linux-kernel - Re: [PATCH v3] PCI: Enable runtime pm of the host bridge

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Message-ID: <31f75b20-7e7b-c070-bd52-9343276a43e3@quicinc.com>
Date: Mon, 8 Jul 2024 09:58:40 +0530
From: Krishna Chaitanya Chundru <quic_krichai@...cinc.com>
To: Mayank Rana <quic_mrana@...cinc.com>, Bjorn Helgaas <bhelgaas@...gle.com>
CC: <linux-pci@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        "Rafael J.
 Wysocki" <rjw@...ysocki.net>, <quic_vbadigan@...cinc.com>,
        <quic_ramkri@...cinc.com>, <quic_nitegupt@...cinc.com>,
        <quic_skananth@...cinc.com>, <quic_parass@...cinc.com>
Subject: Re: [PATCH v3] PCI: Enable runtime pm of the host bridge



On 6/11/2024 12:04 AM, Mayank Rana wrote:
> 
> On 6/8/2024 8:14 PM, Krishna chaitanya chundru wrote:
>> The Controller driver is the parent device of the PCIe host bridge,
>> PCI-PCI bridge and PCIe endpoint as shown below.
>>
>>          PCIe controller(Top level parent & parent of host bridge)
>>                          |
>>                          v
>>          PCIe Host bridge(Parent of PCI-PCI bridge)
>>                          |
>>                          v
>>          PCI-PCI bridge(Parent of endpoint driver)
>>                          |
>>                          v
>>                  PCIe endpoint driver
>>
>> Now, when the controller device goes to runtime suspend, PM framework
>> will check the runtime PM state of the child device (host bridge) and
>> will find it to be disabled. So it will allow the parent (controller
>> device) to go to runtime suspend. Only if the child device's state was
>> 'active' it will prevent the parent to get suspended.
>>
>> Since runtime PM is disabled for host bridge, the state of the child
>> devices under the host bridge is not taken into account by PM framework
>> for the top level parent, PCIe controller. So PM framework, allows
>> the controller driver to enter runtime PM irrespective of the state
>> of the devices under the host bridge. And this causes the topology
>> breakage and also possible PM issues like controller driver goes to
>> runtime suspend while endpoint driver is doing some transfers.
>>
>> So enable runtime PM for the host bridge, so that controller driver
>> goes to suspend only when all child devices goes to runtime suspend.
>>
>> Signed-off-by: Krishna chaitanya chundru <quic_krichai@...cinc.com>
>> ---
>> Changes in v3:
>> - Moved the runtime API call's from the dwc driver to PCI framework
>>    as it is applicable for all (suggested by mani)
>> - Updated the commit message.
>> - Link to v3: 
>> https://lore.kernel.org/all/20240305-runtime_pm_enable-v2-1-a849b74091d1@quicinc.com
>> Changes in v2:
>> - Updated commit message as suggested by mani.
>> - Link to v1: 
>> https://lore.kernel.org/r/20240219-runtime_pm_enable-v1-1-d39660310504@quicinc.com
>> ---
>>
>> ---
>>   drivers/pci/probe.c | 4 ++++
>>   1 file changed, 4 insertions(+)
>>
>> diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
>> index 20475ca30505..b7f9ff75b0b3 100644
>> --- a/drivers/pci/probe.c
>> +++ b/drivers/pci/probe.c
>> @@ -3108,6 +3108,10 @@ int pci_host_probe(struct pci_host_bridge *bridge)
>>           pcie_bus_configure_settings(child);
>>       pci_bus_add_devices(bus);
>> +
>> +    pm_runtime_set_active(&bridge->dev);
>> +    pm_runtime_enable(&bridge->dev);
> Can you consider using devm_pm_runtime_enable() instead of 
> pm_runtime_enable() ?
> It serves calling pm_runtime_disable() when bridge device is removed as 
> seeing pcie driver is using pci_host_probe() after allocating bridge 
> device, and as part of pcie driver removal calls pci_remove_host_bus(), 
> and bridge device would be freed, but it doesn't call 
> pm_runtime_disable() on it. I don't see any specific functionality issue 
> here as bridge device would be freed anyway, although as we have way to 
> undo what is probe() is doing when bridge device is binding with bridge 
> device. Perhaps we can use available mechanism.
> 
> Regards,
> Mayank
Sure I will add as suggested in my next patch.

- Krishna Chaitanya
>>       return 0;
>>   }
>>   EXPORT_SYMBOL_GPL(pci_host_probe);
>>
>> ---
>> base-commit: 30417e6592bfc489a78b3fe564cfe1960e383829
>> change-id: 20240609-runtime_pm-7e6de1190113
>>
>> Best regards,