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From: Stephen Warren <>
To: Alexandre Courbot <>
Cc: Stephen Warren <>,
	Thierry Reding <>,
	Simon Glass <>,
	Grant Likely <>,
	Rob Herring <>,
	Mark Brown <>,
	Anton Vorontsov <>,
	David Woodhouse <>,
	Arnd Bergmann <>,
	Leela Krishna Amudala <>,,,,,,
Subject: Re: [PATCH v5 1/4] Runtime Interpreted Power Sequences
Date: Wed, 05 Sep 2012 11:19:45 -0600	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

On 08/31/2012 05:34 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.

> +++ b/Documentation/devicetree/bindings/power_seq/power_seq.txt

> +Runtime Interpreted Power Sequences
> +===================================
> +
> +Power sequences are sequential descriptions of actions to be performed on
> +power-related resources. Having these descriptions in a precise data format
> +allows us to take much of the board-specific power control code out of the
> +kernel and place it into the device tree instead, making kernels less
> +board-dependant.
> +

> +In the device tree, power sequences are grouped into a set. The set is always
> +declared as the "power-sequences" sub-node of the device node. Power sequences
> +may reference resources declared by that device.

I had to read that a few times to realize this was talking about a
device with multiple power sequences, and not talking about the steps in
a sequence. I think if you add the following sentence at the start of
that paragraph, it will be clearer:

A device may support multiple power sequences, for different events such
as powering on and off.

Also, perhaps add "these" at the first comma, so the above would read:

In the device tree, these power sequences are...

> +Power Sequences Structure
> +-------------------------
> +Every device that makes use of power sequences must have a "power-sequences"
> +sub-node. Power sequences are sub-nodes of this set node, and their node name
> +indicates the id of the sequence.
> +
> +Every power sequence in turn contains its steps as sub-nodes of itself. Step

The last word on that line should be "Steps".

> +must be named sequentially, with the first step named step0, the second step1,
> +etc. Failure to follow this rule will result in a parsing error.
> +
> +Power Sequences Steps
> +---------------------
> +Step of a sequence describes an action to be performed on a resource. They

s/Step/Steps/, s/describes/describe/.

> +always include a "type" property which indicates what kind of resource this
> +step works on. Depending on the resource type, additional properties are defined
> +to control the action to be performed.
> +
> +"delay" type required properties:
> +  - delay_us: delay to wait in microseconds

DT property name should use "-" not "_" to separate words, so "delay-us".

> +"regulator" type required properties:
> +  - id: name of the regulator to use. Regulator is obtained by
> +        regulator_get(dev, id)
> +  - enable / disable: one of these two empty properties must be present to
> +                      enable or disable the resource
> +
> +"pwm" type required properties:
> +  - id: name of the PWM to use. PWM is obtained by pwm_get(dev, id)
> +  - enable / disable: one of these two empty properties must be present to
> +                      enable or disable the resource

For those two, would "name" be a better property name than "id"?

I wonder if we should have "regulator-enable" and "regulator-disable"
step types, rather than a "regulator" step type, with an "enable" or
"disable" property within it. I don't feel strongly though; this is
probably fine.

> +"gpio" type required properties:
> +  - number: phandle of the GPIO to use.

Naming the property "gpio" would seem more consistent with our GPIO

> +  - enable / disable: one of these two empty properties must be present to
> +                      enable or disable the resource

You can't really enable or disable a GPIO (well, perhaps you can, but
I'd consider that to affect tri-state rather than value); it's more that
you're setting the output value to 0 or 1. I think a "value" or
"set-value" property with value <0> or <1> would be better.

> +After the resources declaration, two sequences follow for powering the backlight
> +on and off. Their names are specified by the pwm-backlight driver.

Not the driver, but the binding for the device.

Overall, the general structure of the bindings looks reasonable to me.

> +++ b/Documentation/power/power_seq.txt

> +Platform Data Format
> +--------------------
> +All relevant data structures for declaring power sequences are located in
> +include/linux/power_seq.h.
> +
> +The platform data for a given device is an instance of platform_power_seq_set
> +which points to instances of platform_power_seq. Every platform_power_seq is a
> +single power sequence, and is itself composed of a variable length array of
> +steps.

