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Date:   Thu, 25 Feb 2021 14:46:57 -0700
From:   Mathieu Poirier <mathieu.poirier@linaro.org>
To:     Mike Leach <mike.leach@linaro.org>
Cc:     linux-arm-kernel@lists.infradead.org, coresight@lists.linaro.org,
        linux-doc@vger.kernel.org, suzuki.poulose@arm.com,
        yabinc@google.com, corbet@lwn.net, leo.yan@linaro.org,
        alexander.shishkin@linux.intel.com, tingwei@codeaurora.org,
        gregkh@linuxfoundation.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v4 05/10] coresight: syscfg: Add API to activate and
 enable configurations
Message-ID: <20210225214657.GB3567106@xps15>
References: <20210128170936.9222-1-mike.leach@linaro.org>
 <20210128170936.9222-6-mike.leach@linaro.org>
 <20210225212050.GA3567106@xps15>
MIME-Version: 1.0
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In-Reply-To: <20210225212050.GA3567106@xps15>
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[...]

> > +
> > +/**
> > + * Mark a config descriptor as active.
> > + * This will be seen when csdev devices are activated in the system.
> > + *
> > + * Selection by hash value - generated from the configuration name when it
> > + * was loaded and added to the cs_etm/configurations file system for selection
> > + * by perf.
> > + *
> > + * @cfg_hash: Hash value of the selected configuration name.
> > + */
> > +int cscfg_activate_config(unsigned long cfg_hash)
> > +{
> > +	struct cscfg_config_desc *curr_item, *match_item = 0;
> > +
> > +	mutex_lock(&cscfg_mutex);
> > +
> > +	list_for_each_entry(curr_item, &cscfg_mgr->data.config_desc_list, item) {
> > +		if ((unsigned long)curr_item->id_ea->var == cfg_hash) {
> > +			match_item = curr_item;
> > +			atomic_inc(&cscfg_mgr->data.sys_active_cnt);
> 
> It would be nice to have a comment that mentions why this is needed.  I had to
> go look in patch 09 to see that it prevents a feature from being changed when
> any configuration is active.  And since patch 09 is the only place where
> @sys_active_cnt is used, please move the declaration and handling of the
> variable there.
> 
> > +			break;
> > +		}
> > +	}
> > +	mutex_unlock(&cscfg_mutex);
> > +
> > +	if (!match_item)
> > +		return -EINVAL;
> > +
> > +	dev_dbg(to_device_cscfg(), "Activate config %s.\n", match_item->name);
> > +
> > +	/* mark the descriptors as active so enable config will use them */
> > +	mutex_lock(&cscfg_csdev_mutex);
> > +	atomic_inc(&match_item->active_cnt);
> 
> What is ->active_cnt used for?  I see it referenced in
> cscfg_csdev_enable_active_config() but it doesn't do much other than to confirm
> the configuration has been activated by someone.  
> 
> There is also a chance that a caller would call cscfg_activate_config() once and
> call cscfg_csdev_enable_active_config() multiple time, which would really create
> problems.  I would move the incrementation of sys_active_cnt within the mutex
> hold in cscfg_csdev_enable_active_config() and get rid of ->active_cnt.  If I am
> correct we wouldn't need cscfg_activate_config() after that.
>

I looked at patch 06 and I understand why incrementing sys_active_cnt has to be
done before enabling it.  If it wasn't so someone could update the feature
between the time setup_aux() is called and the event is installed on a CPU.  As
such cscfg_activate_config() should stay but I still think ->active_cnt should
go. 
 
