Date:	Thu, 18 Jun 2015 11:42:01 +0200
From:	Andrzej Hajda <a.hajda@...sung.com>
To:	Tomeu Vizoso <tomeu.vizoso@...labora.com>,
	linux-arm-kernel@...ts.infradead.org
Cc:	Alexander Holler <holler@...oftware.de>,
	Alexandre Courbot <gnurou@...il.com>,
	Arnd Bergmann <arnd@...db.de>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	Grant Likely <grant.likely@...aro.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Javier Martinez Canillas <javier.martinez@...labora.co.uk>,
	Krzysztof Kozlowski <k.kozlowski@...sung.com>,
	Kumar Gala <galak@...eaurora.org>, Len Brown <lenb@...nel.org>,
	Linus Walleij <linus.walleij@...aro.org>,
	linux-kernel@...r.kernel.org, Lv Zheng <lv.zheng@...el.com>,
	Mark Brown <broonie@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	Pawel Moll <pawel.moll@....com>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Robert Moore <robert.moore@...el.com>,
	Rob Herring <robh+dt@...nel.org>,
	Russell King <linux@....linux.org.uk>,
	Stephen Warren <swarren@...dotorg.org>,
	Terje Bergström 
	<tbergstrom@...dia.com>, Thierry Reding <thierry.reding@...il.com>
Subject: Re: [PATCH 00/13] Discover and probe dependencies

Hi Tomeu,

I have few comments about the design.

On 06/17/2015 03:42 PM, Tomeu Vizoso wrote:
> Hi,
>
> this is another attempt at preventing deferred probe from obscuring why your
> devices aren't probing and from delaying to the end of the boot process the
> probe of the device you care the most.
>
> The major differences with my previous approach [0] are:
>
> * Dependencies are probed before the target is probed, so we don't have nested
>   probe() calls, avoiding a series of deadlock situations.
>
> * Dependencies could be stored and reused for other purposes such as for
>   passing resources to drivers ala devm_probe, or for warning when a device is
>   going to be unbound and has dependencies active, etc.

With this approach we should assume many things, for example:
1. Dependencies are explicitly described in firmware (dts/dtb).
    It will not work for example with lookup tables present in
gpios/clocks/regulators.
2. Provider create/register their resources only during probe.
    It is not always the case - for example componentized drivers in
probe often
    calls only component_add, the real initialization is performed in
bind callback.
3. Dependencies are mandatory, ie without it driver will not be able to
successfully finish
the probe.
    It should be not true. Sometimes device will require given resource
only in specific
    scenario, or it can still probe successfully and ask for the
resource later.
    I can also imagine that firmware can describe more information than
given driver require,
    some resources even if they are present in the dts, will be not
requested by the driver, it
    can be the case of drivers providing limited functionality, or just
obsolete bindings.
  

I have also more general design objection, which should not be
necessarily true:
Device node describes piece of the hardware which should be mainly
interpreted by the driver.
Parsing it in external code [1] violates this idea. Additionally we will
have the same information
parsed and interpreted in two different places (discovery framework and
the driver),
it does not look good to me.
But as I said earlier it is just my opinion, not a solid evidence :)

[1]: I know that for example clk_get is also located in external
framework but currently the driver
decides that it should call clk_get, my_private_clk_get,
other_framework_clk_get or do not call it at all,
this framework assumes that it will be always clk_get, or at least
something compatible with it at binding level.

Regards
Andrzej


>
> * I have tried to keep it firmware-agnostic. The previous approach (on-demand
>   probing) could be done like this as well, but would require adding fwnode
>   APIs to all affected subsystems first.
>
> I have only implemented the class.get_dependencies() callback for the GPIO
> subsystem and for the host1x bus because that's all that was needed on my Tegra
> Chromebook to avoid deferred probes, but if this approach is deemed worthwhile
> I will add more implementations so that deferred probes are avoided on the
> other boards I have access to.
>
> [0] http://thread.gmane.org/gmane.linux.kernel.gpio/8465
>
> Thanks,
>
> Tomeu
>
> Tomeu Vizoso (13):
>   gpiolib: Fix docs for gpiochip_add_pingroup_range
>   driver-core: defer all probes until late_initcall
>   ARM: tegra: Add gpio-ranges property
>   pinctrl: tegra: Only set the gpio range if needed
>   driver core: fix docbook for device_private.device
>   of/platform: Set fwnode field for new devices
>   driver-core: Add class.get_dependencies() callback
>   gpio: sysfs: implement class.get_dependencies()
>   gpu: host1x: implement class.get_dependencies()
>   driver-core: add for_each_class()
>   device property: add fwnode_get_parent()
>   device property: add fwnode_get_name()
>   driver-core: probe dependencies before probing
>
>  arch/arm/boot/dts/tegra114.dtsi |   1 +
>  arch/arm/boot/dts/tegra124.dtsi |   1 +
>  arch/arm/boot/dts/tegra20.dtsi  |   1 +
>  arch/arm/boot/dts/tegra30.dtsi  |   1 +
>  drivers/base/base.h             |   4 +-
>  drivers/base/class.c            |  16 +++++
>  drivers/base/dd.c               | 128 +++++++++++++++++++++++++++++++++++++++-
>  drivers/base/property.c         |  38 ++++++++++++
>  drivers/gpio/gpiolib-sysfs.c    |  81 +++++++++++++++++++++++++
>  drivers/gpio/gpiolib.c          |   2 +-
>  drivers/gpu/host1x/dev.c        |  47 +++++++++++++++
>  drivers/of/platform.c           |   1 +
>  drivers/pinctrl/pinctrl-tegra.c |  19 +++++-
>  include/acpi/acpi_bus.h         |   5 ++
>  include/linux/acpi.h            |   5 ++
>  include/linux/device.h          |   6 ++
>  include/linux/fwnode.h          |   5 ++
>  include/linux/property.h        |   4 ++
>  18 files changed, 361 insertions(+), 4 deletions(-)
>

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