Subject: Re: [PATCH v5 3/3] coresight: etm4x: save/restore state for external agents

From: Mathieu Poirier <[email-masked]>
Date: Tue, 20 Aug 2019 16:01:58 -0600

On Fri, Aug 16, 2019 at 04:46:15PM +0100, Andrew Murray wrote:
> Some hardware will ignore bit TRCPDCR.PU which is used to signal
> to hardware that power should not be removed from the trace unit. Much like
> self-hosted debug, we should also save/restore the trace unit state when
> it is in use by external agents.
> 
> We wish to avoid saving the hardware state when coresight isn't in use
> to reduce PM latency - However as external trace/debug is designed to be
> unintrusive to the CPU, the only way of determining that an external agent is
> present is to read the claim tags (TRCCLAIMCLR). Unfortunately this register
> needs power and clocking - something it won't have when coresight isn't in use.
> We also don't want to temporarily enable it due to the latency and PM context.
> 
> Let's compromise by adding a module parameter that will keep the trace unit
> powered and clocked, thus allowing us to only save/restore state when external
> trace (or self-hosted) is in use. Though please note that this doesn't allow
> for tracing from boot on hardware that needs save/restore as the CPU may idle
> prior to the ETMv4 driver starting and adding PM hooks to save/restore.
> 
> Signed-off-by: Andrew Murray <[email-masked]>
> ---
>  drivers/hwtracing/coresight/coresight-etm4x.c | 27 ++++++++++++++++---
>  drivers/hwtracing/coresight/coresight.c       |  2 +-
>  include/linux/coresight.h                     |  7 +++++
>  3 files changed, 31 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/hwtracing/coresight/coresight-etm4x.c b/drivers/hwtracing/coresight/coresight-etm4x.c
> index 35a524eec36d..c5d527f7cbd5 100644
> --- a/drivers/hwtracing/coresight/coresight-etm4x.c
> +++ b/drivers/hwtracing/coresight/coresight-etm4x.c
> @@ -42,11 +42,12 @@ MODULE_PARM_DESC(boot_enable, "Enable tracing on boot");
>  #define PARAM_PM_SAVE_FIRMWARE	  0 /* save self-hosted state as per firmware */
>  #define PARAM_PM_SAVE_NEVER	  1 /* never save any state */
>  #define PARAM_PM_SAVE_SELF_HOSTED 2 /* save self-hosted state only */
> +#define PARAM_PM_SAVE_EXTERNAL	  3 /* save all state (keeps power on) */
>  
>  static int pm_save_enable = PARAM_PM_SAVE_FIRMWARE;
>  module_param(pm_save_enable, int, 0444);
>  MODULE_PARM_DESC(pm_save_enable,
> -	"Save/restore state on power down: 1 = never, 2 = self-hosted");
> +	"Save/restore state on power down: 1 = never, 2 = self-hosted, 3 = self-hosted/external");
>  
>  /* The number of ETMv4 currently registered */
>  static int etm4_count;
> @@ -1331,6 +1332,22 @@ static void etm4_cpu_restore(struct etmv4_drvdata *drvdata)
>  	CS_LOCK(drvdata->base);
>  }
>  
> +static bool etm4_coresight_in_use(struct etmv4_drvdata *drvdata)
> +{
> +	/* Self-hosted session in progress? */
> +	if (local_read(&drvdata->mode))
> +		return true;
> +
> +	/* External agents can be detected through claim tags however we
> +	 * only read these tags if the trace unit is powered.
> +	 */

Multi-line comment format error

> +	if (drvdata->csdev && pm_runtime_active(drvdata->csdev->dev.parent))
> +		if (coresight_is_claimed_any(drvdata->base))
> +			return true;
> +
> +	return false;
> +}
> +
>  static int etm4_cpu_pm_notify(struct notifier_block *nb, unsigned long cmd,
>  			      void *v)
>  {
> @@ -1350,8 +1367,8 @@ static int etm4_cpu_pm_notify(struct notifier_block *nb, unsigned long cmd,
>  
>  	switch (cmd) {
>  	case CPU_PM_ENTER:
> -		/* save the state if self-hosted coresight is in use */
> -		if (local_read(&drvdata->mode))
> +		/* Save the state if coresight is in use */
> +		if (etm4_coresight_in_use(drvdata))
>  			if (etm4_cpu_save(drvdata))
>  				return NOTIFY_BAD;
>  		break;
> @@ -1488,7 +1505,9 @@ static int etm4_probe(struct amba_device *adev, const struct amba_id *id)
>  		goto err_arch_supported;
>  	}
>  
> -	pm_runtime_put(&adev->dev);
> +	if (pm_save_enable != PARAM_PM_SAVE_EXTERNAL)
> +		pm_runtime_put(&adev->dev);
> +
>  	dev_info(&drvdata->csdev->dev, "CPU%d: ETM v%d.%d initialized\n",
>  		 drvdata->cpu, drvdata->arch >> 4, drvdata->arch & 0xf);
>  
> diff --git a/drivers/hwtracing/coresight/coresight.c b/drivers/hwtracing/coresight/coresight.c
> index e6ca899fea4e..474b7372864e 100644
> --- a/drivers/hwtracing/coresight/coresight.c
> +++ b/drivers/hwtracing/coresight/coresight.c
> @@ -140,7 +140,7 @@ static inline bool coresight_is_claimed_self_hosted(void __iomem *base)
>  	return coresight_read_claim_tags(base) == CORESIGHT_CLAIM_SELF_HOSTED;
>  }
>  
> -static inline bool coresight_is_claimed_any(void __iomem *base)
> +bool coresight_is_claimed_any(void __iomem *base)
>  {
>  	return coresight_read_claim_tags(base) != 0;
>  }
> diff --git a/include/linux/coresight.h b/include/linux/coresight.h
> index 44e552de419c..65bfd2cb0283 100644
> --- a/include/linux/coresight.h
> +++ b/include/linux/coresight.h
> @@ -286,6 +286,8 @@ extern void coresight_disclaim_device_unlocked(void __iomem *base);
>  extern char *coresight_alloc_device_name(struct coresight_dev_list *devs,
>  					 struct device *dev);
>  
> +extern bool coresight_is_claimed_any(void __iomem *base);
> +
>  extern bool coresight_loses_context_with_cpu(struct device *dev);
>  #else
>  static inline struct coresight_device *
> @@ -309,6 +311,11 @@ static inline int coresight_claim_device(void __iomem *base)
>  static inline void coresight_disclaim_device(void __iomem *base) {}
>  static inline void coresight_disclaim_device_unlocked(void __iomem *base) {}
>  
> +static inline bool coresight_is_claimed_any(void __iomem *base)
> +{
> +	return false;
> +}
> +
>  static inline bool coresight_loses_context_with_cpu(struct device *dev)
>  {
>  	return false;
> -- 
> 2.21.0
> 

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