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Subject[PATCH v6 0/22] On-demand device probing

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][6][7] with these patches on top of today's
linux-next (20150921) to and I don't see any issues that
could be caused by them.

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











Changes in v6:
- Drop bus_type.pre_probe and read the periphid in match() instead as
suggested by Alan Stern.
- Merge changes to the regulator subsystem's locking so no references
are leaked between commits.

Changes in v5:
- Set the pointer to struct device also for AMBA devices
- Unset the pointer to struct device when the platform device is about
to be unregistered
- Increase the reference count of the device before returning from
- Move the assignment to device_node->device for AMBA devices to another
- Hold a reference to the struct device while it's in use in
- Use regulator_class' klist of devices instead of regulator_list to
store and lookup regulator devices.

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
- Rename of_platform_probe to of_device_probe
- Use device_node.device instead of device_node.platform_dev
- Add Kconfig DELAY_DEVICE_PROBES to allow disabling delayed probing in
machines with initcalls that depend on devices probing at a given time.
- Start processing deferred probes in device_initcall_sync
- Also defer probes of AMBA devices registered from the DT as they can
also request resources.

Changes in v3:
- Set and use device_node.platform_dev instead of reversing the logic to
find the platform device that encloses a device node.
- Drop the fwnode API to probe firmware nodes and add OF-only API for
now. I think this same scheme could be used for machines with ACPI,
but I haven't been able to find one that had to defer its probes because
of the device probe order.

Tomeu Vizoso (22):
driver core: handle -EPROBE_DEFER from bus_type.match()
ARM: amba: Move reading of periphid to amba_match()
of/platform: Point to struct device from device node
of: add function to allow probing a device from a OF node
gpio: Probe GPIO drivers on demand
gpio: Probe pinctrl devices on demand
regulator: core: Remove regulator_list
regulator: core: Probe regulators on demand
drm: Probe panels on demand
drm/tegra: Probe dpaux devices on demand
i2c: core: Probe i2c adapters and devices on demand
pwm: Probe PWM chip devices on demand
backlight: Probe backlight devices on demand
usb: phy: Probe phy devices on demand
clk: Probe clk providers on demand
pinctrl: Probe pinctrl devices on demand
phy: core: Probe phy providers on demand
dma: of: Probe DMA controllers on demand
power-supply: Probe power supplies on demand
driver core: Allow deferring probes until late init
driver core: Start processing deferred probes earlier
of/platform: Defer probes of registered devices

drivers/amba/bus.c | 88 ++++++------
drivers/base/Kconfig | 18 +++
drivers/base/dd.c | 35 ++++-
drivers/clk/clk.c | 3 +
drivers/dma/of-dma.c | 3 +
drivers/gpio/gpiolib-of.c | 5 +
drivers/gpu/drm/drm_panel.c | 3 +
drivers/gpu/drm/tegra/dpaux.c | 3 +
drivers/i2c/i2c-core.c | 4 +
drivers/of/device.c | 61 +++++++++
drivers/of/platform.c | 30 +++--
drivers/phy/phy-core.c | 3 +
drivers/pinctrl/devicetree.c | 3 +
drivers/power/power_supply_core.c | 3 +
drivers/pwm/core.c | 3 +
drivers/regulator/core.c | 257 +++++++++++++++++++++++-------------
drivers/usb/phy/phy.c | 3 +
drivers/video/backlight/backlight.c | 3 +
include/linux/device.h | 4 +-
include/linux/of.h | 1 +
include/linux/of_device.h | 3 +
21 files changed, 386 insertions(+), 150 deletions(-)


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