Re: [Xen-devel] [PATCH v2 00/22] Use MSI chip framework to configure MSI/MSI-X in all platforms

[Yijing Wang <wangyijing@xxxxxxxxxx>]

>> The PCI core can already deal with that. An MSI chip can be set per bus
>> and the weak pcibios_add_bus() can be used to set that. Often it might
>> not even be necessary to do it via pcibios_add_bus() if you create the
>> root bus directly, since you can attach the MSI chip at that time.
> 
> But I'm thinking that we need to move away from pcibios_add_bus() interface 
> to do
> something that should be generic. You don't need to be called for every bus 
> when all
> you want is just the root bus in order to add the MSI chip. Also, from 
> looking at
> the current patchset, a lot of architectures would set the MSI chip to a 
> global
> variable, which means you don't have an option to choose the MSI chip based 
> on the
> bus.

I also agree to remove the pcibios_add_bus() in arm which call .add_bus() to 
associate msi_chip
and PCI bus.

In my opinions, all PCI devices under the same PCI hostbridge must share same 
msi chip, right ?
So if we can associate msi chip and PCI hostbridge, every PCI device can find 
correct msi chip.
PCI hostbridge private attributes can be saved in PCI sysdata, and this data 
will be propagate to
PCI root bus and its child buses.

> 
>>
>>> What I would like to see is a way of creating the pci_host_bridge structure 
>>> outside
>>> the pci_create_root_bus(). That would then allow us to pass this sort of 
>>> platform
>>> details like associated msi_chip into the host bridge and the child busses 
>>> will
>>> have an easy way of finding the information needed by finding the root bus 
>>> and then
>>> the host bridge structure. Then the generic pci_scan_root_bus() can be used 
>>> by (mostly)
>>> everyone and the drivers can remove their kludges that try to work around 
>>> the
>>> current limitations.
>>
>> I think both issues are orthogonal. Last time I checked a lot of work
>> was still necessary to unify host bridges enough so that it could be
>> shared across architectures. But perhaps some of that work has
>> happened in the meantime.
> 
> Breaking out the host bridge creation from root bus creation is not 
> difficult, just not
> agree upon. That would be the first step in making the generic host brige 
> structure
> useful for sharing, specially if used as a sort of "parent" structure that 
> you can
> wrap with your actual host bridge structure.

Breaking out the host bridge creation is a good idea, but there need a lot of 
changes, we can
do it in another series. And if we save msi chip in pci sysdata now, it will be 
easy to
move it to generic pci host bridge. We can also move the pci domain number and 
other common info to it.

Thanks!
Yijing.

> 
>>
>> But like I said, when you create the root bus, you can easily attach the
>> MSI chip to it.
> 
> Not if you want to use the generic pci_scan_root_bus() function. One needs to 
> copy the code
> and add their own needed modifications. Which makes it hard to fix bugs and 
> prevents code
> reuse.
> 
> Best regards,
> Liviu
> 
>>
>> Thierry
> 
> 
> 


-- 
Thanks!
Yijing


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