[alsa-devel] [PATCH v2 0/22] On-demand device probing

[Tomeu Vizoso]

On 30 July 2015 at 05:06, Rob Herring <robherring2 at gmail.com> wrote:
> On Tue, Jul 28, 2015 at 8:19 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
>> 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.
>
>> 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