Measurements: Massive Display-related power savings

Harald Welte laforge at
Wed Oct 8 20:43:44 CEST 2008

Hi all (Cc'ing mickey since I don't know if you read the kernel list)!

I was using the FSO daily build from two days ago as a base reference, and
using the 5second average current reading from the bq27000 battery coulomb
counter to obtain the measurements (with apropriate at least 10 second delays
to ensure correct readings).

I did not insert a SIM card to ensure there is no (and particularly no dynamic)
power consumption by the GSM modem

FSO image, screen backlight full on, CPU idle		177mA
same using frameworkd/sysfs based screen blank		 99mA
same using manual "sleep" into jbt sysfs file		 83mA
same using manual "deep-standby" into jbt		 82mA
same plus disabling glamo DCLK pixel clock		 59mA

Since the patches that I sent implement all this based on the framebuffer
blanking ioctl(), I could verify that unmodified xglamo (using 'xset s 5')
confirms the above measurements, i.e. it automatically goes down to 59mA.

In percent, this is a 40% reduction of idle system power consumption, i.e.
in the case where you use the device as music player without any screen
output but CPU running.

The measurements should apply to both ASU and FSO, since both just used
backlight based blanking so far.  ASU will directly benefit since it uses
the x screen blanking.

FSO should switch the backlight control from the sysfs method to the
fbdev-ioctl method.  It's backwards compatible to old kernels (which will just
backlight-blank) and future-compatible with kernels that add the jbt6k74
notifier and glamo fb_blank() patches.

This additional battery life time was brougt to you by Swisscom ;)
- Harald Welte <laforge at>         
Software for the world's first truly open Free Software mobile phone
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