[PATCH v4 0/22] On-demand device probing

[Tomeu Vizoso]

On 7 September 2015 at 22:50, Rob Herring <robherring2 at gmail.com> wrote:
> On Mon, Sep 7, 2015 at 7:23 AM, Tomeu Vizoso <tomeu.vizoso at collabora.com> wrote:
>> Hello,
>>
>> I have a problem with the panel on my Tegra Chromebook taking longer
>> than expected to be ready during boot (Stéphane Marchesin reported what
>> is basically the same issue in [0]), and have looked into ordered
>> probing as a better way of solving this than moving nodes around in the
>> DT or playing with initcall levels and linking order.
>>
>> While reading the thread [1] that Alexander Holler started with his
>> series to make probing order deterministic, it occurred to me that it
>> should be possible to achieve the same by probing devices as they are
>> referenced by other devices.
>>
>> This basically reuses the information that is already implicit in the
>> probe() implementations, saving us from refactoring existing drivers or
>> adding information to DTBs.
>>
>> During review of v1 of this series Linus Walleij suggested that it
>> should be the device driver core to make sure that dependencies are
>> ready before probing a device. I gave this idea a try [2] but Mark Brown
>> pointed out to the logic duplication between the resource acquisition
>> and dependency discovery code paths (though I think it's fairly minor).
>>
>> To address that code duplication I experimented with Arnd's devm_probe
>> [3] concept of having drivers declare their dependencies instead of
>> acquiring them during probe, and while it worked [4], I don't think we
>> end up winning anything when compared to just probing devices on-demand
>> from resource getters.
>>
>> One remaining objection is to the "sprinkling" of calls to
>> of_device_probe() in the resource getters of each subsystem, but I think
>> it's the right thing to do given that the storage of resources is
>> currently subsystem-specific.
>>
>> We could avoid the above by moving resource storage into the core, but I
>> don't think there's a compelling case for that.
>>
>> I have tested this on boards with Tegra, iMX.6, Exynos, Rockchip and
>> OMAP SoCs, and these patches were enough to eliminate all the deferred
>> probes (except one in PandaBoard because omap_dma_system doesn't have a
>> firmware node as of yet).
>>
>> Have submitted a branch [5] with only these patches on top of thursday's
>> linux-next to kernelci.org and I don't see any issues that could be
>> caused by them. For some reason it currently has more passes than the
>> version of -next it's based on!
>>
>> With this series I get the kernel to output to the panel in 0.5s,
>> instead of 2.8s.
>>
>> Regards,
>>
>> Tomeu
>>
>> [0] http://lists.freedesktop.org/archives/dri-devel/2014-August/066527.html
>>
>> [1] https://lkml.org/lkml/2014/5/12/452
>>
>> [2] https://lkml.org/lkml/2015/6/17/305
>>
>> [3] http://article.gmane.org/gmane.linux.ports.arm.kernel/277689
>>
>> [4] https://lkml.org/lkml/2015/7/21/441a
>>
>> [5] https://git.collabora.com/cgit/user/tomeu/linux.git/log/?h=on-demand-probes-v6
>>
>> [6] http://kernelci.org/boot/all/job/collabora/kernel/v4.2-11902-g25d80c927f8b/
>>
>> [7] http://kernelci.org/boot/all/job/next/kernel/next-20150903/
>>
>> Changes in v4:
>> - Added bus.pre_probe callback so the probes of Primecell devices can be
>>   deferred if their device IDs cannot be yet read because of the clock
>>   driver not having probed when they are registered. Maybe this goes
>>   overboard and the matching information should be in the DT if there is
>>   one.
>
> Seems overboard to me or at least a separate problem.

It's a separate problem but this was preventing the series from
working on a few boards.

> Most clocks have
> to be setup before the driver model simply because timers depend on
> clocks usually.

Yes, but in this case the apb clocks for the primecell devices are
implemented in a normal platform driver (vexpress_osc_driver), instead
of using CLK_OF_DECLARE.

Regards,

Tomeu

> Rob
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