FiiO M3K: disable CPU frequency switching

After conducting some simplistic tests, I found that the power usage
did not appear to be affected by the CPU frequency.

I tested by playing back a 44.1 KHz FLAC file on single track repeat,
and measured current with the AXP173's battery discharge current ADC.
The button and LCD backlights were set to always on. Headphones were
unplugged and the volume was muted to eliminate any influence from
the headphone amp.

On average the current usage was between 78-81 mA at 1008 MHz, 252 MHz,
and 112 MHz. If anything, 1008 MHz drew _less_ current than the lower
frequencies, by about 1-3 mA.

A possible explanation for this, assuming it's not just a bias of the
test, is that the CPU idle state saves so much power that it's better
to maximize the real time that the CPU spends idling. More systematic
testing is needed to confirm this.

Change-Id: I527473e8c4c12bc1e94f8d4e849fecc108022abe

1 files changed, 0 insertions, 20 deletions

diff --git a/firmware/target/mips/ingenic_x1000/kernel-x1000.c b/firmware/target/mips/ingenic_x1000/kernel-x1000.c
index eb7c3d6124..8d272bdaa9 100644
--- a/firmware/target/mips/ingenic_x1000/kernel-x1000.c
+++ b/firmware/target/mips/ingenic_x1000/kernel-x1000.c
@@ -23,26 +23,6 @@
 #include "system.h"
 #include "x1000/ost.h"
 
-/* TODO: implement a CPU frequency switching policy based on CPU utilization
- *
- * The basic assumption is that the workload consumes a fixed number of CPU
- * cycles per second on average (= utilization), so we can set the frequency
- * based on that value. Audio playback should fit this usage pattern well, so
- * it's a good fit for Rockbox.
- *
- * It's easier to understand in terms of fluid flow -- we need to keep
- * a reservoir of water topped up, CPU frequency is the inflow rate, and
- * CPU utilization is the outflow rate. The goal is to avoid running dry
- * and minimize the inflow rate.
- *
- * The only tricky part here is handing usage spikes -- CPU frequency has to
- * increase faster than utilization or there's a risk of audio dropouts.
- *
- * Rockbox CPU boost could be used as a hint to scale up frequency faster.
- * If that's not necessary to get good results, HAVE_ADJUSTABLE_CPU_FREQ can
- * be disabled entirely.
- */
-
 #define CPU_IDLE_SAMPLES 100
 
 void tick_start(unsigned interval_in_ms)