linux-kernel - Re: [PATCH 5/7] coresight: Clear self hosted claim tag on probe

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Message-ID: <20250317180945.GK2487211@e132581.arm.com>
Date: Mon, 17 Mar 2025 18:09:45 +0000
From: Leo Yan <leo.yan@....com>
To: James Clark <james.clark@...aro.org>
Cc: lcherian@...vell.com, coresight@...ts.linaro.org,
	Mike Leach <mike.leach@...aro.org>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Maxime Coquelin <mcoquelin.stm32@...il.com>,
	Alexandre Torgue <alexandre.torgue@...s.st.com>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	linux-stm32@...md-mailman.stormreply.com
Subject: Re: [PATCH 5/7] coresight: Clear self hosted claim tag on probe

On Mon, Mar 17, 2025 at 03:05:41PM +0000, James Clark wrote:

[...]

> > >   /*
> > > - * coresight_disclaim_device_unlocked : Clear the claim tag for the device.
> > > + * Clear the claim tag for the device.
> > > + * Returns an error if the device wasn't already claimed.
> > > + */
> > > +int coresight_reset_claim(struct csdev_access *csa)
> > > +{
> > > +       int ret;
> > > +
> > > +       CS_UNLOCK(csa->base);
> > > +       ret = coresight_reset_claim_unlocked(csa);
> > > +       CS_LOCK(csa->base);
> > > +       return ret;
> > > +}
> > > +EXPORT_SYMBOL_GPL(coresight_reset_claim);
> > 
> > Maybe my question is overlapping with Mike's comment.
> > 
> > Callers never check the return values from coresight_reset_claim(). I am
> 
> I can remove the return value if it's confusing. The thought process was
> probably that it could be useful somewhere in the future, and
> coresight_reset_claim_unlocked() returns something anyway so might as well
> pass it up.
> 
> > wandering if coresight_reset_claim() can directly call
> > coresight_clear_self_claim_tag() for _trying_ to clear self-host tag in
> > probe phase. Any self claim tag issues will be deferred to detect until
> > enable the component.
> > 
> 
> Maybe, the spec does a read before setting which I assumed should be done
> for clearing as well.

If you mean PSCI doc, the flow for claiming tag is a handshake between
self-host software and external debugger, this is why a read is
required.

For a cleaning up, it is not about syncing with external debugger.  And
writing bit 0 to TRCCLAIMCLR will not impact external debugger.  So I
don't think a read prior to cleaning self claim tag is needed.

> As in to not touch anything if it's in use externally.
> It doesn't specifically describe any clearing sequence, but if we assume
> it's ok to blindly clear self hosted flag even when it's in use then yes we
> can directly call coresight_clear_self_claim_tag().

As a result, directly calling coresight_clear_self_claim_tag() works
for me.

Thanks,
Leo