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Add Perf Feedback Fastchannel in CPPC ServiceGroup
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The Performance Feedback fast channel for
each application processor will be used by the
application processor to fetch frequency whenever
its changes. This is required for the estimation
of the delivered performance in cases when platform
does not provide any hardware counter for the same

Signed-off-by: Rahul Pathak <[email protected]>
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@pathakraul
pathakraul committed Oct 2, 2024
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145 changes: 96 additions & 49 deletions src/srvgrp-cppc.adoc
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Expand Up @@ -33,7 +33,7 @@ The platform may have multiple application processors that share the actual
performance controls like clock, voltage regulator and others depending on the
platform. In such cases a performance level change for one application processor
will affect the entire the group sharing the controls. Its the responsibility of
the power/performance management software running on the application processor
the power and performance management software running on the application processor
and the platform to coordinate and manage the group level performance changes.

The following table lists the services in the CPPC service group:
Expand Down Expand Up @@ -77,21 +77,34 @@ The following table lists the services in the CPPC service group:

==== CPPC Fast-channel
The CPPC service group defines the fast-channels to be used by the application
processor for requesting performance level change. A fast-channel is a shared memory
with a layout specific to the CPPC service group and the data written in a
fast-channel do not follow the conventional RPMI message format. The simple data
format supported by the fast-channel allows faster processing of the requests
made through a fast-channel.
processor to request performance changes and to obtain performance change feedback
for an application processor from the platform microcontroller.

A fast-channel shared memory layout is specific to the CPPC service
group. The data written in a fast-channel do not follow the conventional RPMI
message format. The simple data format supported by the fast-channel allows
faster processing of the performance change requests made through a fast-channel
and faster read of the performance feedback values supported over the
fast-channel.

The CPPC service group defines two types of fast-channel for each application
processor.

If the fast-channels are implemented, the application processor will either write
===== Performance Request fast-channel.
In this fast-channel the application processor will either write
the desired performance level in case of normal mode or the minimum
and maximum level in case of Autonomous (CPPC2) mode in the fast-channel.
Otherwise the application processor can call the service `CPPC_WRITE_REG` for the
`DesiredPerformanceRegister` or `MinimumPerformanceRegister` and
`MaximumPerformanceRegister`.
and maximum performance level in case of Autonomous (CPPC2) mode in the
fast-channel. Otherwise the application processor can call the service
`CPPC_WRITE_REG` for the `DesiredPerformanceRegister` or
`MinimumPerformanceRegister` and `MaximumPerformanceRegister`.

The layout of this fast-channel which depends on the CPPC mode, either normal or
autonomous mode is discoverable through a defined service.

[#table_cppc_fastchan]
.CPPC Fast-channel Layout
The size of this fast-channel type is of 8-byte.

[#table_cppc_performance_change_fastchan]
.CPPC Performance Request Fast-channel Layout
[cols="2, 3a", width=100%, align="center", options="header"]
|===
| CPPC Mode
Expand Down Expand Up @@ -122,33 +135,58 @@ Otherwise the application processor can call the service `CPPC_WRITE_REG` for th
! 0x4
! Maximum performance level
!===
|===

===== Performance Feedback fast-channel.
In this fast-channel the application processor will read the
supported value for estimating the delivered performance as performance feedback
for an application processor. The application processor current frequency (Hz)
is used for performance feedback in this fast-channel. The platform
microcontroller must write the frequency of an application processor in the
fast-channel whenever it changes.

The size of this fast-channel type is of 8-byte.

[#table_cppc_performance_feedback_fastchan]
.CPPC Performance Feedback Fast-channel Layout
[cols="2, 3a", width=100%, align="center", options="header"]
|===
Each application processor must be assigned a fast-channel, which is managed by
the CPPC service group for performance control.
| Offset
| Value (32-bit)

The size of the shared memory region for fast-channels for all the managed
application processors must be a `power-of-2` and a multiple of the size of each
fast-channel. The `base-address` and `size` (bytes) of the fast-channels shared
memory region can be discovered using the service `CPPC_GET_FAST_CHANNEL_REGION`.
The layout of each fast-channel which depends on the CPPC mode, either normal or
autonomous mode can also be discovered through the same service.

