[PATCH v6 22/22] of/platform: Defer probes of registered devices

[Tomeu Vizoso]

On 22 October 2015 at 23:27, Scott Wood <scottwood at freescale.com> wrote:
> On Thu, 2015-10-22 at 15:04 +0200, Tomeu Vizoso wrote:
>> On 22 October 2015 at 00:51, Scott Wood <scottwood at freescale.com> wrote:
>> > On Wed, 2015-10-21 at 08:44 -0500, Rob Herring wrote:
>> > > On Wed, Oct 21, 2015 at 12:54 AM, Scott Wood <scottwood at freescale.com>
>> > > wrote:
>> > > > On Mon, 2015-09-21 at 16:03 +0200, Tomeu Vizoso wrote:
>> > > > > Instead of trying to match and probe platform and AMBA devices right
>> > > > > after each is registered, delay their probes until
>> > > > > device_initcall_sync.
>> > > > >
>> > > > > This means that devices will start probing once all built-in drivers
>> > > > > have registered, and after all platform and AMBA devices from the DT
>> > > > > have been registered already.
>> > > > >
>> > > > > This allows us to prevent deferred probes by probing dependencies on
>> > > > > demand.
>> > > > >
>> > > > > Signed-off-by: Tomeu Vizoso <tomeu.vizoso at collabora.com>
>> > > > > ---
>> > > > >
>> > > > > Changes in v4:
>> > > > > - Also defer probes of AMBA devices registered from the DT as they
>> > > > > can
>> > > > >   also request resources.
>> > > > >
>> > > > >  drivers/of/platform.c | 11 ++++++++---
>> > > > >  1 file changed, 8 insertions(+), 3 deletions(-)
>> > > >
>> > > > This breaks arch/powerpc/sysdev/fsl_pci.c.  The PCI bus is an OF
>> > > > platform
>> > > > device, and it must be probed before pcibios_init() which is a
>> > > > subsys_initcall(), or else the PCI bus never gets scanned.
>> > >
>> > > Thanks for the report. This is probably getting dropped, but it could
>> > > be disabled for PPC.
>> >
>> > I don't think that adding another arbitrary arch difference would be the
>> > right solution.
>>
>> I think Rob meant temporarily disable it while things get fixed. At
>> least,
>
> So, what is the permanent fix for the swiotlb issue (or more generally, the
> inability to have a late_initcall that runs after non-module, non-hotplug
> platform devices have been probed)?

If the code in pcibios_init() depends on the PCI bus device having
probed, then I would recommend making that dependency explicit by
calling of_device_probe() on the OF node of the PCI controller when
looking it up.

>> I don't see any reason why PPC wouldn't benefit from this
>> series.
>
> It's not clear to me what the benefit of this is at all, much less for PPC.
> What is the fundamental problem with deferred probes?  In the cover letter
> you say this change saves 2.3 seconds, but where is that time being consumed?
>  Are the drivers taking too long in their probe function trying to initialize
> and then deferring, rather than checking for dependencies up front?  Or are
> there really so many devices and such a pessimal ordering that most of the
> time is spent iterating through and reordering the list, with each defer
> happening quickly?

The problem is that a device that defers its probe is currently sent
to the back of the queue, and that's undesired in some use cases in
which there's a device that should be up as soon as possible during
boot (and boot takes a long time). So the goal is to change the order
in which devices with dependencies end up probing.

> Even if something different does need to be done at this level, forcing all
> OF platform devices to be probed at the late_initcall level seems quite
> intrusive.  You limited it to OF because people complained that other things
> will break.  Things still broke.  Surely there's a better way to address the
> problem.  Can't the delay be requested by drivers that might otherwise need
> to defer (which could be done incrementally, focusing on the worst
> performance problems), rather than enabling it for everything?

Yes, given the amount of breakage that's a sensible option. But in any
case and even if this series is most likely to be dropped, I recommend
to make explicit as many implicit dependencies as possible.

Regards,

Tomeu

> -Scott
>
>
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