linux-kernel - Re: [PATCH 1/3] arm64: dts: qcom: sdm845: Add Coresight support

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Date:   Sat, 12 Jan 2019 18:21:00 +0530
From:   saiprakash.ranjan@...eaurora.org
To:     Mathieu Poirier <mathieu.poirier@...aro.org>
Cc:     Rob Herring <robh+dt@...nel.org>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Leo Yan <leo.yan@...aro.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Andy Gross <andy.gross@...aro.org>,
        David Brown <david.brown@...aro.org>,
        devicetree@...r.kernel.org, Mark Rutland <mark.rutland@....com>,
        Rajendra Nayak <rnayak@...eaurora.org>,
        Vivek Gautam <vivek.gautam@...eaurora.org>,
        Sibi Sankar <sibis@...eaurora.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-arm-msm@...r.kernel.org, linux-arm-msm-owner@...r.kernel.org
Subject: Re: [PATCH 1/3] arm64: dts: qcom: sdm845: Add Coresight support

Hi Mathieu,

>> +
>> +	etm@...0000 {
>> +		compatible = "arm,coresight-etm4x", "arm,primecell";
>> +		arm,primecell-periphid = <0x000bb95d>;
> 
> I'm a little curious as to why you need to bypass the normal AMBA bus 
> discovery
> method by forcing the peripheral ID.  Tracers don't show up the way 
> other
> coresight devices do at boot time?
> 

Yes on some Qcom SoC's like SDM845 and also on some previous ones, for 
ETM(only) amba bus discovery method fails because of wrong pid read from 
the registers. So we have to force this primecell peripheral ids to 
probe etm.

- Sai

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