Generic System Interconnect Subsystem¶
Introduction¶
This framework is designed to provide a standard kernel interface to controlthe settings of the interconnects on an SoC. These settings can be throughput,latency and priority between multiple interconnected devices or functionalblocks. This can be controlled dynamically in order to save power or providemaximum performance.
The interconnect bus is hardware with configurable parameters, which can beset on a data path according to the requests received from various drivers.An example of interconnect buses are the interconnects between variouscomponents or functional blocks in chipsets. There can be multiple interconnectson 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 NoCand Mem NoC.
Interconnect node is the software definition of the interconnect hardwareport. Each interconnect provider consists of multiple interconnect nodes,which are connected to other SoC components including other interconnectproviders. The point on the diagram where the CPUs connect to the memory iscalled an interconnect node, which belongs to the Mem NoC interconnect provider.
Interconnect endpoints are the first or the last element of the path. Everyendpoint is a node, but not every node is an endpoint.
Interconnect path is everything between two endpoints including all the nodesthat have to be traversed to reach from a source to destination node. It mayinclude multiple master-slave pairs across several interconnect providers.
Interconnect consumers are the entities which make use of the data paths exposedby the providers. The consumers send requests to providers requesting variousthroughput, latency and priority. Usually the consumers are device drivers, thatsend request based on their needs. An example for a consumer is a video decoderthat supports various formats and image sizes.
Interconnect providers¶
Interconnect provider is an entity that implements methods to initialize andconfigure interconnect bus hardware. The interconnect provider drivers shouldbe registered with the interconnect provider core.
- 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); struct icc_node* (*xlate)(struct of_phandle_args *spec, void *data); struct device *dev; int users; 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 calledbefore the aggregation begins (optional)
xlate- provider-specific callback for mapping nodes from phandle arguments
dev- the device this interconnect provider belongs to
users- count of active users
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; 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
data- pointer to private data
Interconnect consumers¶
Interconnect consumers are the clients which use the interconnect APIs toget paths between endpoints and set their bandwidth/latency/QoS requirementsfor these interconnect paths. These interfaces are not currentlydocumented.
Interconnect debugfs interfaces¶
Like several other subsystems interconnect will create some files for debuggingand introspection. Files in debugfs are not considered ABI so applicationsoftware 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 bandwidthrequest. 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 allinterconnect nodes and links in the system and groups together nodes from thesame provider as subgraphs. The format is human-readable and can also be pipedthrough dot to generate diagrams in many graphical formats:
$ cat /sys/kernel/debug/interconnect/interconnect_graph | \ dot -Tsvg > interconnect_graph.svg