Generic System Interconnect Subsystem

Introduction

This framework is designed to provide a standard kernel interface to control the settings of the interconnects on an SoC. These settings can be throughput, latency and priority between multiple interconnected devices or functional blocks. This can be controlled dynamically in order to save power or provide maximum performance.

The interconnect bus is hardware with configurable parameters, which can be set on a data path according to the requests received from various drivers. An example of interconnect buses are the interconnects between various components or functional blocks in chipsets. There can be multiple interconnects on an SoC that can be multi-tiered.

Below is a simplified diagram of a real-world SoC interconnect bus topology.

+----------------+    +----------------+
| HW Accelerator |--->|      M NoC     |<---------------+
+----------------+    +----------------+                |
                        |      |                    +------------+
 +-----+  +-------------+      V       +------+     |            |
 | DDR |  |                +--------+  | PCIe |     |            |
 +-----+  |                | Slaves |  +------+     |            |
   ^ ^    |                +--------+     |         |   C NoC    |
   | |    V                               V         |            |
+------------------+   +------------------------+   |            |   +-----+
|                  |-->|                        |-->|            |-->| CPU |
|                  |-->|                        |<--|            |   +-----+
|     Mem NoC      |   |         S NoC          |   +------------+
|                  |<--|                        |---------+    |
|                  |<--|                        |<------+ |    |   +--------+
+------------------+   +------------------------+       | |    +-->| Slaves |
  ^  ^    ^    ^          ^                             | |        +--------+
  |  |    |    |          |                             | V
+------+  |  +-----+   +-----+  +---------+   +----------------+   +--------+
| CPUs |  |  | GPU |   | DSP |  | Masters |-->|       P NoC    |-->| Slaves |
+------+  |  +-----+   +-----+  +---------+   +----------------+   +--------+
          |
      +-------+
      | Modem |
      +-------+

Terminology

Interconnect provider is the software definition of the interconnect hardware. The interconnect providers on the above diagram are M NoC, S NoC, C NoC, P NoC and Mem NoC.

Interconnect node is the software definition of the interconnect hardware port. Each interconnect provider consists of multiple interconnect nodes, which are connected to other SoC components including other interconnect providers. The point on the diagram where the CPUs connect to the memory is called an interconnect node, which belongs to the Mem NoC interconnect provider.

Interconnect endpoints are the first or the last element of the path. Every endpoint is a node, but not every node is an endpoint.

Interconnect path is everything between two endpoints including all the nodes that have to be traversed to reach from a source to destination node. It may include multiple master-slave pairs across several interconnect providers.

Interconnect consumers are the entities which make use of the data paths exposed by the providers. The consumers send requests to providers requesting various throughput, latency and priority. Usually the consumers are device drivers, that send request based on their needs. An example for a consumer is a video decoder that supports various formats and image sizes.

Interconnect providers

Interconnect provider is an entity that implements methods to initialize and configure interconnect bus hardware. The interconnect provider drivers should be registered with the interconnect provider core.

struct icc_node_data

icc node data

Definition:

struct icc_node_data {
    struct icc_node *node;
    u32 tag;
};

Members

node

icc node

tag

tag

struct icc_onecell_data

driver data for onecell interconnect providers

Definition:

struct icc_onecell_data {
    unsigned int num_nodes;
    struct icc_node *nodes[] ;
};

Members

num_nodes

number of nodes in this device

nodes

array of pointers to the nodes in this device

struct icc_provider

interconnect provider (controller) entity that might provide multiple interconnect controls

Definition:

struct icc_provider {
    struct list_head        provider_list;
    struct list_head        nodes;
    int (*set)(struct icc_node *src, struct icc_node *dst);
    int (*aggregate)(struct icc_node *node, u32 tag, u32 avg_bw, u32 peak_bw, u32 *agg_avg, u32 *agg_peak);
    void (*pre_aggregate)(struct icc_node *node);
    int (*get_bw)(struct icc_node *node, u32 *avg, u32 *peak);
    struct icc_node* (*xlate)(const struct of_phandle_args *spec, void *data);
    struct icc_node_data* (*xlate_extended)(const struct of_phandle_args *spec, void *data);
    struct device           *dev;
    int users;
    bool inter_set;
    void *data;
};

Members

provider_list

list of the registered interconnect providers

nodes

internal list of the interconnect provider nodes

set

pointer to device specific set operation function

aggregate

pointer to device specific aggregate operation function

pre_aggregate

pointer to device specific function that is called before the aggregation begins (optional)

get_bw

pointer to device specific function to get current bandwidth

xlate

provider-specific callback for mapping nodes from phandle arguments

xlate_extended

vendor-specific callback for mapping node data from phandle arguments

dev

the device this interconnect provider belongs to

users

count of active users

inter_set

whether inter-provider pairs will be configured with set

data

pointer to private data

struct icc_node

entity that is part of the interconnect topology

Definition:

struct icc_node {
    int id;
    const char              *name;
    struct icc_node         **links;
    size_t num_links;
    struct icc_provider     *provider;
    struct list_head        node_list;
    struct list_head        search_list;
    struct icc_node         *reverse;
    u8 is_traversed:1;
    struct hlist_head       req_list;
    u32 avg_bw;
    u32 peak_bw;
    u32 init_avg;
    u32 init_peak;
    void *data;
};

