Re: [PATCH V7 1/2] PM / Domains: Add support to select performance-state of domains

[Ulf Hansson]

On 17 May 2017 at 09:02, Viresh Kumar <viresh.kumar@xxxxxxxxxx> wrote:
> Some platforms have the capability to configure the performance state of
> their Power Domains. The performance levels are identified by positive
> integer values, a lower value represents lower performance state.
>
> This patch adds a new genpd API: pm_genpd_update_performance_state().
> The caller passes the affected device and the frequency representing its
> next DVFS state.
>
> The power domains get two new callbacks:
>
> - get_performance_state(): This is called by the genpd core to retrieve
>   the performance state (integer value) corresponding to a target
>   frequency for the device. This state is used by the genpd core to find
>   the highest requested state by all the devices powered by a domain.
>
> - set_performance_state(): The highest state retrieved from above
>   interface is then passed to this callback to finally program the
>   performance state of the power domain.
>
> The power domains can avoid supplying these callbacks, if they don't
> support setting performance-states.
>
> A power domain may have only get_performance_state() callback, if it
> doesn't have the capability of changing the performance state itself but
> someone in its parent hierarchy has.
>
> A power domain may have only set_performance_state(), it doesn't have
> any direct devices below it and only subdomains. And so the
> get_performance_state() will never get called.
>
> The more common case would be to have both the callbacks set.
>
> Another API, pm_genpd_has_performance_state(), is also added to let
> other parts of the kernel check if the power domain of a device supports
> performance-states or not.
>
> Note that the same performance level as returned by the parent domain of
> a device is used for every other domain in parent hierarchy.
>
> Signed-off-by: Viresh Kumar <viresh.kumar@xxxxxxxxxx>

Apologize for the delay! I was kind of waiting for Kevin to jump in as
he has already been in the loop.

Anyway, let me give you my thoughts about his. Starting with some
comment, then an overall thought about this.

> ---
>  drivers/base/power/domain.c | 172 ++++++++++++++++++++++++++++++++++++++++++++
>  include/linux/pm_domain.h   |  19 +++++
>  2 files changed, 191 insertions(+)
>
> diff --git a/drivers/base/power/domain.c b/drivers/base/power/domain.c
> index 71c95ad808d5..56b666eb1a71 100644
> --- a/drivers/base/power/domain.c
> +++ b/drivers/base/power/domain.c
> @@ -466,6 +466,166 @@ static int genpd_dev_pm_qos_notifier(struct notifier_block *nb,
>         return NOTIFY_DONE;
>  }
>
> +/*
> + * Returns true if anyone in genpd's parent hierarchy has
> + * set_performance_state() set.
> + */
> +static bool genpd_has_set_performance_state(struct generic_pm_domain *genpd)
> +{
> +       struct gpd_link *link;
> +
> +       if (genpd->set_performance_state)
> +               return true;
> +
> +       list_for_each_entry(link, &genpd->slave_links, slave_node) {
> +               if (genpd_has_set_performance_state(link->master))
> +                       return true;
> +       }
> +
> +       return false;
> +}
> +
> +/**
> + * pm_genpd_has_performance_state - Checks if power domain does performance
> + * state management.
> + *
> + * @dev: Device whose power domain is getting inquired.
> + */
> +bool pm_genpd_has_performance_state(struct device *dev)
> +{
> +       struct generic_pm_domain *genpd = dev_to_genpd(dev);
> +
> +       /* The parent domain must have set get_performance_state() */
> +       if (!IS_ERR(genpd) && genpd->get_performance_state)
> +               return true;
> +
> +       return false;
> +}
> +EXPORT_SYMBOL_GPL(pm_genpd_has_performance_state);

There is no protection from locking point view and there is no
validation the pm_domain pointer being a valid genpd.

Please study the locks that is used in genpd more carefully and try to
figure out what is needed here. I know it's a bit tricky, but the
above just isn't sufficient.

> +
> +static int genpd_update_performance_state(struct generic_pm_domain *genpd,
> +                                         int depth)
> +{
> +       struct generic_pm_domain_data *pd_data;
> +       struct generic_pm_domain *subdomain;
> +       struct pm_domain_data *pdd;
> +       unsigned int state = 0, prev;
> +       struct gpd_link *link;
> +       int ret;
> +
> +       /* Traverse all devices within the domain */
> +       list_for_each_entry(pdd, &genpd->dev_list, list_node) {
> +               pd_data = to_gpd_data(pdd);
> +
> +               if (pd_data->performance_state > state)
> +                       state = pd_data->performance_state;
> +       }
> +
> +       /* Traverse all subdomains within the domain */
> +       list_for_each_entry(link, &genpd->master_links, master_node) {
> +               subdomain = link->slave;
> +
> +               if (subdomain->performance_state > state)
> +                       state = subdomain->performance_state;

You need to take the genpd lock of the subdomain before assigning this, right?

