On 30 July 2015 at 05:06, Rob Herring [off-list ref] wrote:

On Tue, Jul 28, 2015 at 8:19 AM, Tomeu Vizoso
[off-list ref] wrote:

quoted

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.

Seems like a minor change to me.

quoted

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).

With this series I get the kernel to output to the panel in 0.5s,
instead of 2.8s.

Generally, I think this looks pretty good. It is simple and the error
path is simply falling back to deferred probe.

One overall comment is I'm not so sure if fwnode_ensure_device
shouldn't just be of_ensure_device. At least currently, it looks like
all the calling locations are DT specific functions anyway. There's
very little logic within the function to really benefit sharing with
ACPI. It is basically just a call to of_platform_device_find and then
bus_probe_device. I expect the get functions will always call into
DT/ACPI specific functions which can then call the firmware specific
device find function.

That's fine with me. I just went that way because I assumed the plan
was for subsystems to move to consume fw data through fwnode and drop
as much fw-specific code as possible.

But I have just looked at fwnode_get_named_gpiod and the OF and ACPI
code paths are so dissimilar that I guess that's not so and would be
better to do as you say.

Thanks,

Tomeu

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