linux-kernel - Re: [PATCH V3 13/18] coresight: tmc: make sysFS and Perf mode mutually exclusive

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Message-ID: <571F3388.8090700@arm.com>
Date:	Tue, 26 Apr 2016 10:23:20 +0100
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>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V3 13/18] coresight: tmc: make sysFS and Perf mode
 mutually exclusive

On 25/04/16 16:18, Mathieu Poirier wrote:
> On 25 April 2016 at 09:11, Suzuki K Poulose <Suzuki.Poulose@....com> wrote:
>> On 25/04/16 16:05, Mathieu Poirier wrote:
>>>
>>> On 25 April 2016 at 08:52, Suzuki K Poulose <Suzuki.Poulose@....com>
>>> wrote:
>>>>
>>>> On 25/04/16 15:48, Mathieu Poirier wrote:
>>>>>
>>>>>
>>>>> On 25 April 2016 at 08:32, Suzuki K Poulose <Suzuki.Poulose@....com>
>>>>> wrote:
>>>>>>
>>>>>>
>>>>>> On 22/04/16 18:14, Mathieu Poirier wrote:
>>>>
>>>>
>>>>
>>>>>>> +       spin_lock_irqsave(&drvdata->spinlock, flags);
>>>>>>> +       if (drvdata->reading) {
>>>>>>> +               ret = -EINVAL;
>>>>>>> +               goto out;
>>>>>>> +       }
>>>>>>> +
>>>>>>> +       val = local_xchg(&drvdata->mode, mode);
>>>>>>> +       /*
>>>>>>> +        * In Perf mode there can be only one writer per sink.  There
>>>>>>> +        * is also no need to continue if the ETR is already operated
>>>>>>> +        * from sysFS.
>>>>>>> +        */
>>>>>>> +       if (val != CS_MODE_DISABLED) {
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>> Could val be CS_MODE_PERF ? In other words, should we be checking :
>>>>>>            if (val == CS_MODE_SYSFS)  instead ?
>>>>>
>>>>>
>>>>>
>>>>> If we check for CS_MODE_SYSFS we also have to check for CS_MODE_PERF,
>>>>> which is two checks rather than a single one with the current
>>>>> solution.
>>>>
>>>>
>>>>
>>>> I am confused now. The comment says, we want to check for sysfs mode and
>>>> don't continue in that case. So, we shouldn't be worried about PERF mode.
>>>
>>>
>>> You are correct about the sysFS part, but the first sentence of the
>>> comment also mention that in perf mode there can only be one writer
>>> per sink.  Otherwise ring buffers for one session would end up with
>>> traces from other ongoing sessions, and that is not taking into
>>> account the buffer management nightmares it would cause.
>>
>>
>> OK, in either case, val == CS_MODE_SYSFS is much better check there, to
>> what we want to do
>
> If we check for SYSFS we also need to check for PERF.  Otherwise
> nothing prevents another session from using the sink buffer, which is
> not supported.

Ah, I got wrong. Sorry for the noise. The current check makes sense.

Suzuki