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

  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.

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(+)
  

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 ?

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);
  

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);
> 
> 
> .
  

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
  

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@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.
> 

Sure. I'll add the tag in v3.

Thanks.