Date:   Fri, 25 May 2018 09:31:32 +0100
From:   Jon Hunter <jonathanh@...dia.com>
To:     Ulf Hansson <ulf.hansson@...aro.org>
CC:     "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Linux PM <linux-pm@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Todor Tomov <todor.tomov@...aro.org>,
        Rajendra Nayak <rnayak@...eaurora.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Kevin Hilman <khilman@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        <linux-tegra@...r.kernel.org>
Subject: Re: [PATCH 9/9] PM / Domains: Add dev_pm_domain_attach_by_id() to
 manage multi PM domains


On 24/05/18 22:11, Ulf Hansson wrote:
> On 24 May 2018 at 17:48, Jon Hunter <jonathanh@...dia.com> wrote:
>>
>> On 18/05/18 11:31, Ulf Hansson wrote:
>>>
>>> The existing dev_pm_domain_attach() function, allows a single PM domain to
>>> be attached per device. To be able to support devices that are partitioned
>>> across multiple PM domains, let's introduce a new interface,
>>> dev_pm_domain_attach_by_id().
>>>
>>> The dev_pm_domain_attach_by_id() returns a new allocated struct device
>>> with
>>> the corresponding attached PM domain. This enables for example a driver to
>>> operate on the new device from a power management point of view. The
>>> driver
>>> may then also benefit from using the received device, to set up so called
>>> device-links towards its original device. Depending on the situation,
>>> these
>>> links may then be dynamically changed.
>>>
>>> The new interface is typically called by drivers during their probe phase,
>>> in case they manages devices which uses multiple PM domains. If that is
>>> the
>>> case, the driver also becomes responsible of managing the detaching of the
>>> PM domains, which typically should be done at the remove phase. Detaching
>>> is done by calling the existing dev_pm_domain_detach() function and for
>>> each of the received devices from dev_pm_domain_attach_by_id().
>>>
>>> Note, currently its only genpd that supports multiple PM domains per
>>> device, but dev_pm_domain_attach_by_id() can easily by extended to cover
>>> other PM domain types, if/when needed.
>>>
>>> Signed-off-by: Ulf Hansson <ulf.hansson@...aro.org>
>>> ---
>>>    drivers/base/power/common.c | 33 ++++++++++++++++++++++++++++++++-
>>>    include/linux/pm_domain.h   |  7 +++++++
>>>    2 files changed, 39 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/base/power/common.c b/drivers/base/power/common.c
>>> index 7ae62b6..d3db974 100644
>>> --- a/drivers/base/power/common.c
>>> +++ b/drivers/base/power/common.c
>>> @@ -117,13 +117,44 @@ int dev_pm_domain_attach(struct device *dev, bool
>>> power_on)
>>>    EXPORT_SYMBOL_GPL(dev_pm_domain_attach);
>>>      /**
>>> + * dev_pm_domain_attach_by_id - Attach a device to one of its PM domains.
>>
>>
>> Isn't this more of a 'get'?
> 
> I don't think so, at least according to the common understanding of
> how we use get and put APIs.
> 
> For example, clk_get() returns a cookie to a clk, which you then can
> do a hole bunch of different clk specific operations on.
> 
> This is different, there are no specific PM domain operations the
> caller can or should do. Instead the idea is to keep drivers more or
> less transparent, still using runtime PM as before. The only care the
> caller need to take is to use device links, which BTW isn't a PM
> domain specific thing.

Yes, but a user can still call pm_runtime_get/put with the device 
returned if they wish to, right?

>>
>>> + * @index: The index of the PM domain.
>>> + * @dev: Device to attach.
>>
>>
>> Isn't this just the device associated with the PM domain we are getting?
> 
> Correct, but please note, as stated above, I don't think it's a good
> idea to return a special PM domain cookie, because we don't want the
> caller to run special PM domain operations.
> 
>>
>>> + *
>>> + * As @dev may only be attached to a single PM domain, the backend PM
>>> domain
>>> + * provider should create a virtual device to attach instead. As
>>> attachment
>>> + * succeeds, the ->detach() callback in the struct dev_pm_domain should
>>> be
>>> + * assigned by the corresponding backend attach function.
>>> + *
>>> + * This function should typically be invoked from drivers during probe
>>> phase.
>>> + * Especially for those that manages devices which requires power
>>> management
>>> + * through more than one PM domain.
>>> + *
>>> + * Callers must ensure proper synchronization of this function with power
>>> + * management callbacks.
>>> + *
>>> + * Returns the virtual attached device in case successfully attached PM
>>> domain,
>>> + * NULL in case @dev don't need a PM domain, else a PTR_ERR().
>>
>>
>> Should this be 'NULL in the case where the @dev already has a power-domain'?
> 
> Yes, I think so. The intent is to be consistent with how
> dev_pm_domain_attach() works and according to the latest changes I did
> for it. It's queued for 4.18, please have a look in Rafael's tree and
> you will see.

Ah, I see your change now.

>>
>>> + */
>>> +struct device *dev_pm_domain_attach_by_id(struct device *dev,
>>> +                                         unsigned int index)
>>> +{
>>> +       if (dev->pm_domain)
>>
>>
>> I wonder if this is worthy of a ...
>>
>>          if (WARN_ON(dev->pm_domain))
>>
>>> +               return NULL;
>>
>>
>> Don't we consider this an error case? I wonder why not return PTR_ERR here
>> as well? This would be consistent with dev_pm_domain_attach().
> 
> Please see above comment.

Right, but this case still seems like an error. My understanding is that 
only drivers will use this API directly and it will not be used by the 
device driver core (unlike dev_pm_domain_attach), so if anyone calls 
this attempting to attach another PM domain when one is already 
attached, they are doing something wrong.

Cheers
Jon

-- 
nvpublic

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