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SubjectRe: [PATCH v4 0/22] On-demand device probing
On 09/08/2015 02:30 AM, Tomeu Vizoso wrote:
> On 7 September 2015 at 22:50, Rob Herring <> wrote:
>> On Mon, Sep 7, 2015 at 7:23 AM, Tomeu Vizoso <> 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 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]
>>> [1]
>>> [2]
>>> [3]
>>> [4]
>>> [5]
>>> [6]
>>> [7]
>>> 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.

What is the failure? Not booting? Fixing not working would certainly not
be overboard.

>> 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.



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