linux-kernel - Re: [PATCH 1/8] coresight: Fix issue where a source device's helpers aren't disabled

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Date:   Wed, 13 Dec 2023 16:28:49 +0000
From:   Suzuki K Poulose <suzuki.poulose@....com>
To:     James Clark <james.clark@....com>, coresight@...ts.linaro.org
Cc:     Mike Leach <mike.leach@...aro.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Maxime Coquelin <mcoquelin.stm32@...il.com>,
        Alexandre Torgue <alexandre.torgue@...s.st.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-stm32@...md-mailman.stormreply.com
Subject: Re: [PATCH 1/8] coresight: Fix issue where a source device's helpers
 aren't disabled

On 13/12/2023 13:54, James Clark wrote:
> 
> 
> On 12/12/2023 17:44, Suzuki K Poulose wrote:
>> Hi James
>>
>> On 12/12/2023 15:53, James Clark wrote:
>>> The linked commit reverts the change that accidentally used some sysfs
>>> enable/disable functions from Perf which broke the refcounting, but it
>>> also removes the fact that the sysfs disable function disabled the
>>> helpers.
>>
>>
>>>
>>> Add a new wrapper function that does both which is used by both Perf and
>>> sysfs, and label the sysfs disable function appropriately. The naming of
>>> all of the functions will be tidied up later to avoid this happening
>>> again.
>>>
>>> Fixes: 287e82cf69aa ("coresight: Fix crash when Perf and sysfs modes
>>> are used concurrently")
>>
>> But we still don't "enable" the helpers from perf mode with this patch.
>> i.e., we use source_ops()->enable directly. So, I guess this patch
>> doesn't fix a bug as such. But that said, it would be good to
>> enable/disable helpers for sources, in perf mode.
>>
>> Suzuki
> 
> We do, it happens in coresight_enable_path() which Perf uses. I added
> the comment below about that.

Ah, I see. That indeed is a bit confusing. And I think all users of 
coresight_enable_path() enables the source right after. So, I don't
see any point in having it separate. That said, this fix makes sense
and apologies for the confusion. We could may be cleanup the 
enable_path() to enable the source too, in a separate patch.

Suzuki


> 
>   [...]
> 
>>>    +/*
>>> + * Helper function to call source_ops(csdev)->disable and also
>>> disable the
>>> + * helpers.
>>> + *
>>> + * There is an imbalance between coresight_enable_path() and
>>> + * coresight_disable_path(). Enabling also enables the source's
>>> helpers as part
>>> + * of the path, but disabling always skips the first item in the path
>>> (which is
>>> + * the source), so sources and their helpers don't get disabled as
>>> part of that
>>> + * function and we need the extra step here.
>>> + */
> 
> The reason coresight_disable_path() skips the first item is because it's
> used after errors where a path is only partially enabled and it unwinds,
> skipping the last item, because the last item didn't enable.
> 
> It seemed a bit more intrusive to change that, and it's already working.