Is there a way to do this the opposite direction? It's important that
we walk upwards in the topology, as we need to preserve the order for
how genpd locks are taken.

The way we do it is always taking the lock of the subdomain first,
then its master, then the master's master and so own. You can have
look at for example genpd_power_on().

> +       }
> +
> +       if (genpd->performance_state == state)
> +               return 0;
> +
> +       /*
> +        * Not all domains support updating performance state. Move on to their
> +        * parent domains in that case.
> +        */
> +       if (genpd->set_performance_state) {
> +               ret = genpd->set_performance_state(genpd, state);
> +               if (!ret)
> +                       genpd->performance_state = state;
> +
> +               return ret;
> +       }
> +
> +       prev = genpd->performance_state;
> +       genpd->performance_state = state;
> +
> +       list_for_each_entry(link, &genpd->slave_links, slave_node) {
> +               struct generic_pm_domain *master = link->master;
> +
> +               genpd_lock_nested(master, depth + 1);
> +               ret = genpd_update_performance_state(master, depth + 1);
> +               genpd_unlock(master);
> +
> +               if (!ret)
> +                       continue;
> +
> +               /*
> +                * Preserve earlier state for all the power domains which have
> +                * been updated.
> +                */
> +               genpd->performance_state = prev;
> +
> +               list_for_each_entry(link, &genpd->slave_links, slave_node) {
> +                       struct generic_pm_domain *nmaster = link->master;
> +
> +                       if (nmaster == master)
> +                               break;
> +
> +                       genpd_lock_nested(nmaster, depth + 1);
> +                       genpd_update_performance_state(master, depth + 1);
> +                       genpd_unlock(master);
> +               }

I think some comment on how to walk the genpd topology to fetch the
current selected state would be nice. I am not sure I get it by just
looking at the code.

First you are walking master links then slave links. Then you start
over again with the slave links in the error path.

Normally in the error path we use
list_for_each_entry_continue_reverse() to restore data, so this looks
a bit weird to me. Are you sure the error path is correct?

> +
> +               return ret;
> +       }
> +
> +       return 0;
> +}
> +
> +/**
> + * pm_genpd_update_performance_state - Update performance state of parent power
> + * domain for the target frequency for the device.
> + *
> + * @dev: Device for which the performance-state needs to be adjusted.
> + * @rate: Device's next frequency.
> + */
> +int pm_genpd_update_performance_state(struct device *dev, unsigned long rate)
> +{
> +       struct generic_pm_domain *genpd = ERR_PTR(-ENODATA);
> +       struct generic_pm_domain_data *gpd_data;
> +       int ret, prev;
> +
> +       spin_lock_irq(&dev->power.lock);
> +       if (dev->power.subsys_data && dev->power.subsys_data->domain_data) {
> +               gpd_data = to_gpd_data(dev->power.subsys_data->domain_data);
> +               genpd = dev_to_genpd(dev);
> +       }
> +       spin_unlock_irq(&dev->power.lock);

So even if you fetch the genpd pointer here, you don't protect the
device from detached from its genpd after this point (thus freeing the
domain data).

To move this issue even further, there is no guarantee that the genpd
don't get removed after this point, but you are still operating on the
pointer...

