linux-kernel - Re: [PATCH 5/9] perf: Adding sysfs group format attribute for pmu device

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Date:	Fri, 27 Jan 2012 22:19:42 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Corey Ashford <cjashfor@...ux.vnet.ibm.com>
Cc:	Jiri Olsa <jolsa@...hat.com>, acme@...hat.com, mingo@...e.hu,
	paulus@...ba.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 5/9] perf: Adding sysfs group format attribute for pmu
 device

On Fri, 2012-01-27 at 13:08 -0800, Corey Ashford wrote:
> s an example, the IBM PowerEN processor has roughly 20 different PMU's
> on it.  Some of those PMU's are quite complex and divide their events up
> into subsets, each with different fields.  For example, some events may
> have a PID matching field, and others may have an bus number matching
> field, or matching mode field, etc.  The fields are different widths,
> and may overlap in the config/1/2 space.
> 
> It seems that there are two approaches you could take:
> 
> 1) Keep your format, but allow the fields to overlap in the bit space.
> For example:
> 
>   "/sys/...<dev>/format/event" contains "config:0-7"
>   "/sys/...<dev>/format/pidmatch" contains "config:8-15"
>   "/sys/...<dev>/format/busmatch"   contains "config:8-13"
> 
> Note that busmatch overlaps pidmatch
> 
> 2) Create event groups that have their overlapping config space
> separated out:
> 
>   "/sys/...<dev>/format/event" contains "config:0-7"
> 
>   "/sys/...<dev>.1/format/pidmatch" contains "config:8-15"
> 
>   "/sys/...<dev>.2/format/busmatch"   contains "config:8-13"
> 
> 
> Notice the .1 and .2 on the <dev>.
> 
> This might help the user understand which fields go together.  I'm not
> sold on the .1 syntax... you could do it as <dev>.<event-group-name>/ or
> <dev>/<event-group-name>/... or whatever seems to make the most sense
> and is relatively easy to implement and use. 

Why try and stuff those 20 in a single driver? Have 20 drivers and each
their own format/ hierarchy.
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