On 3 April 2014 14:30, Tomasz Figa [off-list ref] wrote:

Hi Ulf,


On 03.04.2014 14:16, Ulf Hansson wrote:

quoted

On 3 March 2014 17:02, Tomasz Figa [off-list ref] wrote:

quoted

This patch introduces generic code to perform power domain look-up using
device tree and automatically bind devices to their power domains.
Generic device tree binding is introduced to specify power domains of
devices in their device tree nodes.

Backwards compatibility with legacy Samsung-specific power domain
bindings is provided, but for now the new code is not compiled when
CONFIG_ARCH_EXYNOS is selected to avoid collision with legacy code. This
will change as soon as Exynos power domain code gets converted to use
the generic framework in further patch.

Signed-off-by: Tomasz Figa <redacted>
---
  .../devicetree/bindings/power/power_domain.txt     |  51 ++++
  drivers/base/power/domain.c                        | 298
+++++++++++++++++++++
  include/linux/pm_domain.h                          |  46 ++++
  kernel/power/Kconfig                               |   4 +
  4 files changed, 399 insertions(+)
  create mode 100644
Documentation/devicetree/bindings/power/power_domain.txt

diff --git a/Documentation/devicetree/bindings/power/power_domain.txt

b/Documentation/devicetree/bindings/power/power_domain.txt
new file mode 100644
index 0000000..93be5d9

--- /dev/null
+++ b/Documentation/devicetree/bindings/power/power_domain.txt

@@ -0,0 +1,51 @@
+* Generic power domains
+
+System on chip designs are often divided into multiple power domains

that
+can be used for power gating of selected IP blocks for power saving by
+reduced leakage current.
+
+This device tree binding can be used to bind power domain consumer
devices
+with their power domains provided by power domain providers. A power
domain
+provider can be represented by any node in the device tree and can
provide
+one or more power domains. A consumer node can refer to the provider by
+a phandle and a set of phandle arguments (so called power domain
specifier)
+of length specified by #power-domain-cells property in the power domain
+provider node.
+
+==Power domain providers==
+
+Required properties:
+ - #power-domain-cells : Number of cells in a power domain specifier;
+   Typically 0 for nodes representing a single power domain and 1 for
nodes
+   providing multiple power domains (e.g. power controllers), but can be
+   any value as specified by device tree binding documentation of
particular
+   provider.

I am trying to understand if using a value > 1, ever would make sense.
Wouldn't that mean each consumer (device) would actually be a part of
more than one power domain? That won't work, right!?

Not exactly. Each phandle + #power-domain-cells cells are used just for
single power domain.

The cells here are used merely as the identifier used by power domain driver
to translate a power domain specifier from DT to a kernel pointer. It's up
to driver bindings to select the number of cells to properly identify a
power domain.

As an example (from different world, but showing the same mechanism), let's
see a common pattern of GPIO banks on some SoC:

GPA0
GPA1
GPB0
GPB1
GPC0
GPC1

One might assign a single-cell ID to each bank ending with a namespace of
integers from 0 to 5 that would be used as follows:

#define GPA0 0
#define GPA1 1
#define GPB0 2
#define GPB1 3
#define GPC0 4
#define GPC1 5

reset-gpios = <&gpio GPA0 4>;

However one might also consider assigning one cell to bank set (e.g. GPA)
and one cell to identify the bank inside of a set, like on the following
example:

#define GPA 0
#define GPB 1
#define GPC 2

reset-gpios = <&gpio GPA 0 4>;

Good bindings should allow arbitrary identification schemes to let a driver
developer use the one that suits the hardware he's working on.

quoted

Additionally, there are no corresponding parsing method (like
of_genpd_xlate_onecell() ) that support more than one cell.

There are two generic xlate helpers provided for the most common cases that
are likely to be reused by most drivers. For more complex cases it's up to
the driver to implement its own mapping function. It can be promoted to a
generic one later if such need shows up.

Best regards,
Tomasz

Tomasz, thanks for the clarification! I still have more to learn about DT. :-)

Not sure if some additional comments would make this more clear though
- or if it's juts my untrained eye that had a few problems
understanding.

Kind regards
Ulf Hansson