> +
> +       if (IS_ERR(genpd))
> +               return -ENODEV;
> +
> +       if (!genpd->get_performance_state) {
> +               dev_err(dev, "Performance states aren't supported by power domain\n");
> +               return -EINVAL;
> +       }
> +
> +       genpd_lock(genpd);
> +       ret = genpd->get_performance_state(dev, rate);
> +       if (ret < 0)
> +               goto unlock;
> +
> +       prev = gpd_data->performance_state;
> +       gpd_data->performance_state = ret;
> +       ret = genpd_update_performance_state(genpd, 0);
> +       if (ret)
> +               gpd_data->performance_state = prev;
> +
> +unlock:
> +       genpd_unlock(genpd);
> +
> +       return ret;
> +}
> +EXPORT_SYMBOL_GPL(pm_genpd_update_performance_state);
> +
>  /**
>   * genpd_power_off_work_fn - Power off PM domain whose subdomain count is 0.
>   * @work: Work structure used for scheduling the execution of this function.
> @@ -1156,6 +1316,7 @@ static struct generic_pm_domain_data *genpd_alloc_dev_data(struct device *dev,
>         gpd_data->td.constraint_changed = true;
>         gpd_data->td.effective_constraint_ns = -1;
>         gpd_data->nb.notifier_call = genpd_dev_pm_qos_notifier;
> +       gpd_data->performance_state = 0;
>
>         spin_lock_irq(&dev->power.lock);
>
> @@ -1502,6 +1663,17 @@ int pm_genpd_init(struct generic_pm_domain *genpd,
>                 genpd->dev_ops.start = pm_clk_resume;
>         }
>
> +       /*
> +        * If this genpd supports getting performance state, then someone in its
> +        * hierarchy must support setting it too.
> +        */
> +       if (genpd->get_performance_state &&
> +           !genpd_has_set_performance_state(genpd)) {
> +               pr_err("%s: genpd doesn't support updating performance state\n",
> +                      genpd->name);
> +               return -ENODEV;
> +       }
> +
>         /* Always-on domains must be powered on at initialization. */
>         if (genpd_is_always_on(genpd) && !genpd_status_on(genpd))
>                 return -EINVAL;
> diff --git a/include/linux/pm_domain.h b/include/linux/pm_domain.h
> index b7803a251044..74502664ea33 100644
> --- a/include/linux/pm_domain.h
> +++ b/include/linux/pm_domain.h
> @@ -63,8 +63,12 @@ struct generic_pm_domain {
>         unsigned int device_count;      /* Number of devices */
>         unsigned int suspended_count;   /* System suspend device counter */
>         unsigned int prepared_count;    /* Suspend counter of prepared devices */
> +       unsigned int performance_state; /* Max requested performance state */
>         int (*power_off)(struct generic_pm_domain *domain);
>         int (*power_on)(struct generic_pm_domain *domain);
> +       int (*get_performance_state)(struct device *dev, unsigned long rate);
> +       int (*set_performance_state)(struct generic_pm_domain *domain,
> +                                    unsigned int state);
>         struct gpd_dev_ops dev_ops;
>         s64 max_off_time_ns;    /* Maximum allowed "suspended" time. */
>         bool max_off_time_changed;
> @@ -118,6 +122,7 @@ struct generic_pm_domain_data {
>         struct pm_domain_data base;
>         struct gpd_timing_data td;
>         struct notifier_block nb;
> +       unsigned int performance_state;
>         void *data;
>  };
>
> @@ -148,6 +153,9 @@ extern int pm_genpd_remove(struct generic_pm_domain *genpd);
>
>  extern struct dev_power_governor simple_qos_governor;
>  extern struct dev_power_governor pm_domain_always_on_gov;
> +extern bool pm_genpd_has_performance_state(struct device *dev);
> +extern int pm_genpd_update_performance_state(struct device *dev,
> +                                            unsigned long rate);
>  #else
>
>  static inline struct generic_pm_domain_data *dev_gpd_data(struct device *dev)
> @@ -185,6 +193,17 @@ static inline int pm_genpd_remove(struct generic_pm_domain *genpd)
>         return -ENOTSUPP;
>  }
>
> +static inline bool pm_genpd_has_performance_state(struct device *dev)
> +{
> +       return false;
> +}
> +
> +static inline int pm_genpd_update_performance_state(struct device *dev,
> +                                                   unsigned long rate)
> +{
> +       return -ENOTSUPP;
> +}
> +
>  #define simple_qos_governor            (*(struct dev_power_governor *)(NULL))
>  #define pm_domain_always_on_gov                (*(struct dev_power_governor *)(NULL))
>  #endif
> --
> 2.13.0.303.g4ebf3021692d
>

As you probably figured out from my above comments, the solution here
isn't really working.

Adding these new APIs, more or less requires us to manage reference
counting on the genpd for the device. I haven't thought about that in
great detail, just that we of course need to be careful if we want to
introduce something like that, to make sure all aspect of locking
becomes correct.

Moreover adding reference counting touches the idea of adding APIs to
genpd, to allow users/drivers to control their genpd explicitly. This
is required to support > 1 pm_domain per device. I assume you have
been following that discussion for genpd on linux-pm as well. My point
is that, we should consider that while inventing *any* new APIs.

Kind regards
Uffe