On Mon, May 25, 2015 at 10:53 AM, Tomeu Vizoso
[off-list ref] 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.

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 registering 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.

Something I'm not completely happy with is that I have had to move the call to
of_platform_populate after all platform drivers have been registered.
Otherwise I don't see how I could register drivers on demand as we don't have
yet each driver's compatible strings.

For machs that don't move of_platform_populate() to a later point, these
patches shouldn't cause any problems but it's not guaranteed that we'll avoid
all the deferred probes as some drivers may not be registered yet.

I have tested this on boards with Tegra, iMX.6 and Exynos SoCs, and these
patches were enough to eliminate all the deferred probes.

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

So, complete drive-by comment (and I won't claim to be a DT expert,
etc, etc, so take this with a few grains of salt), but why not push
the problem to the DT compiler (or a pre-process step that could be
run on existing DT blobs), which generates an optional DT node that is
the recommended probe order?  That seems like it avoids adding
complexity into the early boot code (which seems like a good thing)..

As a bonus, a bootarg (or something like that) which runs through the
recommended probe order in reverse (to continue our current state of
ensuring that EPROBE_DEFER error paths are well tested)

At any rate, for a device like a drm driver that has multiple
sub-components, and depends on various other clk/gpio/regulator/etc
drivers, the current EPROBE_DEFER situation is pretty comical, so any
solution that improves on things is very much welcome :-)

BR,
-R

Regards,

Tomeu

[0] http://lists.freedesktop.org/archives/dri-devel/2014-August/066527.html

[1] https://lkml.org/lkml/2014/5/12/452

Tomeu Vizoso (21):
  regulator: core: Reduce critical area in _regulator_get
  ARM: tegra: Add gpio-ranges property
  ARM: tegra: Register drivers before devices
  ARM: EXYNOS: Register drivers before devices
  ARM i.MX6q: Register drivers before devices
  of/platform: Add of_platform_device_ensure()
  of/platform: Ensure device registration on lookup
  gpio: Probe GPIO drivers on demand
  gpio: Probe pinctrl devices on demand
  regulator: core: Probe regulators on demand
  drm: Probe panels on demand
  drm/tegra: Probe dpaux devices on demand
  i2c: core: Probe i2c master 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

 arch/arm/boot/dts/tegra124.dtsi     |  1 +
 arch/arm/mach-exynos/exynos.c       |  4 +--
 arch/arm/mach-imx/mach-imx6q.c      | 12 ++++-----
 arch/arm/mach-tegra/tegra.c         | 21 ++++++---------
 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              |  3 +++
 drivers/of/platform.c               | 53 +++++++++++++++++++++++++++++++++++++
 drivers/phy/phy-core.c              |  3 +++
 drivers/pinctrl/devicetree.c        |  2 ++
 drivers/power/power_supply_core.c   |  3 +++
 drivers/pwm/core.c                  |  3 +++
 drivers/regulator/core.c            | 45 +++++++++++++++----------------
 drivers/usb/phy/phy.c               |  3 +++
 drivers/video/backlight/backlight.c |  3 +++
 include/linux/of_platform.h         |  2 ++
 19 files changed, 130 insertions(+), 45 deletions(-)

--
2.4.1


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