Date:	Mon, 30 Jul 2012 10:44:29 -0500
From:	Rob Herring <robherring2@...il.com>
To:	Alexandre Courbot <acourbot@...dia.com>
CC:	Stephen Warren <swarren@...dia.com>,
	Thierry Reding <thierry.reding@...onic-design.de>,
	Simon Glass <sjg@...omium.org>,
	Grant Likely <grant.likely@...retlab.ca>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Mark Brown <broonie@...nsource.wolfsonmicro.com>,
	Arnd Bergmann <arnd@...db.de>, linux-tegra@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-fbdev@...r.kernel.org,
	devicetree-discuss@...ts.ozlabs.org
Subject: Re: [RFC][PATCH v3 1/3] runtime interpreted power sequences

On 07/27/2012 07:05 AM, Alexandre Courbot wrote:
> Some device drivers (panel backlights especially) need to follow precise
> sequences for powering on and off, involving gpios, regulators, PWMs
> with a precise powering order and delays to respect between each steps.
> These sequences are board-specific, and do not belong to a particular
> driver - therefore they have been performed by board-specific hook
> functions to far.
> 
> With the advent of the device tree and of ARM kernels that are not
> board-tied, we cannot rely on these board-specific hooks anymore but
> need a way to implement these sequences in a portable manner. This patch
> introduces a simple interpreter that can execute such power sequences
> encoded either as platform data or within the device tree.
> 

Why not? We'll always have some amount of board code. The key is to
limit parts that are just data. I'm not sure this is something that
should be in devicetree.

Perhaps what is needed is a better way to hook into the driver like
notifiers?

> Signed-off-by: Alexandre Courbot <acourbot@...dia.com>
> ---
>  Documentation/power/power_seq.txt | 120 +++++++++++++++
>  drivers/base/Kconfig              |   4 +
>  drivers/base/Makefile             |   1 +
>  drivers/base/power_seq.c          | 300 ++++++++++++++++++++++++++++++++++++++
>  include/linux/power_seq.h         | 139 ++++++++++++++++++
>  5 files changed, 564 insertions(+)
>  create mode 100644 Documentation/power/power_seq.txt
>  create mode 100644 drivers/base/power_seq.c
>  create mode 100644 include/linux/power_seq.h
> 
> diff --git a/Documentation/power/power_seq.txt b/Documentation/power/power_seq.txt
> new file mode 100644
> index 0000000..aa2ceb5
> --- /dev/null
> +++ b/Documentation/power/power_seq.txt
> @@ -0,0 +1,120 @@
> +Runtime Interpreted Power Sequences
> +-----------------------------------
> +
> +Problem
> +-------
> +One very common board-dependent code is the out-of-driver code that is used to
> +turn a device on or off. For instance, SoC boards very commonly use a GPIO
> +(abstracted to a regulator or not) to control the power supply of a backlight,
> +disabling it when the backlight is not used in order to save power. The GPIO
> +that should be used, however, as well as the exact power sequence that may
> +involve different resources, is board-dependent and thus unknown of the driver.
> +
> +This has been addressed so far by using hooks in the device's platform data that
> +are called whenever the state of the device might reflect a power change. This
> +approach, however, introduces board-dependant code into the kernel and is not
> +compatible with the device tree.
> +
> +The Runtime Interpreted Power Sequences (or power sequences for short) aim at
> +turning this code into platform data or device tree nodes. Power sequences are
> +described using a simple format and run by a simple interpreter whenever needed.
> +This allows to remove the callback mechanism and makes the kernel less
> +board-dependant.
> +
> +Sequences Format
> +----------------
> +Power sequences are a series of sequential steps during which an action is
> +performed on a resource. The supported resources so far are:
> +- GPIOs
> +- Regulators
> +- PWMs
> +
> +Each step designates a resource and the following parameters:
> +- Whether the step should enable or disable the resource,
> +- Delay to wait before performing the action,
> +- Delay to wait after performing the action.
> +
> +Both new resources and parameters can be introduced, but the goal is of course
> +to keep things as simple and compact as possible.
> +
> +The platform data is a simple array of platform_power_seq_step instances, each
> +instance describing a step. The type as well as one of id or gpio members
> +(depending on the type) must be specified. The last step must be of type
> +POWER_SEQ_STOP. Regulator and PWM resources are identified by name. GPIO are
> +identified by number. For example, the following sequence will turn on the
> +"power" regulator of the device, wait 10ms, and set GPIO number 110 to 1:
> +
> +struct platform_power_seq_step power_on_seq[] = {
> +	{
> +		.type = POWER_SEQ_REGULATOR,
> +		.id = "power",
> +		.params = {
> +			.enable = 1,
> +			.post_delay = 10,
> +		},
> +	},
> +	{
> +		.type = POWER_SEQ_GPIO,
> +		.gpio = 110,
> +		.params = {
> +			.enable = 1,
> +		},
> +	},
> +	{
> +		.type = POWER_SEQ_STOP,
> +	},
> +};
> +
> +Usage by Drivers and Resources Management
> +-----------------------------------------
> +Power sequences make use of resources that must be properly allocated and
> +managed. The power_seq_build() function takes care of resolving the resources as
> +they are met in the sequence and to allocate them if needed:
> +
> +power_seq *power_seq_build(struct device *dev, power_seq_resources *ress,
> +			   platform_power_seq *pseq);
> +
> +You will need an instance of power_seq_resources to keep track of the resources
> +that are already allocated. On success, the function returns a devm allocated
> +resolved sequence that is ready to be passed to power_seq_run(). In case of
> +failure, and error code is returned.
> +
> +A resolved power sequence returned by power_seq_build can be run by
> +power_run_run():
> +
> +int power_seq_run(struct device *dev, power_seq *seq);
> +
> +It returns 0 if the sequence has successfully been run, or an error code if a
> +problem occured.
> +
> +Finally, some resources that cannot be allocated through devm need to be freed
> +manually. Therefore, be sure to call power_seq_free_resources() in your device
> +remove function:
> +
> +void power_seq_free_resources(power_seq_resources *ress);
> +
> +Device tree

Bindings need to documented in Documentation/devicetree

> +-----------
> +All the same, power sequences can be encoded as device tree nodes. The following
> +properties and nodes are equivalent to the platform data defined previously:
> +
> +		power-supply = <&mydevice_reg>;
> +		enable-gpio = <&gpio 6 0>;
> +
> +		power-on-sequence {
> +			regulator@0 {
> +				id = "power";
> +				enable;

What do this mean? Isn't this implied for a regulator?

> +				post-delay = <10>;
> +			};
> +			gpio@1 {
> +				id = "enable-gpio";
> +				enable;
> +			};
> +		};

This looks like you designed the platform_data structs first and then
came up with device nodes to mirror the struct.

Rob
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