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

[Théo Lebrun]

Hello,

On Wed Nov 22, 2023 at 11:23 PM CET, Kevin Hilman wrote:
> Théo Lebrun <theo.lebrun@xxxxxxxxxxx> 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".
Are there platforms where RPM must get enabled for the attached
power-domains to be turned on?

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?

> >> Even if driver doesn't have runtime_suspend/resume hooks, wouldn't 
> >> the USB Power domain turn off if we enable runtime PM and allow runtime
> >> autosuspend and all children have runtime suspended?
> >
> > That cannot be the currently desired behavior as it would require a
> > runtime_resume implementation that restores this wrapper to its desired
> > state.
>
> Or, for this USB IP block to be in a power domain that has memory
> retention, in which case the power domain can still go off to save
> power, but not lose context.
>
> > It could however be something that could be implemented. It would be a
> > matter of enabling PM runtime and that is it in the probe. No need to
> > even init the hardware in the probe. Then the runtime_resume
> > implementation would call the new cdns_ti_init_hw.
>
> This is the way.

I agree & I have a prototype, but that requires some work on the child
devices as from what I remember they are not eager to go to runtime
suspend (ie a driver might be holding a reference).

I feel this leans outside the scope of this patch series though.

Regards,

--
Théo Lebrun, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com