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

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Message-ID: <adf0211e-3e7d-8d33-bff4-cf064bfb20af@huawei.com>
Date:   Wed, 20 Sep 2023 15:10:25 +0800
From:   Yicong Yang <yangyicong@...wei.com>
To:     Suzuki K Poulose <suzuki.poulose@....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 2023/9/20 1:01, Suzuki K Poulose wrote:
> 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.
> 

Sure. I'll add the tag in v3.

Thanks.