linux-kernel - Re: [PATCH 00/10] perf tools: Add support for CoreSight trace decoding

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Date:   Thu, 11 Jan 2018 12:23:17 +0000
From:   Mark Brown <broonie@...nel.org>
To:     Kim Phillips <kim.phillips@....com>
Cc:     Mathieu Poirier <mathieu.poirier@...aro.org>, acme@...nel.org,
        peterz@...radead.org, mingo@...hat.com,
        alexander.shishkin@...ux.intel.com, namhyung@...nel.org,
        adrian.hunter@...el.com, mike.leach@....com,
        suzuki.poulosi@....com, tor@...com, jolsa@...hat.com,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 00/10] perf tools: Add support for CoreSight trace
 decoding

On Wed, Jan 10, 2018 at 06:08:21PM -0600, Kim Phillips wrote:
> Mathieu Poirier <mathieu.poirier@...aro.org> wrote:

> > Instructions on how to build and install the openCSD library are provided
> > in the HOWTO.md of the project repository.  

> Usually when a perf builder sees something they need "on," they - or,
> at least I - start querying the host's package manager for something
> that provides it (e.g., apt search/install libopencsd), but since no
> distro provides libopencsd, this is bad because it misleads the user.

It's on the radar to push this at distros fairly soon.  Part of the
discussion was wanting to get things to the point where the tools using
the library were far enough along that we could be reasonably sure that
there weren't any problems that were going to require ABI breaks to fix
before pushing the library at distros since ABI churn isn't nice for
packagers to deal with.  There's also a bit of a chicken and egg problem
in that it's a lot easier to get distros to package libraries that have
users available (some are not really bothered about this of course but
it still helps).  

> Keeping the library external will also inevitably introduce more
> source level synchronization problems because the perf sources being
> built may not be compatible with their version of the library, whether
> due to new features like new trace hardware support, or API changes.

Perf users installing from source rather than from a package (who do
tend to the more technical side even for kernel developers) already have
to cope with potentially installing at least dwarf, gtk2, libaudit,
libbfd, libelf, libnuma, libperl, libpython, libslang, libcrypto,
libunwind, libdw-dwarf-unwind, zlib, lzma, bpf and OpenJDK depending on
which features they want.  I'm not sure that adding one more library is
going to be the end of the world here, especially once the packaging
starts to filter through distros.  Until that happens at least people
are no worse off for not having the feature.

> As Mark Brown (cc'd) mentioned on the Coresight mailing list, this may
> be able to be done the same way the dtc is incorporated into the
> kernel, where only its relevant sources are included and updated as
> needed:  see linux/scripts/dtc/update-dtc-source.sh.

Bear in mind that we need dtc for essentially all kernel development on
ARM and when it was introduced it was a new requirement for existing
systems, it's a bit of a different case here where it's an optional
feature in an optional tool.

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