linux-kernel - Re: [RESEND PATCH v5] PCI: Make sure the bus bridge powered on when scanning bus

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Date:   Wed, 25 May 2022 23:53:15 +0800
From:   Yicong Yang <yangyccccc@...il.com>
To:     Bjorn Helgaas <helgaas@...nel.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     Yicong Yang <yangyicong@...ilicon.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Linux PCI <linux-pci@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>
Subject: Re: [RESEND PATCH v5] PCI: Make sure the bus bridge powered on when
 scanning bus

在 2022/5/25 22:19, Bjorn Helgaas 写道:
> On Wed, May 25, 2022 at 01:27:21PM +0200, Rafael J. Wysocki wrote:
>> On Tue, May 24, 2022 at 10:58 PM Bjorn Helgaas <helgaas@...nel.org> wrote:
>>> On Tue, May 17, 2022 at 08:43:19PM +0800, Yicong Yang wrote:
>>>> When the bus bridge is runtime suspended, we'll fail to rescan
>>>> the devices through sysfs as we cannot access the configuration
>>>> space correctly when the bridge is in D3hot.
>>> Is the "D3hot" above a typo?  I think devices are supposed to respond
>>> to config accesses when in D3hot.  PCIe r6.0, sec 5.3.1.4.1:
>>>
>>>   Configuration and Message requests are the only TLPs accepted by a
>>>   Function in the D3Hot state. ...
>>>
>>>   Functions that are in D3Hot are permitted to be transitioned by
>>>   software (writing to their PMCSR PowerState field) to the D0active
>>>   state or the D0uninitialized state. Functions that are in D3Hot must
>>>   respond to Configuration Space accesses as long as power and clock
>>>   are supplied so that they can be returned to D0 by software.
>> That applies to the device itself, though, and not to the bus under it.
>>
>> In general, a bridge in D3hot causes the bus segment on the other side
>> of it to be inaccessible even for config space accesses AFAICS.
> Oh, thank you!  That was the piece I was missing.  I'll tweak the
> commit log to say something like:
>
>   A bridge in a non-D0 power state does not forward config accesses to
>   its secondary side (PCIe r6.0, sec 5.3.1).  Make sure the bridge is
>   in D0 while we enumerate devices below it.

Thanks for the illustration and tweaking of this. I should have qualified
the "devices" in the commit with downstream or secondary to make it
less ambiguous.

>>>> It can be reproduced like:
>>>>
>>>> $ echo 1 > /sys/bus/pci/devices/0000:80:00.0/0000:81:00.1/remove
>>>> $ echo 1 > /sys/bus/pci/devices/0000:80:00.0/pci_bus/0000:81/rescan
>>>>
>>>> 0000:80:00.0 is a Root Port and it is runtime-suspended, so
>>>> 0000:81:00.1 is unreachable after a rescan.
>>>>
>>>> Power up the bridge when scanning the child bus and allow it to
>>>> suspend again by adding pm_runtime_get_sync()/pm_runtime_put()
>>>> in pci_scan_child_bus_extend().
>>>>
>>>> Cc: Rafael J. Wysocki <rafael@...nel.org>
>>>> Cc: Mika Westerberg <mika.westerberg@...ux.intel.com>
>>>> Cc: Bjorn Helgaas <bhelgaas@...gle.com>
>>>> Signed-off-by: Yicong Yang <yangyicong@...ilicon.com>
>>>> Reviewed-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
>>>> ---
>>>>  drivers/pci/probe.c | 12 ++++++++++++
>>>>  1 file changed, 12 insertions(+)
>>>>
>>>> diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
>>>> index 17a969942d37..b108e72b6586 100644
>>>> --- a/drivers/pci/probe.c
>>>> +++ b/drivers/pci/probe.c
>>>> @@ -2859,11 +2859,20 @@ static unsigned int pci_scan_child_bus_extend(struct pci_bus *bus,
>>>>       unsigned int used_buses, normal_bridges = 0, hotplug_bridges = 0;
>>>>       unsigned int start = bus->busn_res.start;
>>>>       unsigned int devfn, fn, cmax, max = start;
>>>> +     struct pci_dev *bridge = bus->self;
>>>>       struct pci_dev *dev;
>>>>       int nr_devs;
>>>>
>>>>       dev_dbg(&bus->dev, "scanning bus\n");
>>>>
>>>> +     /*
>>>> +      * Make sure the bus bridge is powered on, otherwise we may not be
>>>> +      * able to scan the devices as we may fail to access the configuration
>>>> +      * space of subordinates.
>>>> +      */
>>>> +     if (bridge)
>>>> +             pm_runtime_get_sync(&bridge->dev);
>>>> +
>>>>       /* Go find them, Rover! */
>>>>       for (devfn = 0; devfn < 256; devfn += 8) {
>>>>               nr_devs = pci_scan_slot(bus, devfn);
>>>> @@ -2976,6 +2985,9 @@ static unsigned int pci_scan_child_bus_extend(struct pci_bus *bus,
>>>>               }
>>>>       }
>>>>
>>>> +     if (bridge)
>>>> +             pm_runtime_put(&bridge->dev);
>>>> +
>>>>       /*
>>>>        * We've scanned the bus and so we know all about what's on
>>>>        * the other side of any bridges that may be on this bus plus
>>>> --
>>>> 2.24.0
>>>>