linux-kernel - Re: [PATCH v2] PCI: dwc: Enable runtime pm of the host bridge

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Date: Fri, 12 Apr 2024 16:02:48 +0530
From: Krishna Chaitanya Chundru <quic_krichai@...cinc.com>
To: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>,
        Bjorn Helgaas
	<helgaas@...nel.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>
CC: Jingoo Han <jingoohan1@...il.com>,
        Gustavo Pimentel
	<gustavo.pimentel@...opsys.com>,
        Lorenzo Pieralisi <lpieralisi@...nel.org>,
        Krzysztof Wilczyński <kw@...ux.com>,
        Rob Herring
	<robh@...nel.org>, Bjorn Helgaas <bhelgaas@...gle.com>,
        <linux-pci@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <quic_vbadigan@...cinc.com>, <quic_ramkri@...cinc.com>,
        <quic_nitegupt@...cinc.com>, <quic_skananth@...cinc.com>,
        <quic_parass@...cinc.com>
Subject: Re: [PATCH v2] PCI: dwc: Enable runtime pm of the host bridge



On 3/25/2024 4:39 PM, Manivannan Sadhasivam wrote:
> On Fri, Mar 22, 2024 at 05:04:56PM -0500, Bjorn Helgaas wrote:
>> On Tue, Mar 19, 2024 at 04:41:48PM +0530, Manivannan Sadhasivam wrote:
>>> On Fri, Mar 08, 2024 at 11:12:48AM -0600, Bjorn Helgaas wrote:
>>>> On Fri, Mar 08, 2024 at 08:38:52AM +0530, Krishna Chaitanya Chundru wrote:
>>>>> On 3/8/2024 3:25 AM, Bjorn Helgaas wrote:
>>>>>> [+to Rafael, sorry, another runtime PM question, beginning of thread:
>>>>>> https://lore.kernel.org/r/20240305-runtime_pm_enable-v2-1-a849b74091d1@quicinc.com]
>>>>>>
>>>>>> On Thu, Mar 07, 2024 at 07:28:54AM +0530, Krishna Chaitanya Chundru wrote:
>>>>>>> On 3/6/2024 1:27 AM, Bjorn Helgaas wrote:
>>>>>>>> On Tue, Mar 05, 2024 at 03:19:01PM +0530, 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
>>>>>>>>>
>>>>>>>>> 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.
>>>>>>>>
>>>>>>>> IIUC this says that we runtime suspend the controller even though
>>>>>>>> runtime PM is disabled for the host bridge?  I have a hard time
>>>>>>>> parsing this; can you cite a function that does this or some relevant
>>>>>>>> documentation about how this part of runtime PM works?
>>>>>>>>
>>>>>>> Generally controller should go to runtime suspend when endpoint client
>>>>>>> drivers and pci-pci host bridge drivers goes to runtime suspend as the
>>>>>>> controller driver is the parent, but we are observing controller driver
>>>>>>> goes to runtime suspend even when client drivers and PCI-PCI bridge are
>>>>>>> in active state.
>>>>>>
>>>>>> It surprises me that a device could be suspended while children are
>>>>>> active.  A PCI-PCI bridge must be in D0 for any devices below it to be
>>>>>> active.  The controller is a platform device, not a PCI device, but I
>>>>>> am similarly surprised that we would suspend it when children are
>>>>>> active, which makes me think we didn't set the hierarchy up correctly.
>>>>>>
>>>>>> It doesn't seem like we should need to enable runtime PM for a parent
>>>>>> to keep it from being suspended when children are active.
>>>>>
>>>>> Here we are not enabling runtime PM of the controller device, we are
>>>>> enabling runtime PM for the bridge device which is maintained by the
>>>>> PCIe framework. The bridge device is the parent of the PCI-PCI
>>>>> bridge and child of the controller device. As the bridge device's
>>>>> runtime PM is not enabled the PM framework is ignoring the child's
>>>>> runtime status.
>>>>
>>>> OK, it's the host bridge, not the controller.
>>>>
>>>> I'm still surprised that the PM framework will runtime suspend a
>>>> device when child devices are active.
>>>
>>> There is a catch here. Even though the child devices are funtionally
>>> active, PM framework will only consider their runtime_pm state,
>>> which is initially set to 'disabled' for all devices. It is upto the
>>> device drivers to enable it when required.
>>>
>>> Here is the initial runtime PM status of each device post boot:
>>>
>>> Controller device -> disabled initially but enabled by pcie-qcom.c
>>> Host bridge -> disabled initially
>>> PCIe bridge -> disabled initially but conditionally enabled by portdrv.c
>>> PCIe devices -> disabled initially but enabled by respective drivers like WLAN
>>>
>>> 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.
>>>
>>> But you may wonder if this is ideal? IMO NO. But we cannot blame the
>>> PM framework here. The responsibility is within the device drivers
>>> to handle the PM state based on the usecase. Ideally, the host
>>> bridge driver should've handled runtime PM state during the probe
>>> time. Otherwise, PM framework wouldn't know when would be the best
>>> time to suspend the devices.
>>
>> My expectation is that adding new functionality should only require
>> changes in drivers that want to take advantage of it.  For example, if
>> we add runtime PM support in the controller driver, the result should
>> be functionally correct even if we don't update drivers for downstream
>> devices.
>>
> 
> Well, IMO PM framework should disable runtime PM for the parent if the child's
> runtime PM state is disabled.
> 
> It'd be good to get the opinion of Rafael.
>
Hi Rafeal,

can you please comment on this.

> - Mani
> 
>> If that's not the way it works, I suggest that would be a problem in
>> the PM framework.
>>
>> The host bridge might be a special case because we don't have a
>> separate "host bridge" driver; that code is kind of integrated with
>> the controller drivers.  So maybe it's OK to do controller + host
>> bridge runtime PM support at the same time, as long as any time we add
>> runtime PM to a controller, we sure it's also set up for the host
>> bridge.
>>
>> Bjorn
>