linux-kernel - Re: [PATCH 1/3] perf, pt, coresight: Clean up address filter structure

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Date:   Wed, 1 Feb 2017 14:33:46 -0700
From:   Mathieu Poirier <mathieu.poirier@...aro.org>
To:     Alexander Shishkin <alexander.shishkin@...ux.intel.com>
Cc:     Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Ingo Molnar <mingo@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Vince Weaver <vince@...ter.net>,
        Stephane Eranian <eranian@...gle.com>,
        Arnaldo Carvalho de Melo <acme@...radead.org>,
        Will Deacon <will.deacon@....com>,
        Mark Rutland <mark.rutland@....com>
Subject: Re: [PATCH 1/3] perf, pt, coresight: Clean up address filter structure

)

On 1 February 2017 at 05:46, Alexander Shishkin
<alexander.shishkin@...ux.intel.com> wrote:
> Mathieu Poirier <mathieu.poirier@...aro.org> writes:
>
>> On 27 January 2017 at 05:12, Alexander Shishkin
>> <alexander.shishkin@...ux.intel.com> wrote:
>>> But "range" is not an action, it's a type of a filter. It determines the
>>> condition that triggers an action. An action, however, is what we do
>>> when the condition comes true.
>>
>> Then filter->action could be renamed 'type'.
>
> No. Again, *action* is what we *do*. *Type* is *how* we detect that
> something needs to be done.

If this is what you want to convey then

+ * @action:    filter/start/stop

needs to be fixed.  This can be interpreted as "use range filter,
start filter or stop filter" - which is exactly what I did.  Something
like

+ * @action:    1: start filtering 0: stop filtering

will avoid any confusion.

>
>> In the end filters on PT
>> are range filters, the same way they are on CS.  But changing the
>
> No. The CS driver supports both single address and address range
> filters at least acconding to my reading of the code. Now that I look
> more at it, I see that it also gets the range filters wrong: it
> disregards filter->filter for range filters, assuming that since it's a
> range, it means that the user wants to trace what's in the range
> (filter->filter == 1), but it may also mean "stop if you end up in this
> range" (filter->filter == 0).

Exactly.  The code does the right thing based on my interpretation of
the comment found in the code:

* @range:      1: range, 0: address
* @filter:     1: filter/start, 0: stop

That is @range to determine if we are using a range or an address
filter and @filter to specify what kind of address filter to use
(start or stop).  Ignoring range filters when ->filter == 0 was done
on purpose as I simply couldn't see how to fit it in.

> The fact that the CS driver gets it wrong
> just proves the point that "filter->filter" is confusing and misleading
> and needs to be replaced.
>

I could not agree more.

On the flip side it doesn't change anything to my original argument:
the code should not be made to be smart.  If a range filter is used
then a size of zero should be treated as an error.

To move forward please keep the current functionality on the CS side,
i.e return -EINVAL when a size of zero is used with a range filter.
Once it is queued I'll send a set of patches to support the exclusion
of address ranges.

> In the case of CS, I think that a -EOPNOTSUPP is also appropriate for
> the type==range&&action==stop combination.

That will also be part of said patches.

Thanks,
Mathieu

>
> Regards,
> --
> Alex