Re: [RFC/RFT PATCH v2] PCI: move pci_read_bridge_bases to the generic PCI layer

[Guenter Roeck <linux@roeck-us.net>]

On 06/13/2015 02:12 AM, Lorenzo Pieralisi wrote:
> On Sat, Jun 13, 2015 at 02:47:55AM +0100, Guenter Roeck wrote:
>> On Tue, Jun 09, 2015 at 10:01:45AM +0100, Lorenzo Pieralisi wrote:
>>> When a PCI bus is scanned, upon PCI bridge detection the kernel
>>> has to read the bridge registers to set-up its resources so that
>>> the PCI resource hierarchy can be validated properly.
>>>
>>> Most if not all architectures read PCI bridge registers in the
>>> pcibios_fixup_bus hook, that is called by the PCI generic layer
>>> whenever a PCI bus is scanned.
>>>
>>> Since pci_read_bridge_bases is an arch agnostic operation (and it
>>> is carried out on all architectures) it can be moved to the generic
>>> PCI layer in order to consolidate code and remove the respective
>>> calls from the architectures back-ends.
>>>
>>> The PCI_PROBE_ONLY flag is not checked before calling
>>> pci_read_bridge_buses in the generic layer since reading the bridge
>>> bases is not related to resources assignment; this implies that it
>>> can be carried out safely on PCI_PROBE_ONLY systems too and should
>>> not affect architectures (alpha, mips) that check the PCI_PROBE_ONLY
>>> flag before reading the bridge bases.
>>>
>>> In order to validate the resource hierarchy as soon as the resources
>>> themselves are probed (ie read from the bridge), this patch also adds
>>> code to pci_read_bridge_bases that claims the bridge resources, so that
>>> they are validated and inserted in the resource hierarchy as soon as
>>> the bridge bases are probed.
>>>
>>
>> Hi Lorenzo,
>>
>> on one of our systems, I see a lot of messages with your patch applied.
>>
>> bart kernel: pci 0000:b0:00.0: can't claim BAR 7 [io 0x0000-0x0fff]: no compatible bridge window
>> bart kernel: pci 0000:b0:00.0: can't claim BAR 8 [mem 0x94000000-0x941fffff]: no compatible bridge window
>> bart kernel: pci 0000:b1:03.0: can't claim BAR 7 [io 0x0000-0x0fff]: no compatible bridge window
>> bart kernel: pci 0000:b1:03.0: can't claim BAR 8 [mem 0x95800000-0x959fffff]: no compatible bridge window
>>
>> and so on. The final IO memory assignment is the same, though,
>> before and after your patch.
>>
>>        95800000-95bfffff : PCI Bus 0000:b0
>>          95800000-959fffff : PCI Bus 0000:b1
>> 	  95800000-959fffff : PCI Bus 0000:b2
>> 	95a00000-95a3ffff : 0000:b0:00.0
>>
>>
>> Does that have any relevance or is it just nuisance messages ?
>
> Yes, I knew this could happen. It should be just nuisance messages,
> since we are claming bridge resources even on systems where they
> are reassigned. We should remove those messages, this means that I
> have to craft a function that claims resources without spitting too
> much unwanted noise, I can't use pci_claim_bridge_resource for this
> purpose as you have noticed, unless I refactor it, open to suggestions
> (we claim bridge resources by default, regardless of PROBE_ONLY flag).
>
> Thanks a lot for testing it, appreciated, I will prepare a v3.
>

Hi Lorenzo,

There is no need for v3 because of this. My patch set addresses the BAR 7
message, and the BAR 8 message is actually warranted since the memory
window on the upstream port is too small.

Guenter