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From:   Ulf Hansson <ulf.hansson@linaro.org>
Date:   Wed, 23 May 2018 08:12:27 +0200
Message-ID: <CAPDyKFo_69kSS6UzdsJeyeiiED=S=chsdkP+R43qMHDEbZ8n_A@mail.gmail.com>
Subject: Re: [PATCH 8/9] PM / Domains: Add support for multi PM domains per
 device to genpd
To:     Rajendra Nayak <rnayak@codeaurora.org>,
        Jon Hunter <jonathanh@nvidia.com>
Cc:     Geert Uytterhoeven <geert+renesas@glider.be>,
        Linux PM <linux-pm@vger.kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Kevin Hilman <khilman@kernel.org>,
        "Rafael J . Wysocki" <rjw@rjwysocki.net>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Todor Tomov <todor.tomov@linaro.org>,
        Viresh Kumar <viresh.kumar@linaro.org>,
        linux-tegra@vger.kernel.org,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>
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Rajendra, Jon,

On 23 May 2018 at 06:51, Rajendra Nayak <rnayak@codeaurora.org> wrote:
>
>
> On 05/23/2018 02:25 AM, Jon Hunter wrote:
>>
>> On 22/05/18 15:47, Ulf Hansson wrote:
>>> [...]
>>>
>>>>>
>>>>> +/**
>>>>> + * genpd_dev_pm_attach_by_id() - Attach a device to one of its PM domain.
>>>>> + * @dev: Device to attach.
>>>>> + * @index: The index of the PM domain.
>>>>> + *
>>>>> + * Parse device's OF node to find a PM domain specifier at the provided @index.
>>>>> + * If such is found, allocates a new device and attaches it to retrieved
>>>>> + * pm_domain ops.
>>>>> + *
>>>>> + * Returns the allocated device if successfully attached PM domain, NULL when
>>>>> + * the device don't need a PM domain or have a single PM domain, else PTR_ERR()
>>>>> + * in case of failures. Note that if a power-domain exists for the device, but
>>>>> + * cannot be found or turned on, then return PTR_ERR(-EPROBE_DEFER) to ensure
>>>>> + * that the device is not probed and to re-try again later.
>>>>> + */
>>>>> +struct device *genpd_dev_pm_attach_by_id(struct device *dev,
>>>>> +                                      unsigned int index)
>>>>> +{
>>>>> +     struct device *genpd_dev;
>>>>> +     int num_domains;
>>>>> +     int ret;
>>>>> +
>>>>> +     if (!dev->of_node)
>>>>> +             return NULL;
>>>>> +
>>>>> +     /* Deal only with devices using multiple PM domains. */
>>>>> +     num_domains = of_count_phandle_with_args(dev->of_node, "power-domains",
>>>>> +                                              "#power-domain-cells");
>>>>> +     if (num_domains < 2 || index >= num_domains)
>>>>> +             return NULL;
>>>>> +
>>>>> +     /* Allocate and register device on the genpd bus. */
>>>>> +     genpd_dev = kzalloc(sizeof(*genpd_dev), GFP_KERNEL);
>>>>> +     if (!genpd_dev)
>>>>> +             return ERR_PTR(-ENOMEM);
>>>>> +
>>>>> +     dev_set_name(genpd_dev, "genpd:%u:%s", index, dev_name(dev));
>>>>> +     genpd_dev->bus = &genpd_bus_type;
>>>>> +     genpd_dev->release = genpd_release_dev;
>>>>> +
>>>>> +     ret = device_register(genpd_dev);
>>>>> +     if (ret) {
>>>>> +             kfree(genpd_dev);
>>>>> +             return ERR_PTR(ret);
>>>>> +     }
>>>>> +
>>>>> +     /* Try to attach the device to the PM domain at the specified index. */
>>>>> +     ret = __genpd_dev_pm_attach(genpd_dev, dev->of_node, index);
>>>>> +     if (ret < 1) {
>>>>> +             device_unregister(genpd_dev);
>>>>> +             return ret ? ERR_PTR(ret) : NULL;
>>>>> +     }
>>>>> +
>>>>> +     pm_runtime_set_active(genpd_dev);
>>>>> +     pm_runtime_enable(genpd_dev);
>>>>> +
>>>>> +     return genpd_dev;
>>>>> +}
>>>>> +EXPORT_SYMBOL_GPL(genpd_dev_pm_attach_by_id);
>>>>
>>>> Thanks for sending this. Believe it or not this has still been on my to-do list
>>>> and so we definitely need a solution for Tegra.
>>>>
>>>> Looking at the above it appears that additional power-domains exposed as devices
>>>> to the client device. So I assume that this means that the drivers for devices
>>>> with multiple power-domains will need to call RPM APIs for each of these
>>>> additional power-domains. Is that correct?
>>>
>>> They can, but should not!
>>>
>>> Instead, the driver shall use device_link_add() and device_link_del(),
>>> dynamically, depending on what PM domain that their original device
>>> needs for the current running use case.
>>>
>>> In that way, they keep existing runtime PM deployment, operating on
>>> its original device.
>>
>> OK, sounds good. Any reason why the linking cannot be handled by the above API? Is there a use-case where you would not want it linked?
>
> I am guessing the linking is what would give the driver the ability to decide which subset of powerdomains it actually wants to control
> at any point using runtime PM. If we have cases wherein the driver would want to turn on/off _all_ its associated powerdomains _always_
> then a default linking of all would help.

First, I think we need to decide on *where* the linking should be
done, not at both places, as that would just mess up synchronization
of who is responsible for calling the device_link_del() at detach.

Second, It would in principle be fine to call device_link_add() and
device_link_del() as a part of the attach/detach APIs. However, there
is a downside to such solution, which would be that the driver then
needs call the detach API, just to do device_link_del(). Of course
then it would also needs to call the attach API later if/when needed.
Doing this adds unnecessary overhead - comparing to just let the
driver call device_link_add|del() when needed. On the upside, yes, it
would put less burden on the drivers as it then only needs to care
about using one set of functions.

Which solution do you prefer?

Kind regards
Uffe