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

[Tomeu Vizoso]

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][6][7] with these patches on top of today's
linux-next (20150921) to kernelci.org 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.

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-v8

[6] http://kernelci.org/boot/all/job/collabora/kernel/v4.3-rc2-2587-gf92b0ab33d14/

[7] http://kernelci.org/boot/all/job/next/kernel/next-20150921

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
  of_find_device_by_node()
- Move the assignment to device_node->device for AMBA devices to another
  commit.
- Hold a reference to the struct device while it's in use in
  of_device_probe().
- 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
  one.
- 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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2.4.3