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(-)