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

[Leo Yan <leo.yan@arm.com>]

On Thu, Dec 04, 2025 at 03:07:10PM +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.

QCOM CTI does not follow the spec in two aspects:

- Given the patch's comment: "QCOM CTI does not implement Claim tag
  functionality as per CoreSight specification", I am suspect the CLAIM
  registers are not implemented at all in QCOM CTI.

- It neither follows up the "unimplemented registers must be RAZ/WI" -
  the patch says its reset value is 0xF and then even can write 0 to it.

> > 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.

If an IP violates both the rules for implemented claim registers and
the rules for non-implemented claim registers, how can we rely on
these registers to detect the claim feature?

My feeling is we are building a house on sand - these registers are not
used for claim tags purpose at all.

Thanks,
Leo