PM / devfreq: bw_hwmon: Update to low latency, high sampling rate algorithm
The existing bw_hwmon governor samples the bandwidth every polling_interval
milliseconds and makes decisions. Polling interval of 50ms or even 10ms
gives a very low resolution picture of the DDR/bus traffic. Due to the
lower resolution picture, the existing governor algorithm has to be biased
aggressively towards performance to avoid any performance degradation
compared to using a static mapping between bus master (CPU, GPU, etc)
frequency to DDR/bus BW votes.
While the existing governor uses IRQ to get early notification of traffic
increase, even a 4x early notification for a 50ms polling interval still
takes 12.5ms. This kind of reaction time is still too slow for some bus
masters like CPU.
To take care of these limitations, rewrite the governor algorithm to take
multiple short samples of BW within a decision window (polling interval)
and use that higher resolution picture to make much better and faster
decisions. Doing so allows the governor to have the following features:
- Very low reaction time
- Over voting to stay ahead of increasing traffic
- Historic peak tracking to limit over voting
- Being power aware when doing over voting
- Pattern detection and intelligent hysteresis
- Detection low traffic modes and being less aggressive about BW votes
Change-Id: I69886b7fbeea0b64d10b5a1fb23fcb5f3918f0ce
Signed-off-by: Saravana Kannan <skannan@codeaurora.org>
3 files changed