Re: [PATCH] driver core: Disable late probes by default

[Rob Herring]

On Wed, Oct 21, 2015 at 1:45 PM, Greg Kroah-Hartman
<gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
> On Wed, Oct 21, 2015 at 01:09:55PM -0500, Rob Herring wrote:
>> On Wed, Oct 21, 2015 at 11:06 AM, Greg Kroah-Hartman
>> <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
>> > On Wed, Oct 21, 2015 at 05:53:13PM +0200, Tomeu Vizoso wrote:
>> >> On 21 October 2015 at 17:14, Greg Kroah-Hartman
>> >> <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
>> >> > On Wed, Oct 21, 2015 at 04:35:58PM +0200, Tomeu Vizoso wrote:
>> >> >> On 21 October 2015 at 05:39, Greg Kroah-Hartman
>> >> >> <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
>> >> >> > On Tue, Oct 20, 2015 at 06:17:39PM +0200, Tomeu Vizoso wrote:
>> >> >> >> On 20 October 2015 at 16:05, Greg Kroah-Hartman
>> >> >> >> <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:

[...]

>> >> >> >> Because of that, ChromeOS had to use their own bindings for the panel
>> >> >> >> node so that the panel probe wouldn't be deferred, introducing a
>> >> >> >> sizable delta that is a barrier to rebasing on newer mainline releases
>> >> >> >> and for vendors to upstream their HW adaptation for chrome devices.
>> >> >> >
>> >> >> > 1.5 second delay is crazy (again, my laptop boots to X in less time than
>> >> >> > that),
>> >> >>
>> >> >> 1.5 seconds isn't crazy at all for the kernel to initialize all the
>> >> >> devices in an embedded board. That's the current state of affairs
>> >> >> today.
>> >> >
>> >> > Then someone needs to fix that, that really is crazy.  What takes so
>> >> > long here?  Why aren't you using async probing to do things in parallel
>> >> > when you need to sleep in device probe (I'm hoping you are sleeping in
>> >> > device probe, otherwise that's really broken)?
>> >>
>> >> I'm a bit surprised now. During all the time that I have been pushing
>> >> this forward I have been regularly testing on more than a dozen boards
>> >> with different socs and 1.5 seconds to probe all the devices isn't
>> >> that much. This is basically due to having to wait for the hardware a
>> >> bit here and there, and to the sheer number of devices involved.
>> >>
>> >> Of course people have been looking at speeding up boot on ARM devices
>> >> for years now and this is what we have come with up to now.
>> >>
>> >> > Have you used the tools we have to find where the time is being spent?
>> >>
>> >> Have to recognize that my starting point has been that probe order was
>> >> the cause of the problem and haven't profiled the whole boot process,
>> >> but I don't see how probe ordering would become irrelevant unless we
>> >> got total probing time down to 200ms. And that would give us a
>> >> fabulously fast boot, which I don't think is as realistic as you seem
>> >> to believe.
>> >
>> > So you aren't using the tools that we have today that were created years
>> > ago, to help to reduce boot time problems like this and instead work on
>> > changing the driver core to try to guess at what the real issue is here?
>> >
>> > Come on, until you really know where you are taking so long, how can you
>> > know what you need to fix?  I strongly recommend doing that here first,
>> > that's why those tools were written in the first place.
>>
>> For something everyone is or should be doing for years, there is
>> surprisingly zero information I can find. It is perf timechart you are
>> talking about, right? Everything I find on it is all after userspace
>> starts. I know perf has command line options, but I never could get it
>> to do what I wanted (which was dumping events up until a boot hang).
>
> scripts/bootgraph.pl combined with 'initcall_debug' on the kernel
> command line.  Landed in Linus's tree back in 2008, perf is not needed.

Okay, well that I have used. I was thinking something more granular
than that. It will get you driver probe times, but the problem could
be some underlying dependency causing the actual problem. For example,
a driver enabling its regulator which happens to be connected via a
bit-banged I2C bus. Obviously we can dig down from there, but it is
not as simple as enabling the tool, running it, and instantly
identifying the problem.

Rob
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