Members

id

platform specific node id

name

node name used in debugfs

links

a list of targets pointing to where we can go next when traversing

num_links

number of links to other interconnect nodes

provider

points to the interconnect provider of this node

node_list

the list entry in the parent provider’s “nodes” list

search_list

list used when walking the nodes graph

reverse

pointer to previous node when walking the nodes graph

is_traversed

flag that is used when walking the nodes graph

req_list

a list of QoS constraint requests associated with this node

avg_bw

aggregated value of average bandwidth requests from all consumers

peak_bw

aggregated value of peak bandwidth requests from all consumers

init_avg

average bandwidth value that is read from the hardware during init

init_peak

peak bandwidth value that is read from the hardware during init

data

pointer to private data

struct icc_node *icc_node_create_dyn(void)

create a node with dynamic id

Parameters

void

no arguments

Return

icc_node pointer on success, or ERR_PTR() on error

struct icc_node *icc_node_create(int id)

create a node

Parameters

int id

node id

Return

icc_node pointer on success, or ERR_PTR() on error

void icc_node_destroy(int id)

destroy a node

Parameters

int id

node id

int icc_node_set_name(struct icc_node *node, const struct icc_provider *provider, const char *name)

set node name

Parameters

struct icc_node *node

node

const struct icc_provider *provider

node provider

const char *name

node name

Return

0 on success, or -ENOMEM on allocation failure

int icc_link_nodes(struct icc_node *src_node, struct icc_node **dst_node)

create link between two nodes

Parameters

struct icc_node *src_node

source node

struct icc_node **dst_node

destination node

Description

Create a link between two nodes. The nodes might belong to different interconnect providers and the dst_node might not exist (if the provider driver has not probed yet). So just create the dst_node and when the actual provider driver is probed, the rest of the node data is filled.

Return

0 on success, or an error code otherwise

int icc_link_create(struct icc_node *node, const int dst_id)

create a link between two nodes

Parameters

struct icc_node *node

source node id

const int dst_id

destination node id

Description

Create a link between two nodes. The nodes might belong to different interconnect providers and the dst_id node might not exist (if the provider driver has not probed yet). So just create the dst_id node and when the actual provider driver is probed, the rest of the node data is filled.

Return

0 on success, or an error code otherwise

void icc_node_add(struct icc_node *node, struct icc_provider *provider)

add interconnect node to interconnect provider

Parameters

struct icc_node *node

pointer to the interconnect node

struct icc_provider *provider

pointer to the interconnect provider

void icc_node_del(struct icc_node *node)

delete interconnect node from interconnect provider

Parameters

struct icc_node *node

pointer to the interconnect node

int icc_nodes_remove(struct icc_provider *provider)

remove all previously added nodes from provider

Parameters

struct icc_provider *provider

the interconnect provider we are removing nodes from

Return

0 on success, or an error code otherwise

void icc_provider_init(struct icc_provider *provider)

initialize a new interconnect provider

Parameters

struct icc_provider *provider

the interconnect provider to initialize

Description

Must be called before adding nodes to the provider.

int icc_provider_register(struct icc_provider *provider)

register a new interconnect provider

Parameters

struct icc_provider *provider

the interconnect provider to register

Return

0 on success, or an error code otherwise

void icc_provider_deregister(struct icc_provider *provider)

deregister an interconnect provider

Parameters

struct icc_provider *provider

the interconnect provider to deregister

Interconnect consumers

Interconnect consumers are the clients which use the interconnect APIs to get paths between endpoints and set their bandwidth/latency/QoS requirements for these interconnect paths.

struct icc_path *of_icc_get_by_index(struct device *dev, int idx)

get a path handle from a DT node based on index

Parameters

struct device *dev

device pointer for the consumer device

int idx

interconnect path index

Description

This function will search for a path between two endpoints and return an icc_path handle on success. Use icc_put() to release constraints when they are not needed anymore. If the interconnect API is disabled, NULL is returned and the consumer drivers will still build. Drivers are free to handle this specifically, but they don’t have to.

Return

icc_path pointer on success or ERR_PTR() on error. NULL is returned when the API is disabled or the “interconnects” DT property is missing.

struct icc_path *of_icc_get(struct device *dev, const char *name)

get a path handle from a DT node based on name

Parameters

struct device *dev

device pointer for the consumer device

const char *name

interconnect path name

Description

This function will search for a path between two endpoints and return an icc_path handle on success. Use icc_put() to release constraints when they are not needed anymore. If the interconnect API is disabled, NULL is returned and the consumer drivers will still build. Drivers are free to handle this specifically, but they don’t have to.

Return

icc_path pointer on success or ERR_PTR() on error. NULL is returned when the API is disabled or the “interconnects” DT property is missing.

struct icc_path *icc_get(struct device *dev, const char *src, const char *dst)

get a path handle between two endpoints

Parameters

struct device *dev

device pointer for the consumer device

const char *src

source node name

const char *dst

destination node name

Description

This function will search for a path between two endpoints and return an icc_path handle on success. Use icc_put() to release constraints when they are not needed anymore.

