Re: [PATCH v2 07/22] drm/msm: Do rpm get sooner in the submit path

[Viresh Kumar]

On 27-10-20, 17:05, Viresh Kumar wrote:
> It isn't that straight forward unfortunately, we need to make sure the
> table doesn't get allocated for the same device twice, so
> find+allocate needs to happen within a locked region.
> 
> I have taken, not so straight forward, approach to fixing this issue,
> lets see if this fixes it or not.
> 
> -------------------------8<-------------------------
> 
> diff --git a/drivers/opp/core.c b/drivers/opp/core.c
> index 4ac4e7ce6b8b..6f4a73a6391f 100644
> --- a/drivers/opp/core.c
> +++ b/drivers/opp/core.c
> @@ -29,6 +29,8 @@
>  LIST_HEAD(opp_tables);
>  /* Lock to allow exclusive modification to the device and opp lists */
>  DEFINE_MUTEX(opp_table_lock);
> +/* Flag indicating that opp_tables list is being updated at the moment */
> +static bool opp_tables_busy;
>  
>  static struct opp_device *_find_opp_dev(const struct device *dev,
>  					struct opp_table *opp_table)
> @@ -1036,8 +1038,8 @@ static void _remove_opp_dev(struct opp_device *opp_dev,
>  	kfree(opp_dev);
>  }
>  
> -static struct opp_device *_add_opp_dev_unlocked(const struct device *dev,
> -						struct opp_table *opp_table)
> +struct opp_device *_add_opp_dev(const struct device *dev,
> +				struct opp_table *opp_table)
>  {
>  	struct opp_device *opp_dev;
>  
> @@ -1048,7 +1050,9 @@ static struct opp_device *_add_opp_dev_unlocked(const struct device *dev,
>  	/* Initialize opp-dev */
>  	opp_dev->dev = dev;
>  
> +	mutex_lock(&opp_table->lock);
>  	list_add(&opp_dev->node, &opp_table->dev_list);
> +	mutex_unlock(&opp_table->lock);
>  
>  	/* Create debugfs entries for the opp_table */
>  	opp_debug_register(opp_dev, opp_table);
> @@ -1056,18 +1060,6 @@ static struct opp_device *_add_opp_dev_unlocked(const struct device *dev,
>  	return opp_dev;
>  }
>  
> -struct opp_device *_add_opp_dev(const struct device *dev,
> -				struct opp_table *opp_table)
> -{
> -	struct opp_device *opp_dev;
> -
> -	mutex_lock(&opp_table->lock);
> -	opp_dev = _add_opp_dev_unlocked(dev, opp_table);
> -	mutex_unlock(&opp_table->lock);
> -
> -	return opp_dev;
> -}
> -
>  static struct opp_table *_allocate_opp_table(struct device *dev, int index)
>  {
>  	struct opp_table *opp_table;
> @@ -1121,8 +1113,6 @@ static struct opp_table *_allocate_opp_table(struct device *dev, int index)
>  	INIT_LIST_HEAD(&opp_table->opp_list);
>  	kref_init(&opp_table->kref);
>  
> -	/* Secure the device table modification */
> -	list_add(&opp_table->node, &opp_tables);
>  	return opp_table;
>  
>  err:
> @@ -1135,27 +1125,64 @@ void _get_opp_table_kref(struct opp_table *opp_table)
>  	kref_get(&opp_table->kref);
>  }
>  
> +/*
> + * We need to make sure that the OPP table for a device doesn't get added twice,
> + * if this routine gets called in parallel with the same device pointer.
> + *
> + * The simplest way to enforce that is to perform everything (find existing
> + * table and if not found, create a new one) under the opp_table_lock, so only
> + * one creator gets access to the same. But that expands the critical section
> + * under the lock and may end up causing circular dependencies with frameworks
> + * like debugfs, interconnect or clock framework as they may be direct or
> + * indirect users of OPP core.
> + *
> + * And for that reason we have to go for a bit tricky implementation here, which
> + * uses the opp_tables_busy flag to indicate if another creator is in the middle
> + * of adding an OPP table and others should wait for it to finish.
> + */
>  static struct opp_table *_opp_get_opp_table(struct device *dev, int index)
>  {
>  	struct opp_table *opp_table;
>  
> -	/* Hold our table modification lock here */
> +again:
>  	mutex_lock(&opp_table_lock);
>  
>  	opp_table = _find_opp_table_unlocked(dev);
>  	if (!IS_ERR(opp_table))
>  		goto unlock;
>  
> +	/*
> +	 * The opp_tables list or an OPP table's dev_list is getting updated by
> +	 * another user, wait for it to finish.
> +	 */
> +	if (unlikely(opp_tables_busy)) {
> +		mutex_unlock(&opp_table_lock);
> +		cpu_relax();
> +		goto again;
> +	}
> +
> +	opp_tables_busy = true;
>  	opp_table = _managed_opp(dev, index);
> +
> +	/* Drop the lock to reduce the size of critical section */
> +	mutex_unlock(&opp_table_lock);
> +
>  	if (opp_table) {
> -		if (!_add_opp_dev_unlocked(dev, opp_table)) {
> +		if (!_add_opp_dev(dev, opp_table)) {
>  			dev_pm_opp_put_opp_table(opp_table);
>  			opp_table = ERR_PTR(-ENOMEM);
>  		}
> -		goto unlock;
> +
> +		mutex_lock(&opp_table_lock);
> +	} else {
> +		opp_table = _allocate_opp_table(dev, index);
> +
> +		mutex_lock(&opp_table_lock);
> +		if (!IS_ERR(opp_table))
> +			list_add(&opp_table->node, &opp_tables);
>  	}
>  
> -	opp_table = _allocate_opp_table(dev, index);
> +	opp_tables_busy = false;
>  
>  unlock:
>  	mutex_unlock(&opp_table_lock);
> @@ -1181,6 +1208,10 @@ static void _opp_table_kref_release(struct kref *kref)
>  	struct opp_device *opp_dev, *temp;
>  	int i;
>  
> +	/* Drop the lock as soon as we can */
> +	list_del(&opp_table->node);
> +	mutex_unlock(&opp_table_lock);
> +
>  	_of_clear_opp_table(opp_table);
>  
>  	/* Release clk */
> @@ -1208,10 +1239,7 @@ static void _opp_table_kref_release(struct kref *kref)
>  
>  	mutex_destroy(&opp_table->genpd_virt_dev_lock);
>  	mutex_destroy(&opp_table->lock);
> -	list_del(&opp_table->node);
>  	kfree(opp_table);
> -
> -	mutex_unlock(&opp_table_lock);
>  }
>  
>  void dev_pm_opp_put_opp_table(struct opp_table *opp_table)

Rob, Ping.

-- 
viresh