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

From: Andrew Murray <[email-masked]>
Date: Fri, 13 Sep 2019 11:32:53 +0100

On Thu, Sep 12, 2019 at 04:35:20PM +0100, Suzuki K Poulose wrote:
> Hi Andrew
> 
> On 16/08/2019 16:46, 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.
> > 
> 
> This looks fine to me. Some minor comments below.
> 
> > 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) */
> 
> Should we say PARAM_PM_SAVE_ALWAYS instead ?
> 

Yes I think this is OK.

> >   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");
> 
> similarly here and also mention that the power/clocks are not dropped in that
> case ? I see the comment above, but please could we make it more explicit ?

I'll change it to ... 3 = self-hosted/external (keeps power on)");

> 
> >   /* 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.
> > +	 */
> > +	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);
> > +
> 
> It may be a good idea to explain why we don't drop the power here
> in a comment to help people reading the code. You could paste what
> is in the commit description in here to avoid another lookup.

Good idea. I'll modify it slightly however.

> 
> >   	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;
> >   }
> 
> minor nit: We may retain this as static inline and move this to the header file.

OK.

Thanks,

Andrew Murray

> Kind regards
> Suzuki

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