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

[Roger Quadros <rogerq@kernel.org>]

+Vibhore,

On 16/11/2023 20:56, Théo Lebrun wrote:
> Hello Roger,
> 
> 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:
>>>> On 13/11/2023 16:26, Théo Lebrun wrote:
>>>>> Hardware initialisation is only done at probe. The J7200 USB controller
>>>>> is reset at resume because of power-domains toggling off & on. We
>>>>> therefore (1) toggle PM runtime at suspend/resume & (2) reconfigure the
>>>>> hardware at resume.
>>>>
>>>> at probe we are doing a pm_runtime_get() and never doing a put thus
>>>> preventing any runtime PM.
>>>
>>> Indeed. The get() from probe/resume are in symmetry with the put() from
>>> suspend. Is this wrong in some manner?
>>>
>>>>> index c331bcd2faeb..50b38c4b9c87 100644
>>>>> --- a/drivers/usb/cdns3/cdns3-ti.c
>>>>> +++ b/drivers/usb/cdns3/cdns3-ti.c
>>>>> @@ -197,6 +197,50 @@ static int cdns_ti_probe(struct platform_device *pdev)
>>>>>  	return error;
>>>>>  }
>>>>>  
>>>>> +#ifdef CONFIG_PM
>>>>> +
>>>>> +static int cdns_ti_suspend(struct device *dev)
>>>>> +{
>>>>> +	struct cdns_ti *data = dev_get_drvdata(dev);
>>>>> +
>>>>> +	if (!of_device_is_compatible(dev_of_node(dev), "ti,j7200-usb"))
>>>>> +		return 0;
>>>>> +
>>>>> +	pm_runtime_put_sync(data->dev);
>>>>> +
>>>>> +	return 0;
>>>>
>>>> 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.

> 
> Thinking some more about it and having read parts of the genpd source,
> it's unclear to me why there even is some PM runtime calls in this
> driver. No runtime_suspend/runtime_resume callbacks are registered.
> Also, power-domains work as expected without any PM runtime calls.

Probably it was required when the driver was introduced.

> 
> The power domain is turned on when attached to a device
> (see genpd_dev_pm_attach). It gets turned off automatically at
> suspend_noirq (taking into account the many things that make genpd
> complex: multiple devices per PD, subdomains, flags to customise the
> behavior, etc.). Removing calls to PM runtime at probe keeps the driver
> working.
> 
> So my new proposal would be: remove all all PM runtime calls from this
> driver. Anything wrong with this approach?

Nothing wrong if we don't expect runtime_pm to work with this driver.

> 
> Only possible reason I see for having PM runtime in this wrapper driver
> would be cut the full power-domain when USB isn't used, with some PM
> runtime interaction with the children node. But that cannot work
> currently as we don't register a runtime_resume to init the hardware,
> so this cannot be the current expected behavior.
> 
>> e.g.
>>
>>         pm_runtime_set_active(dev);
>>         pm_runtime_enable(dev);
>>         if (cnds_ti->can_loose_context)
>>                 pm_runtime_forbid(dev);
>>
>>         pm_runtime_set_autosuspend_delay(dev, CNDS_TI_AUTOSUSPEND_DELAY);	/* could be 20ms? */
> 
> Why mention autosuspend in this driver? This will turn the device off in
> CNDS_TI_AUTOSUSPEND_DELAY then nothing enables it back using
> pm_runtime_get. We have nothing to reconfigure the device, ie no
> runtime_resume, so we must not go into runtime suspend.

It would be enabled/disabled based on when the child "cdns3,usb"
does runtime_resume/suspend.

> 
>>         pm_runtime_mark_last_busy(dev);
>>         pm_runtime_use_autosuspend(dev);
>>
>> You will need to modify the suspend/resume handlers accordingly.
>> https://docs.kernel.org/power/runtime_pm.html#runtime-pm-and-system-sleep
>>
>> What I'm not sure of is if there are any TI platforms that retain USB context
>> on power domain off. Let me get back on this. Till then we can assume that
>> all platforms loose USB context on power domain off.
> 
> Good question indeed! Thanks for looking into it. From what I see all 5
> DT nodes which use this driver in upstream devicetrees have a
> power-domain configured. So if the behavior is the same on all three TI
> platforms (which would be the logical thing to assume) it would make
> sense that all controllers lose power at suspend.
> 
> Thanks,
> 
> --
> Théo Lebrun, Bootlin
> Embedded Linux and Kernel engineering
> https://bootlin.com

-- 
cheers,
-roger