Re: [PATCH v3 0/18] On-demand device probing

[Rob Herring <robherring2@gmail.com>]

On Thu, Aug 6, 2015 at 9:11 AM, Tomeu Vizoso <tomeu.vizoso@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
> fwnode_ensure_device() 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 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 these patches to kernelci.org and I'm
> currently trying to fix all regressions, usually due to code assuming
> that devices will be probed in a specific order. Current results [6] are
> 348 passes, 30 fails and 42 unknowns (linux-next [7] is currently
> 387/3/23).

This is a bit worrying. If this causes a high number of boot failures,
fixing the errors you can find is not the path forward as we can't
test a lot of platforms (and many people don't look at -next). We may
want to put this behind a kconfig option so that we can easily restore
old behavior it needed. Otherwise, we could have to revert the series.

Are all the commits before this series fixing boot failures? You can't
do dts updates as the fix or backwards compatibility will be broken.

>
> 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-v5
>
> [6] http://kernelci.org/boot/all/job/collabora/kernel/v4.2-rc5-6548-g632b98c83840/
>
> [7] http://kernelci.org/boot/all/job/next/kernel/next-20150806/
>
> Changes in v3:
> - Only delay platform devices with OF nodes
> - Set and use device_node.platform_dev instead of reversing the logic to
>   find the platform device that encloses a device node.

I still want this to be a struct device and not a struct
platform_device and am not convinced it can't be. It can simply be an
optimization of the existing function:

struct platform_device *of_find_device_by_node(struct device_node *np)
{
  if (node->device && node->device->bus == &platform_bus_type)
    return to_platform_device(node->device);
  return NULL;
}

Rob
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