This document discusses power management beyond the CPU. It provides examples of power consumption for different use cases like GPS, audio, and video streaming. It then summarizes several Linux frameworks for power management including clock trees, power supplies, idle management, active CPU and device power/performance management, dynamic device power management, and constraints. It walks through an example of MP3 playback optimization using these frameworks to turn off components like the MMC, regulate CPU frequency, and use timers and DMA to drive audio while keeping the SoC in a low-power state. It concludes that peripheral power management is important and Linux provides tools to help manage power for the whole system.

1. Power Management beyond the CPU
Amit Kucheria
Power Management Working Group
Linaro Connect, Hong Kong
1st
June 2012

2. The power guzzlers...
Internet Phone
Video streaming
Angry birds
GPS navigation
0
100
200
300
400
500
600
700
800
900
GPS
Audio
Wifi
Display
CPU
Use case
Powerconsumption(mW)

3. Concepts: quick overview
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Clock trees [clock framework]
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Power supplies [regulator framework]
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Idle management [cpuidle framework]
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Active CPU power/performance management
[cpufreq framework]
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Active device power/performance management
[devfreq framework]
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Dynamic device power management [runtime PM
framework]
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Constraints [pm-qos framework]

4. Use case walkthrough
MP3 playback

5. mp3 playback: Assumptions
Music on MMC/SDcard
Decoding on ARM

6. mp3 playback: Simplified HW blocks
DRAM ARM
MMC/SD
Buffer
mp3
Audio
codec
SoC

7. mp3 playback: Flow
mp3
DRAM ARM
MMC/SD
File
Cache
BufferPCM
frames
DMA
decode
Audio
codec
SoC

8. mp3 playback: SW optimisations
Entire file read into main memory
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Run ARM cores at low frequency [cpufreq]
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Turn off MMC [runtime pm hooks in mmc driver]
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What is turned off?
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Clocks immediately
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Regulators turned off lazily
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Ensure system doesn't idle too long [cpuidle]
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Register a latency constraint [pm-qos]

9. mp3 playback: SW optimisations
Decode mp3 to PCM
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Decoded samples in main memory
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Turn off ARM core, no more processing necessary
[cpuidle]
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Peripherals may still be awake

10. mp3 playback: SW optimisations
DMA from main memory to some FIFO
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e.g. FIFO on audio codec or some buffered serial port
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RAM can be put in self-refresh
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FIFO is large (few 100ms)

11. mp3 playback: SW optimisations
Buffer drives Codec while entire SoC is in very
low-power state
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Timer wakes up DMA thread just before buffer
runs out

12. Conclusions
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ARM power consumption is a small component in
a typical usecase
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Surrounding peripherals need to be power-
managed too
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Linux has frameworks to help with this
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Runtime PM
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PM-QoS
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devfreq

13. Thank You