[PATCH] PCIe bridge deferred probe breaks suspend resume

[Feng Kan]

On Sun, Feb 25, 2018 at 1:29 AM, Rafael J. Wysocki <rafael at kernel.org> wrote:
> On Sat, Feb 24, 2018 at 2:59 PM, Lukas Wunner <lukas at wunner.de> wrote:
>> On Sat, Feb 24, 2018 at 10:47:26AM +0100, Rafael J. Wysocki wrote:
>>> On Sat, Feb 24, 2018 at 10:35 AM, Rafael J. Wysocki <rafael at kernel.org> wrote:
>>> > On Sat, Feb 24, 2018 at 1:46 AM, Feng Kan <fkan at apm.com> wrote:
>>> >> This is not a patch, but rather a question regarding the deferred
>>> >> probe's effect on PCIe PM ordering. This happens on our system
>>> >> which defer the probing of root bridge due to  the IOMMU not being
>>> >> ready. Because of the deferred action, the bridge is moved to the
>>> >> end of the dpm_list which results in incorrect suspend and resume
>>> >> sequence.
>>> >>
>>> >> In the cases I have seen, the bridge is always reordered because of
>>> >> startup sequence. They are always place after the endpoint. If that
>>> >> is the case the following code should be able to prevent such cases.
>>> >> However, is there some cases here that would violate such situation?
>>> >
>>> > The code in dd.c assumes that the device being probed has no children
>>> > (or consumers, for that matter) and so it is safe to move it to the
>>> > end of the list.
>>> >
>>> > If the device has children (or consumers), moving it to the end of the
>>> > list by itself doesn't work, which is the case for you.
>>> >
>>> > You can try to replace the device_pm_move_last(dev) in
>>> > deferred_probe_work_func(struct() with device_reorder_to_tail(), but
>>> > that has to be called under device_links_read_lock/unloc () and
>>> > device_pm_lock/unloc() (in the right order).
>>>
>>> Alternatively, you can replace your !dev_is_pci(dev) check with a
>>> check for the presence of children or consumers and only move the
>>> device to the end of the PM list if there are not any.  That would
>>> kind of make sense, but it may break other assumptions in the deferred
>>> probing mechanism which I don't recall ATM.
>>>
>>> Or avoid deferred probing of the host bridge driver entirely.  I guess
>>> you can use it with limited functionality until the IOMMU driver is
>>> ready and switch over to the fully functional operation mode when that
>>> happens, but that would need to hook into the IOMMU code somehow.
>>
>> Or model the root bridge's dependency on the iommu using a device link:
>> https://www.kernel.org/doc/html/latest/driver-api/device_link.html
>
> Apparently, there are children registered under the bridge device
> before the driver for it is probed.

Yes, so the order seems to be like this:
1. root port and endporint is enumerated and added to dpm_list
2. root port probed and deferred
3. iommu probed and successful
4. root port probed again and successful -> moved to last in dpm_list
5. endpoint probed and successful

I guess another approach would be to prevent 1 from happening by not
adding to dpm_list until the probe is successful.

>
> That is kind of unusual and I'm not really sure why it happens at all,
> because (in theory) the bridge driver should be responsible for
> assigning resources to the devices on the bus, among other things.
> Depending on what the reason is, your suggestion may or may not work.