> > +	mutex_unlock(&cscfg_csdev_mutex);
> > +
> > +	return 0;
> > +}
> > +EXPORT_SYMBOL_GPL(cscfg_activate_config);
> > +
> > +void cscfg_deactivate_config(unsigned long cfg_hash)
> 
> I'm fine with either cfg_hash or id_hash, but not both.
> 
> > +{
> > +	struct cscfg_config_desc *curr_item, *match_item = 0;
> > +
> > +	mutex_lock(&cscfg_mutex);
> > +
> > +	list_for_each_entry(curr_item, &cscfg_mgr->data.config_desc_list, item) {
> > +		if ((unsigned long)curr_item->id_ea->var == cfg_hash) {
> > +			match_item = curr_item;
> > +			break;
> > +		}
> > +	}
> > +	mutex_unlock(&cscfg_mutex);
> > +	if (!match_item)
> > +		return;
> > +
> > +	dev_dbg(to_device_cscfg(), "Deactivate config %s.\n", match_item->name);
> > +
> > +	mutex_lock(&cscfg_csdev_mutex);
> > +	atomic_dec(&match_item->active_cnt);
> > +	mutex_unlock(&cscfg_csdev_mutex);
> > +
> > +	atomic_dec(&cscfg_mgr->data.sys_active_cnt);
> > +}
> > +EXPORT_SYMBOL_GPL(cscfg_deactivate_config);
> > +
> > +/* Find and program any active config for the supplied device.*/
> > +int cscfg_csdev_enable_active_config(struct coresight_device *csdev,
> > +				     unsigned long id_hash, int preset)
> > +{
> > +	struct cscfg_config_csdev *cfg = NULL, *item;
> > +	const struct cscfg_config_desc *desc;
> > +	int err = 0;
> > +
> > +	/* quickly check global count */
> > +	if (!atomic_read(&cscfg_mgr->data.sys_active_cnt))
> > +		return 0;
> > +
> > +	mutex_lock(&cscfg_csdev_mutex);
> > +	list_for_each_entry(item, &csdev->config_csdev_list, node) {
> > +		desc = item->desc;
> > +		if ((atomic_read(&desc->active_cnt)) &&
> > +		    ((unsigned long)desc->id_ea->var == id_hash)) {
> > +			cfg = item;
> > +			break;
> > +		}
> > +	}
> > +	if (cfg) {
> > +		err = cscfg_csdev_enable_config(cfg, preset);
> > +		if (!err)
> > +			csdev->active_cfg_ctxt = (void *)cfg;
> > +	}
> > +	mutex_unlock(&cscfg_csdev_mutex);
> > +	return err;
> > +}
> > +EXPORT_SYMBOL_GPL(cscfg_csdev_enable_active_config);
> > +
> > +/* save and disable the active config for the device */
> > +void cscfg_csdev_disable_active_config(struct coresight_device *csdev)
> > +{
> > +	struct cscfg_config_csdev *cfg;
> > +
> > +	mutex_lock(&cscfg_csdev_mutex);
> > +	cfg = (struct cscfg_config_csdev *)csdev->active_cfg_ctxt;
> > +	if (cfg)
> > +		cscfg_csdev_disable_config(cfg);
> > +	mutex_unlock(&cscfg_csdev_mutex);
> > +}
> > +EXPORT_SYMBOL_GPL(cscfg_csdev_disable_active_config);
> > +
> >  /* Initialise system configuration management device. */
> >  
> >  struct device *to_device_cscfg(void)
> > @@ -546,6 +672,7 @@ int __init cscfg_init(void)
> >  	INIT_LIST_HEAD(&cscfg_mgr->data.feat_desc_list);
> >  	INIT_LIST_HEAD(&cscfg_mgr->data.config_desc_list);
> >  	cscfg_mgr->data.nr_csdev = 0;
> > +	atomic_set(&cscfg_mgr->data.sys_active_cnt, 0);
> >  
> >  	dev_info(to_device_cscfg(), "CoreSight Configuration manager initialised");
> >  	return 0;
> > diff --git a/drivers/hwtracing/coresight/coresight-syscfg.h b/drivers/hwtracing/coresight/coresight-syscfg.h
> > index ebf5e1491d86..301e26e1e98f 100644
> > --- a/drivers/hwtracing/coresight/coresight-syscfg.h
> > +++ b/drivers/hwtracing/coresight/coresight-syscfg.h
> > @@ -17,13 +17,15 @@
> >   * @csdev_list:		List of coresight devices registered with the configuration manager.
> >   * @feat_desc_list:	List of feature descriptors to load into registered devices.
> >   * @config_desc_list:	List of system configuration descriptors to load into registered devices.
> > - * @nr_csdev:	Number of registered devices with the cscfg system
> > + * @nr_csdev:		Number of registered devices with the cscfg system
> > + * @sys_active_cnt:	Total number of active config descriptor references.
> >   */
> >  struct cscfg_api_data {
> >  	struct list_head csdev_desc_list;
> >  	struct list_head feat_desc_list;
> >  	struct list_head config_desc_list;
> >  	int nr_csdev;
> > +	atomic_t sys_active_cnt;
> >  };
> >  
> >  /**
> > @@ -53,6 +55,12 @@ int cscfg_register_csdev(struct coresight_device *csdev,
> >  			 struct cscfg_match_desc *info,
> >  			 struct cscfg_csdev_feat_ops *ops);
> >  void cscfg_unregister_csdev(struct coresight_device *csdev);
> > +int cscfg_activate_config(unsigned long cfg_hash);
> > +void cscfg_deactivate_config(unsigned long cfg_hash);
> > +void cscfg_csdev_reset_feats(struct coresight_device *csdev);
> > +int cscfg_csdev_enable_active_config(struct coresight_device *csdev,
> > +				     unsigned long id_hash, int preset);
> > +void cscfg_csdev_disable_active_config(struct coresight_device *csdev);
> >  
> >  /**
> >   * System configuration manager device.
> > diff --git a/include/linux/coresight.h b/include/linux/coresight.h
> > index d0126ed326a6..3941854e8280 100644
> > --- a/include/linux/coresight.h
> > +++ b/include/linux/coresight.h
> > @@ -221,6 +221,7 @@ struct coresight_sysfs_link {
> >   * @has_conns_grp: Have added a "connections" group for sysfs links.
> >   * @feature_csdev_list: List of complex feature programming added to the device.
> >   * @config_csdev_list:  List of system configurations added to the device.
> > + * @active_cfg_ctxt:    Context information for current active congfig.
> >   */
> >  struct coresight_device {
> >  	struct coresight_platform_data *pdata;
> > @@ -245,6 +246,7 @@ struct coresight_device {
> >  	/* system configuration and feature lists */
> >  	struct list_head feature_csdev_list;
> >  	struct list_head config_csdev_list;
> > +	void *active_cfg_ctxt;
> >  };
> >  
> >  /*
> > -- 
> > 2.17.1
> >