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Message-ID: <3cc013bd-c85e-b28a-f921-d5593b3935a4@arm.com>
Date:   Tue, 19 Sep 2023 18:01:38 +0100
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Subject: Re: [PATCH v2 5/5] hwtracing: hisi_ptt: Add dummy callback
 pmu::read()
To:     Yicong Yang <yangyicong@huawei.com>, mathieu.poirier@linaro.org,
        jonathan.cameron@huawei.com, linux-kernel@vger.kernel.org
Cc:     yangyicong@hisilicon.com, alexander.shishkin@linux.intel.com,
        helgaas@kernel.org, linux-pci@vger.kernel.org,
        prime.zeng@hisilicon.com, linuxarm@huawei.com, hejunhao3@huawei.com
References: <20230914112223.27165-1-yangyicong@huawei.com>
 <20230914112223.27165-6-yangyicong@huawei.com>
 <73655f9b-9ea4-cb46-d712-20f1c4ac7c95@arm.com>
 <75b93d15-f099-5d17-caa9-94b0390773cf@huawei.com>
From:   Suzuki K Poulose <suzuki.poulose@arm.com>
In-Reply-To: <75b93d15-f099-5d17-caa9-94b0390773cf@huawei.com>
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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@huawei.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