linux-kernel - Re: [PATCH v2 5/5] hwtracing: hisi_ptt: Add dummy callback pmu::read()

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Date:   Tue, 19 Sep 2023 18:01:38 +0100
From:   Suzuki K Poulose <suzuki.poulose@....com>
To:     Yicong Yang <yangyicong@...wei.com>, mathieu.poirier@...aro.org,
        jonathan.cameron@...wei.com, linux-kernel@...r.kernel.org
Cc:     yangyicong@...ilicon.com, alexander.shishkin@...ux.intel.com,
        helgaas@...nel.org, linux-pci@...r.kernel.org,
        prime.zeng@...ilicon.com, linuxarm@...wei.com, hejunhao3@...wei.com
Subject: Re: [PATCH v2 5/5] hwtracing: hisi_ptt: Add dummy callback
 pmu::read()

On 19/09/2023 14:03, Yicong Yang wrote:
> On 2023/9/15 20:53, Suzuki K Poulose wrote:
>> On 14/09/2023 12:22, Yicong Yang wrote:
>>> From: Junhao He <hejunhao3@...wei.com>
>>>
>>> When start trace with perf option "-C $cpu" and immediately stop it
>>> with SIGTERM or others, the perf core will invoke pmu::read() while
>>> the driver doesn't implement it. Add a dummy pmu::read() to avoid
>>> any issues.
>>
>> What issues are we talking about here ? Shouldn't the core perf
>> skip the call, if pmu::read() is not available ?
>>
> 
> Actually no, the core doesn't check it. So I think that's why some PMUs
> like SPE implements a dummy pmu::read() callback. Otherwise we'll
> dereference a NULL pointer.
> 
> Currently we only met this on emulated platforms with very slow CPUs,
> follow the instructions in the commit above.

Ok, then it calls for a Fixes tag. Please tag it to the commit that
introduced the PMU.

Suzuki