linux-kernel - Re: [2/2] coresight: Fix reference count for software sources

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Message-ID: <ddf386ee-0be5-b1ca-174c-89d9e48ba282@arm.com>
Date:   Tue, 14 Mar 2017 18:06:30 +0000
From:   Suzuki K Poulose <Suzuki.Poulose@....com>
To:     Mathieu Poirier <mathieu.poirier@...aro.org>
Cc:     "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        Chunyan Zhang <zhang.chunyan@...aro.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [2/2] coresight: Fix reference count for software sources

On 14/03/17 17:40, Mathieu Poirier wrote:
> On 14 March 2017 at 11:32, Mathieu Poirier <mathieu.poirier@...aro.org> wrote:
>> From: "Suzuki K. Poulose" <Suzuki.Poulose@....com>
>>
>> For software sources (i.e STM), there could be multiple agents
>> generating the trace data, unlike the ETMs. So we need to
>> properly do the accounting for the active number of users
>> to disable the device when the last user goes away. Right
>> now, the reference counting is broken for sources as we skip
>> the actions when we detect that the source is enabled.
>>
>> This patch fixes the problem by adding the refcounting for
>> software sources, even when they are enabled.
>>
>> Cc: Mathieu Poirier <mathieu.poirier@...aro.org>
>> Reported-by: Robert Walker <robert.walker@....com>
>> Signed-off-by: Suzuki K Poulose <suzuki.poulose@....com>
>> ---
>>  drivers/hwtracing/coresight/coresight.c | 13 +++++++++++--
>>  1 file changed, 11 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/hwtracing/coresight/coresight.c b/drivers/hwtracing/coresight/coresight.c
>> index 34cd1ed..2da9e39 100644
>> --- a/drivers/hwtracing/coresight/coresight.c
>> +++ b/drivers/hwtracing/coresight/coresight.c
>> @@ -552,6 +552,7 @@ int coresight_enable(struct coresight_device *csdev)
>>         int cpu, ret = 0;
>>         struct coresight_device *sink;
>>         struct list_head *path;
>> +       enum coresight_dev_subtype_source subtype = csdev->subtype.source_subtype;
>
> Checkpatch.pl complains about a line over 80 characters.
>
>>
>>         mutex_lock(&coresight_mutex);
>>
>> @@ -559,8 +560,16 @@ int coresight_enable(struct coresight_device *csdev)
>>         if (ret)
>>                 goto out;
>>
>> -       if (csdev->enable)
>> +       if (csdev->enable) {
>> +               /*
>> +                * There could be multiple applications driving the software
>> +                * source. So keep the refcount for each such user when the
>> +                * source is already enabled.
>> +                */
>> +               if (subtype == CORESIGHT_DEV_SUBTYPE_SOURCE_SOFTWARE)
>> +                       atomic_inc(csdev->refcnt);
>>                 goto out;
>> +       }
>>

Btw, should we allow the user to turn on the STM from sysfs (echo 1 > $STM/enable_source) ?
All STM users should set their policy via ioctls and that in turn turns the device on. So
it doesn't make sense for enable_source to really enable the hardware unless someone really
opens it.

Chunyan, Mathieu,

thoughts ?

Suzuki