linux-kernel - Re: [PATCH 3/6] usb: cdns3-ti: add suspend/resume procedures for J7200

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Message-ID: <7hil5odtwl.fsf@baylibre.com>
Date:   Sun, 26 Nov 2023 14:36:10 -0800
From:   Kevin Hilman <khilman@...nel.org>
To:     Théo Lebrun <theo.lebrun@...tlin.com>,
        Roger Quadros <rogerq@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Conor Dooley <conor+dt@...nel.org>,
        Peter Chen <peter.chen@...nel.org>,
        Pawel Laszczak <pawell@...ence.com>,
        Nishanth Menon <nm@...com>,
        Vignesh Raghavendra <vigneshr@...com>,
        Tero Kristo <kristo@...nel.org>,
        "Vardhan, Vibhore" <vibhore@...com>
Cc:     linux-usb@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        Grégory Clement <gregory.clement@...tlin.com>,
        Thomas Petazzoni <thomas.petazzoni@...tlin.com>
Subject: Re: [PATCH 3/6] usb: cdns3-ti: add suspend/resume procedures for J7200

Théo Lebrun <theo.lebrun@...tlin.com> writes:

> Hello,
>
> On Wed Nov 22, 2023 at 11:23 PM CET, Kevin Hilman wrote:
>> Théo Lebrun <theo.lebrun@...tlin.com> writes:
>> > On Fri Nov 17, 2023 at 12:51 PM CET, Roger Quadros wrote:
>> >> On 17/11/2023 12:17, Théo Lebrun wrote:
>> >> > On Thu Nov 16, 2023 at 10:44 PM CET, Roger Quadros wrote:
>> >> >> On 16/11/2023 20:56, Théo Lebrun wrote:
>> >> >>> On Thu Nov 16, 2023 at 1:40 PM CET, Roger Quadros wrote:
>> >> >>>> On 15/11/2023 17:02, Théo Lebrun wrote:
>> >> >>>>> On Wed Nov 15, 2023 at 12:37 PM CET, Roger Quadros wrote:
>> >> >>>>>> You might want to check suspend/resume ops in cdns3-plat and
>> >> >>>>>> do something similar here.
>> >> >>>>>
>> >> >>>>> I'm unsure what you are referring to specifically in cdns3-plat?
>> >> >>>>
>> >> >>>> What I meant is, calling pm_runtime_get/put() from system suspend/resume
>> >> >>>> hooks doesn't seem right.
>> >> >>>>
>> >> >>>> How about using something like pm_runtime_forbid(dev) on devices which
>> >> >>>> loose USB context on runtime suspend e.g. J7200.
>> >> >>>> So at probe we can get rid of the pm_runtime_get_sync() call.
>> >> >>>
>> >> >>> What is the goal of enabling PM runtime to then block (ie forbid) it in
>> >> >>> its enabled state until system suspend?
>> >> >>
>> >> >> If USB controller retains context on runtime_suspend on some platforms
>> >> >> then we don't want to forbid PM runtime.
>> >> > 
>> >> > What's the point of runtime PM if nothing is done based on it? This is
>> >> > the current behavior of the driver.
>>
>> The point is to signal to the power domain the device is in that it can
>> power on/off.  These IP blocks are (re)used on many different SoCs, so
>> the driver should not make any assumptions about what power domain it is
>> in (if any.)
>
> On my platform, when the device is attached to the PD it gets turned on.
> That feels logical to me: if a driver is not RPM aware it "just works".

It "just works"... until the domain gets turned off.

> Are there platforms where RPM must get enabled for the attached
> power-domains to be turned on?

Yes, but but more importantly, there are platforms where RPM must get
enabled for the power domain to *stay* on.  For example, the power
domain might get turned on due to devices probing etc, but as soon as
all the RPM-enabled drivers drop their refcount, the domain will turn
off.  If there is a device in that domain with a non-RPM enabled driver,
that device will be powered off anc cause a crash.

> The call chain that attaches & enables PD is platform_probe ->
> dev_pm_domain_attach. That function takes a bool power_on which is
> always true. In the genpd case, genpd_dev_pm_attach
> calls __genpd_dev_pm_attach which does a genpd_power_on.
>
> Things I've not accounted for:
>
>  - ACPI looks like it does the same but I've not checked. It gets passed
>    the power_on bool argument.
>
>  - genpd_dev_pm_attach early returns with no error if there are multiple
>    PM domains attached to the device. There are many platforms in the
>    case according to some devicetree grepping. I can imagine a behavior
>    difference where dev_pm_domain callpaths handle that differently in
>    the RPM case. Is that what we are discussing?

You're only looking at the attach, power-on phase.  You need to think
about when the domain powers off and powers back on.

Kevin