linux-kernel - Re: [PATCH 11/17] coresight etr: Handle driver mode specific ETR buffers

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Date:   Fri, 3 Nov 2017 14:30:23 -0600
From:   Mathieu Poirier <mathieu.poirier@...aro.org>
To:     Suzuki K Poulose <Suzuki.Poulose@....com>
Cc:     "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Robert Walker <robert.walker@....com>,
        Mike Leach <mike.leach@...aro.org>, coresight@...ts.linaro.org
Subject: Re: [PATCH 11/17] coresight etr: Handle driver mode specific ETR buffers

On 3 November 2017 at 04:08, Suzuki K Poulose <Suzuki.Poulose@....com> wrote:
> On 02/11/17 20:26, Mathieu Poirier wrote:
>>
>> On Thu, Oct 19, 2017 at 06:15:47PM +0100, Suzuki K Poulose wrote:
>>>
>>> Since the ETR could be driven either by SYSFS or by perf, it
>>> becomes complicated how we deal with the buffers used for each
>>> of these modes. The ETR driver cannot simply free the current
>>> attached buffer without knowing the provider (i.e, sysfs vs perf).
>>>
>>> To solve this issue, we provide:
>>> 1) the driver-mode specific etr buffer to be retained in the drvdata
>>> 2) the etr_buf for a session should be passed on when enabling the
>>>     hardware, which will be stored in drvdata->etr_buf. This will be
>>>     replaced (not free'd) as soon as the hardware is disabled, after
>>>     necessary sync operation.
>>
>>
>> If I get you right the problem you're trying to solve is what to do with a
>> sysFS
>> buffer that hasn't been read (and freed) when a perf session is requested.
>> In
>> my opinion it should simply be freed.  Indeed the user probably doesn't
>> care
>> much about that sysFS buffer, if it did the data would have been
>> harvested.
>
>
> Not only that. If we simply use the drvdata->etr_buf, we cannot track the
> mode
> which uses it. If we keep the etr_buf around, how do the new mode user
> decide
> how to free the existing one ? (e.g, the perf etr_buf could be associated
> with
> other perf data structures). This change would allow us to leave the
> handling
> of the etr_buf to its respective modes.

struct etr_buf has a 'mode' and an '*ops', how is that not sufficient?
 I'll try to finish reviewing your patches today, maybe I'll find the
answer later on...

>
> And whether to keep the sysfs etr_buf around is a separate decision from the
> above.
>
>
> Cheers
> Suzuki