linux-kernel - Re: [PATCH v6 2/2] coresight: cti: Add Qualcomm extended CTI support

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Message-ID: <CAJ9a7VgV6chvWmeG2xta11YqyNpdRZqx4=EF7vC7k=J2utpRYQ@mail.gmail.com>
Date: Thu, 4 Dec 2025 15:07:10 +0000
From: Mike Leach <mike.leach@...aro.org>
To: Leo Yan <leo.yan@....com>
Cc: Yingchao Deng <yingchao.deng@....qualcomm.com>, 
	Suzuki K Poulose <suzuki.poulose@....com>, James Clark <james.clark@...aro.org>, 
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>, 
	Tingwei Zhang <tingwei.zhang@....qualcomm.com>, quic_yingdeng@...cinc.com, 
	coresight@...ts.linaro.org, linux-arm-kernel@...ts.infradead.org, 
	linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org, 
	Jinlong Mao <jinlong.mao@....qualcomm.com>, Mao Jinlong <quic_jinlmao@...cinc.com>
Subject: Re: [PATCH v6 2/2] coresight: cti: Add Qualcomm extended CTI support

Hi Leo

On Thu, 4 Dec 2025 at 10:47, Leo Yan <leo.yan@....com> wrote:
>
> On Thu, Dec 04, 2025 at 09:15:07AM +0000, Mike Leach wrote:
>
> [...]
>
> > > > +             /*
> > > > +              * QCOM CTI does not implement Claimtag functionality as
> > > > +              * per CoreSight specification, but its CLAIMSET register
> > > > +              * is incorrectly initialized to 0xF. This can mislead
> > > > +              * tools or drivers into thinking the component is claimed.
> > > > +              *
> > > > +              * Reset CLAIMSET to 0 to reflect that no claims are active.
> > > > +              */
> > > > +             writel_relaxed(0, drvdata->base + CORESIGHT_CLAIMSET);
> > >
> > > I am confused for this.  If QCOM CTI does not implement claim tag,
> > > then what is the designed register at the offset CORESIGHT_CLAIMSET?
> > >
> > > Should you bypass all claim tag related operations for QCOM CTI case?
> > > (I don't see you touch anything for claim and declaim tags).
> > >
> >
> > The patch I have created to handle systems without correct claim tag
> > operation is a dependency for this patch set. Thus no need for
> > override here as the core code will handle this correctly.
> >
> > The only issue is ensuring the non-CTI spec implementation will result
> > in the correct detection of no claim tags present.
>
> Your patch works only when a module has implemented claim registers.
>

Per the Coresight spec - unimplemented registers must be RAZ/WI- so
this still works for non implemented claim registers.

> This leads to two issues: we end up clearing an unknown register in the
> CTI driver, and then the coresight core layer assumes it is reading a
> claim register even though it is not.

Again RAZ will simply read 0x0 - which is an indication that there are
no claim bits implemented.

>
> For QCOM CTI, combined with your patch, I would suggest directly
> setting csdev->access.claim_tag_impl to false (perhaps using a helper).

That would be a better solution, though as Qcom appear to have
implemented a pair of standard RW registers rather than the claim tag
functionality, the write solution works for this particular
implementation.

Regards

Mike

> This would be much clearer than the "hacking" way.
>
> Thanks,
> Leo



-- 
Mike Leach
Principal Engineer, ARM Ltd.
Manchester Design Centre. UK