linux-kernel - Re: [GIT PULL] On-demand device probing

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Message-ID: <CAL_Jsq+Due3CgV0Z_m4vYQsa+X30DSxgmqz_r=26OrBV7v+H2g@mail.gmail.com>
Date:	Sat, 17 Oct 2015 10:04:55 -0500
From:	Rob Herring <robh+dt@...nel.org>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:	Tomeu Vizoso <tomeu.vizoso@...labora.com>,
	Russell King <linux@....linux.org.uk>,
	Michael Turquette <mturquette@...libre.com>,
	Stephen Boyd <sboyd@...eaurora.org>,
	Vinod Koul <vinod.koul@...el.com>,
	Dan Williams <dan.j.williams@...el.com>,
	Linus Walleij <linus.walleij@...aro.org>,
	Alexandre Courbot <gnurou@...il.com>,
	Thierry Reding <thierry.reding@...il.com>,
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Subject: Re: [GIT PULL] On-demand device probing

On Sat, Oct 17, 2015 at 1:57 AM, Greg Kroah-Hartman
<gregkh@...uxfoundation.org> wrote:
> On Wed, Oct 14, 2015 at 10:34:00AM +0200, Tomeu Vizoso wrote:
>> Hi Rob,
>>
>> here is the pull request you asked for, with no changes from the version
>> that I posted last to the list.
>>
>> The following changes since commit 6ff33f3902c3b1c5d0db6b1e2c70b6d76fba357f:
>>
>> Linux 4.3-rc1 (2015-09-12 16:35:56 -0700)
>>
>> are available in the git repository at:
>>
>> git+ssh://git.collabora.co.uk/git/user/tomeu/linux.git
>> on-demand-probes-for-next
>
> That's not a signed tag :(
>
> Anyway, I REALLY don't like this series (sorry for the delay in
> reviewing them, normally I trust Rob's judgement...)

We've seen a lot of attempts here. This is really the best solution so
far in that it is simple, uses existing data from DT, and was low risk
for breaking platforms (at least I thought it would be). Anyway,
getting more exposure is why I've put it into -next.

> I can't see adding calls like this all over the tree just to solve a
> bus-specific problem, you are adding of_* calls where they aren't
> needed, or wanted, at all.

I think Linus W, Mark B, and I all said a similar thing initially in
that dependencies should be handled in the driver core. We went down
the path of making this not firmware (aka bus) specific and an earlier
version had just that (with fwnode_* calls). That turned out to be
pointless as the calling locations were almost always in DT specific
code anyway. If you notice, the calls are next to other DT specific
calls generally (usually a "get"). So yes, I'd prefer not to have to
touch every subsystem, but we had to do that anyway to add DT support.

We've generally split the DT code into the core (in drivers/of) and
the binding specific (in subsystems). Extracting dependency
information the DT is going to require binding specific knowledge, so
subsystem changes are probably unavoidable.

The alternative is we put binding specific knowledge into the core DT
code to parse dependencies.

> What is the root-problem of your delay in device probing?  I read your
> last patch series and I can't seem to figure out what the issue is that
> this is solving in any "better" way from the existing deferred probing.

It saves 2 seconds in the boot time as re-probing takes time. That
alone seems compelling to me.

Another downside to deferred probing is you have to touch every driver
and subsystem to support it. This contains the problem to the
subsystems.

Rob
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