Re: [PATCH v11 2/4] PCI: Factor out pci_bus_wait_crs()

[Sinan Kaya <okaya@codeaurora.org>]

On 8/21/2017 4:23 PM, Bjorn Helgaas wrote:
> On Mon, Aug 21, 2017 at 03:37:06PM -0400, Sinan Kaya wrote:
>> On 8/21/2017 3:18 PM, Bjorn Helgaas wrote:
>> ...
>> if (pci_bus_crs_pending(id))
>> 	return pci_bus_wait_crs(dev->bus, dev->devfn, &id, 60000);
>>
>>> I think that makes sense.  We'd want to check for CRS SV being
>>> enabled, e.g., maybe read PCI_EXP_RTCTL_CRSSVE back in
>>> pci_enable_crs() and cache it somewhere.  Maybe a crs_sv_enabled bit
>>> in the root port's pci_dev, and check it with something like what
>>> pcie_root_rcb_set() does?
>>>
>>
>> You can observe CRS under the following conditions
>>
>> 1. root port <-> endpoint 
>> 2. bridge <-> endpoint 
>> 3. root port<->bridge
>>
>> I was relying on the fact that we are reading 0x001 as an indication that
>> this device detected CRS. Maybe, this is too indirect.
>>
>> If we also want to capture the capability, I think the right thing is to
>> check the parent capability.
>>
>> bool pci_bus_crs_vis_supported(struct pci_dev *bridge)
>> {
>> 	if (device type(bridge) == root port)
>> 		return read(root_crs_register_reg);
>>
>> 	if (device type(bridge) == switch)
>> 		return read(switch_crs_register);
> 
> I don't understand this part.  AFAIK, CRS SV is only a feature of root
> ports.  The capability and enable bits are in the Root Capabilities
> and Root Control registers.
> 

No question about it.

> It's certainly true that a device below a switch can respond with a
> CRS completion, but the switch is not the requester, and my
> understanding is that it would not take any action on the completion
> other than passing it upstream.
> 

I saw some bridge references in the spec for CRS. I was going to do
some research for it. You answered my question. I was curious how this
would impact the behavior.

"Bridge Configuration Retry Enable – When Set, this bit enables PCI Express
to PCI/PCI-X bridges to return Configuration Request Retry Status (CRS) in
response to Configuration Requests that target devices below the bridge. 
Refer to the PCI Express to PCI/PCI-X Bridge Specification, Revision 1.0 for
further details."

-- 
Sinan Kaya
Qualcomm Datacenter Technologies, Inc. as an affiliate of Qualcomm Technologies, Inc.
Qualcomm Technologies, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project.