A fast-channel size for a application processor is of `8-byte` with two fields
of `4-byte` each. A fast-channel must be naturally aligned to the size of the
fast-channel.
| 0x0
| Current frequency Low 32-bit (Hz)

The offset of a fast-channel entry belonging to a application processor in that
shared memory region can be discovered through service `CPPC_GET_FAST_CHANNEL_OFFSET`.
The offset can be added into the `base-address` of the shared memory region to
form the address of a application processor fast-channel.
| 0x4
| Current frequency High 32-bit (Hz)
|===

===== Fast-channel Doorbell
A doorbell can also be supported with fast-channels which is shared between
all the application processors. If a doorbell is available then it must be
supported through a read-modify-write sequence to a memory-mapped register.
The doorbell details and attributes can be discovered by the application processor
through the defined service in this service group.
If fast-channels are supported then each application processor must be assigned
both types fast-channel. In the shared memory region allocated by the
platform for fast-channels, the Performance Request fast-channels must be grouped
together for all the application processors. Similarly, the Performance Feedback
fast-channels must be grouped together.

The size of the shared memory region for fast-channels for all the managed
application processors must be a `power-of-2`. The `base-address` and `size`
(bytes) of the fast-channels shared memory region can be discovered using the
service `CPPC_GET_FAST_CHANNEL_REGION`. The `base-address` of the shared memory
region must be aligned to 8-byte which is maximum size of a fast-channel in
both the types.

The offsets of fast-channel of both types for an application processor are aligned
to 8-byte. The offset of both fast-channel types in the shared memory region can
be discovered through service `CPPC_GET_FAST_CHANNEL_OFFSET`. The offsets
discovered can be added into the `base-address` of the shared memory region to
form the address of Performance Request fast-channel and Performance Feedback
fast-channel for an application processor.

===== Performance Request Fast-channel Doorbell
A doorbell can also be supported for this fast-channel type which is shared
between all the application processors. If a doorbell is available then it must
be supported through a read-modify-write sequence to a memory-mapped register.
The doorbell details and attributes can be discovered by the application
processor through the defined service in this service group.

[#cppc-notifications]
==== Notifications
Expand Down Expand Up @@ -444,9 +482,6 @@ doorbell.
! RPMI_SUCCESS
! Service completed successfully.

! RPMI_ERR_INVALID_PARAM
! `HART_ID` is invalid.

! RPMI_ERR_NOT_SUPPORTED
! Fast-channels not supported.
!===
Expand All @@ -469,19 +504,20 @@ doorbell.
Desired performance level for performance change.

0b01: Autonomous mode.
Performance limit change. Platform can choose the level in the requested limit.
Performance limit change. Platform can choose the level in the requested
limit.

0b10 - 0b11: Reserved

! [2:1]
! Doorbell register width
! Performance Request fast-channel doorbell register width.

0b00: 8-bit
0b01: 16-bit
0b10: 32-bit
0b11: 64-bit
! [0]
! Doorbell support.
! Performance Request fast-channel doorbell support.

0b1: Supported
0b0: Not supported
Expand Down Expand Up @@ -539,8 +575,9 @@ doorbell.
|===

==== Service: CPPC_GET_FAST_CHANNEL_OFFSET (SERVICE_ID: 0x06)
This service is used to get the offset of a fast-channel of an application
processor in the shared memory region containing all the fast-channels.
This service is used to get the offsets of Performance Request fast-channel and
Performance Feedback fast-channel for an application processor in the shared
memory region containing all the fast-channels.

[#table_cppc_getfastchanoffset_request_data]
.Request Data
Expand All @@ -559,7 +596,7 @@ processor in the shared memory region containing all the fast-channels.

[#table_cppc_getfastchanoffset_response_data]
.Response Data
[cols="1, 2, 1, 7a", width=100%, align="center", options="header"]
[cols="1, 5, 1, 7a", width=100%, align="center", options="header"]
|===
| Word
| Name
Expand All @@ -570,7 +607,7 @@ processor in the shared memory region containing all the fast-channels.
| STATUS
| int32
| Return error code
[cols="1,1a", options="header"]
[cols="2,1a", options="header"]
!===
! Error Code
! Description
Expand All @@ -587,14 +624,24 @@ processor in the shared memory region containing all the fast-channels.
- Other errors <<table_error_codes>>

| 1
| OFFSET_LOW
| PERF_REQUEST_OFFSET_LOW
| uint32
| Lower 32-bit of a fast-channel offset.
| Lower 32-bit of a Performance Request fast-channel offset.

| 2
| OFFSET_HIGH
| PERF_REQUEST_OFFSET_HIGH
| uint32
| Upper 32-bit of a Performance Request fast-channel offset.

| 3
| PERF_FEEDBACK_OFFSET_LOW
| uint32
| Lower 32-bit of a Performance Feedback fast-channel offset.

| 4
| PERF_FEEDBACK_OFFSET_HIGH
| uint32
| Upper 32-bit of a fast-channel offset.
| Upper 32-bit of a Performance Feedback fast-channel offset.
|===

==== Service: CPPC_GET_HART_LIST (SERVICE_ID: 0x07)
Expand Down

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