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Subject[RFC PATCH 0/8] coresight: Update device tree bindings
Date
Coresight uses DT graph bindings to describe the connections of the
components. However we have some undocumented usage of the bindings
to describe some of the properties of the connections.

The coresight driver needs to know the hardware ports invovled
in the connection and the direction of data flow to effectively
manage the trace sessions. So far we have relied on the "port"
address (as described by the generic graph bindings) to represent
the hardware port of the component for a connection.

The hardware uses separate numbering scheme for input and output
ports, which implies, we could have two different (input and output)
ports with the same port number. This could create problems in the
graph bindings where the label of the port wouldn't match the address.

e.g, with the existing bindings we get :

port@0{ // Output port 0
reg = <0>;
...
};

port@1{
reg = <0>; // Input port 0
endpoint {
slave-mode;
...
};
};

With the new enforcement in the DT rules, mismatches in label and address
are not allowed (as see in the case for port@1). So, we need a new mechanism
to describe the hardware port number reliably.

Also, we relied on an undocumented "slave-mode" property (see the above
example) to indicate if the port is an input port. Let us formalise and
switch to a new property to describe the direction of data flow.

There were three options considered for the hardware port number scheme:

1) Use natural ordering in the DT to infer the hardware port number.
i.e, Mandate that the all ports are listed in the DT and in the ascending
order for each class (input and output respectively).
Pros :
- We don't need new properties and if the existing DTS list them in
order (which most of them do), they work out of the box.
Cons :
- We must list all the ports even if the system cannot/shouldn't use
it.
- It is prone to human errors (if the order is not kept).

2) Use an explicit property to list both the direction and the hw port
number and direction. Define "coresight,hwid" as 2 member array of u32,
where the members are port number and the direction respectively.
e.g

port@0{
reg = <0>;
endpoint {
coresight,hwid = <0 1>; // Port # 0, Output
}
};

port@1{
reg = <1>;
endpoint {
coresight,hwid = <0 0>; // Port # 0, Input
};
};

Pros:
- The bindings are formal but not so reader friendly and could potentially
lead to human errors.
Cons:
- Backward compatiblity is lost.
3) Use explicit properties (implemented in the series) for the hardware
port id and direction. We define a new property "coresight,hwid" for
each endpoint in coresight devices to specify the hardware port number
explicitly. Also use a separate property "direction" to specify the
direction of the data flow.

e.g,

port@0{
reg = <0>;
endpoint {
direction = <1>; // Output
coresight,hwid = <0>; // Port # 0
}
};

port@1{
reg = <1>;
endpoint {
direction = <0>; // Input
coresight,hwid = <0>; // Port # 0
};
};

Pros:
- The bindings are formal and reader friendly, and less prone to errors.
Cons:
- Backward compatibility is lost.


This series achieves implements Option (3) listed above while still retaining
the backward compatibility. The driver now issues a warning (once) when it
encounters the old bindings.
It also cleans up the platform parsing code to reduce the memory usage by
reusing the platform description. The series also includes the
changes for Juno platform as an example. If there are no objections
to the approach, I could post the series, converting all the
in-kernel DTS to the new binding.

Suzuki K Poulose (8):
dts: binding: coresight: Document graph bindings
coresight: Fix remote endpoint parsing
coresight: Cleanup platform description data
coresight: platform: Cleanup coresight connection handling
coresight: Handle errors in finding input/output ports
dts: coresight: Clean up the device tree graph bindings
dts: coresight: Define new bindings for direction of data flow
dts: juno: Update coresight bindings for hw port

.../devicetree/bindings/arm/coresight.txt | 52 ++++++++--
arch/arm64/boot/dts/arm/juno-base.dtsi | 82 +++++++++++----
arch/arm64/boot/dts/arm/juno.dts | 5 +-
drivers/hwtracing/coresight/coresight.c | 28 ++----
drivers/hwtracing/coresight/of_coresight.c | 111 ++++++++++++---------
include/linux/coresight.h | 11 +-
6 files changed, 181 insertions(+), 108 deletions(-)

--
2.7.4

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