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SubjectRe: [PATCH v4 0/22] On-demand device probing
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. Most clocks have
to be setup before the driver model simply because timers depend on
clocks usually.


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