I don't think you can mandate that the entire platform data structure
for a device is a platform_power_seq_set. Instead, I think you should
say that "The platform data for a device may contain an instance of

> +A power sequence can be executed by power_seq_run:
> +
> +  int power_seq_run(struct power_seq *seq);
> +
> +It returns 0 if the sequence has successfully been run, or an error code if a
> +problem occured.


> +Sometimes, you may want to browse the list of resources allocated by a sequence,
> +to for instance ensure that a resource of a given type is present. The
> +power_seq_set_resources() function returns a list head that can be used with
> +the power_seq_for_each_resource() macro to browse all the resources of a set:
> +
> +  struct list_head *power_seq_set_resources(struct power_seq_set *seqs);
> +  power_seq_for_each_resource(pos, seqs)
> +
> +Here "pos" will be of type struct power_seq_resource. This structure contains a
> +"pdata" pointer that can be used to explore the platform data of this resource,
> +as well as the resolved resource, if applicable.

I'm not sure what "pdata" is supposed to point at; platform data applies
to the original "struct device", not one of the resources used by the
power sequences.

> +Finally, users of the device tree can build the platform data corresponding to
> +the tree node using this function:
> +
> +  struct platform_power_seq_set *devm_of_parse_power_seq_set(struct device *dev);

Hmmm. It'd be nice not to need separate functions for the non-DT and DT
cases. That would require that devm_power_seq_set_build() be able to
find the power sequence definitions somewhere other than platform data
in the non-DT case - that's exactly why the regulator and pinctrl
subsystems represent the device<->data mapping table separately from the
device's platform data.

> +++ b/drivers/power/power_seq/Kconfig

> +config POWER_SEQ
> +	bool

Some kind of help text might be useful?

I didn't review any of the .c files.

> +++ b/include/linux/power_seq.h

> +/**
> + * struct platform_power_seq_step - platform data for power sequences steps
> + * @type:	The type of this step. This decides which member of the union is
> + *		valid for this step.
> + * @delay:	Used if type == POWER_SEQ_DELAY
> + * @regulator:	Used if type == POWER_SEQ_REGULATOR
> + * @pwm:	Used if type == POWER_SEQ_PWN
> + * @gpio:	Used if type == POWER_SEQ_GPIO

In those last 4 line, I think s/type/@type/ since you're referencing
another parameter?

> +struct power_seq_resource {
> +	/* relevant for resolving the resource and knowing its type */
> +	struct platform_power_seq_step *pdata;

Aha. So this isn't really platform data for the resource, but actually a
step definition that referenced it. I think it'd be better to rename
this field "step", and amend the documentation above not to refer to
"pdata" but explicitly talk about a step definition; the step may have
been defined in pdata, but isn't in the DT case.

Alternatively, why not just copy the step type enum here, rather than
referencing the step definition?

  reply	other threads:[~2012-09-05 17:19 UTC|newest]

Thread overview: 16+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2012-08-31 11:34 [PATCH v5 0/4] " Alexandre Courbot
2012-08-31 11:34 ` [PATCH v5 1/4] " Alexandre Courbot
2012-09-05 17:19   ` Stephen Warren [this message]
2012-09-07  8:21     ` Alex Courbot
2012-09-06 14:14   ` Heiko Stübner
2012-09-07  8:04     ` Alex Courbot
2012-09-07  8:15       ` Mark Brown
2012-09-07  9:08       ` Heiko Stübner
2012-09-07 16:36         ` Stephen Warren
2012-08-31 11:34 ` [PATCH v5 2/4] pwm_backlight: use power sequences Alexandre Courbot
2012-09-05 17:25   ` Stephen Warren
2012-09-07  8:28     ` Alex Courbot
2012-09-07  8:29       ` Mark Brown
2012-09-07  8:34         ` Alex Courbot
2012-08-31 11:34 ` [PATCH v5 3/4] tegra: dt: add label to tegra20's PWM Alexandre Courbot
2012-08-31 11:34 ` [PATCH v5 4/4] tegra: ventana: add pwm backlight DT nodes Alexandre Courbot

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