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

[Yicong Yang <yangyicong@huawei.com>]

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.

> Suzuki
> 
>>
>> Signed-off-by: Junhao He <hejunhao3@huawei.com>
>> Signed-off-by: Yicong Yang <yangyicong@hisilicon.com>
>> Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
>> ---
>>   drivers/hwtracing/ptt/hisi_ptt.c | 5 +++++
>>   1 file changed, 5 insertions(+)
>>
>> diff --git a/drivers/hwtracing/ptt/hisi_ptt.c b/drivers/hwtracing/ptt/hisi_ptt.c
>> index 62a444f5228e..c1b5fd2b8974 100644
>> --- a/drivers/hwtracing/ptt/hisi_ptt.c
>> +++ b/drivers/hwtracing/ptt/hisi_ptt.c
>> @@ -1184,6 +1184,10 @@ static void hisi_ptt_pmu_del(struct perf_event *event, int flags)
>>       hisi_ptt_pmu_stop(event, PERF_EF_UPDATE);
>>   }
>>   +static void hisi_ptt_pmu_read(struct perf_event *event)
>> +{
>> +}
>> +
>>   static void hisi_ptt_remove_cpuhp_instance(void *hotplug_node)
>>   {
>>       cpuhp_state_remove_instance_nocalls(hisi_ptt_pmu_online, hotplug_node);
>> @@ -1227,6 +1231,7 @@ static int hisi_ptt_register_pmu(struct hisi_ptt *hisi_ptt)
>>           .stop        = hisi_ptt_pmu_stop,
>>           .add        = hisi_ptt_pmu_add,
>>           .del        = hisi_ptt_pmu_del,
>> +        .read        = hisi_ptt_pmu_read,
>>       };
>>         reg = readl(hisi_ptt->iobase + HISI_PTT_LOCATION);
> 
> 
> .