Return

icc_path pointer on success or ERR_PTR() on error. NULL is returned when the API is disabled.

void icc_set_tag(struct icc_path *path, u32 tag)

set an optional tag on a path

Parameters

struct icc_path *path

the path we want to tag

u32 tag

the tag value

Description

This function allows consumers to append a tag to the requests associated with a path, so that a different aggregation could be done based on this tag.

const char *icc_get_name(struct icc_path *path)

Get name of the icc path

Parameters

struct icc_path *path

interconnect path

Description

This function is used by an interconnect consumer to get the name of the icc path.

Returns a valid pointer on success, or NULL otherwise.

int icc_set_bw(struct icc_path *path, u32 avg_bw, u32 peak_bw)

set bandwidth constraints on an interconnect path

Parameters

struct icc_path *path

interconnect path

u32 avg_bw

average bandwidth in kilobytes per second

u32 peak_bw

peak bandwidth in kilobytes per second

Description

This function is used by an interconnect consumer to express its own needs in terms of bandwidth for a previously requested path between two endpoints. The requests are aggregated and each node is updated accordingly. The entire path is locked by a mutex to ensure that the set() is completed. The path can be NULL when the “interconnects” DT properties is missing, which will mean that no constraints will be set.

Returns 0 on success, or an appropriate error code otherwise.

void icc_put(struct icc_path *path)

release the reference to the icc_path

Parameters

struct icc_path *path

interconnect path

Description

Use this function to release the constraints on a path when the path is no longer needed. The constraints will be re-aggregated.

int of_icc_bulk_get(struct device *dev, int num_paths, struct icc_bulk_data *paths)

get interconnect paths

Parameters

struct device *dev

the device requesting the path

int num_paths

the number of icc_bulk_data

struct icc_bulk_data *paths

the table with the paths we want to get

Description

Returns 0 on success or negative errno otherwise.

void icc_bulk_put(int num_paths, struct icc_bulk_data *paths)

put a list of interconnect paths

Parameters

int num_paths

the number of icc_bulk_data

struct icc_bulk_data *paths

the icc_bulk_data table with the paths being put

int icc_bulk_set_bw(int num_paths, const struct icc_bulk_data *paths)

set bandwidth to a set of paths

Parameters

int num_paths

the number of icc_bulk_data

const struct icc_bulk_data *paths

the icc_bulk_data table containing the paths and bandwidth

Description

Returns 0 on success or negative errno otherwise.

int icc_bulk_enable(int num_paths, const struct icc_bulk_data *paths)

enable a previously disabled set of paths

Parameters

int num_paths

the number of icc_bulk_data

const struct icc_bulk_data *paths

the icc_bulk_data table containing the paths and bandwidth

Description

Returns 0 on success or negative errno otherwise.

void icc_bulk_disable(int num_paths, const struct icc_bulk_data *paths)

disable a set of interconnect paths

Parameters

int num_paths

the number of icc_bulk_data

const struct icc_bulk_data *paths

the icc_bulk_data table containing the paths and bandwidth

int devm_of_icc_bulk_get(struct device *dev, int num_paths, struct icc_bulk_data *paths)

resource managed of_icc_bulk_get

Parameters

struct device *dev

the device requesting the path

int num_paths

the number of icc_bulk_data

struct icc_bulk_data *paths

the table with the paths we want to get

Description

Returns 0 on success or negative errno otherwise.

Interconnect debugfs interfaces

Like several other subsystems interconnect will create some files for debugging and introspection. Files in debugfs are not considered ABI so application software shouldn’t rely on format details change between kernel versions.

/sys/kernel/debug/interconnect/interconnect_summary:

Show all interconnect nodes in the system with their aggregated bandwidth request. Indented under each node show bandwidth requests from each device.

/sys/kernel/debug/interconnect/interconnect_graph:

Show the interconnect graph in the graphviz dot format. It shows all interconnect nodes and links in the system and groups together nodes from the same provider as subgraphs. The format is human-readable and can also be piped through dot to generate diagrams in many graphical formats:

$ cat /sys/kernel/debug/interconnect/interconnect_graph | \
        dot -Tsvg > interconnect_graph.svg

The test-client directory provides interfaces for issuing BW requests to any arbitrary path. Note that for safety reasons, this feature is disabled by default without a Kconfig to enable it. Enabling it requires code changes to #define INTERCONNECT_ALLOW_WRITE_DEBUGFS. Example usage:

cd /sys/kernel/debug/interconnect/test-client/

# Configure node endpoints for the path from CPU to DDR on
# qcom/sm8550.
echo chm_apps > src_node
echo ebi > dst_node

# Get path between src_node and dst_node. This is only
# necessary after updating the node endpoints.
echo 1 > get

# Set desired BW to 1GBps avg and 2GBps peak.
echo 1000000 > avg_bw
echo 2000000 > peak_bw

# Vote for avg_bw and peak_bw on the latest path from "get".
# Voting for multiple paths is possible by repeating this
# process for different nodes endpoints.
echo 1 > commit