the pci-pci
bridge cannot be connected to the root complex. So both were needed.

Thanks
Laszlo