SATA Aggressive Link Power Management (ALPM) - call for testing

Tim Gardner tim.gardner at
Tue Dec 6 20:58:30 UTC 2011

On 12/06/2011 01:44 PM, Colin Ian King wrote:
> On 06/12/11 17:14, Tim Gardner wrote:
>> On 12/06/2011 06:25 AM, Colin Ian King wrote:


>> How about screen backlight management? I've noticed that my screen at
>> full brightness consumes about 4W compared to screen off. Should we at
>> least be measuring that ?
> Tim,
> measuring this is fairly easy but I'm not sure what you'd like tested.
> Backlights are adjusted on AC or battery automatically and users can
> tweak these using hotkeys too. So, do we want to accurately know just
> how much power
> we consume, with different levels?

Knowing what the backlight is consuming at various levels gives you 
quantitative input to your screen blanking interval decision. I find it 
quite annoying to have my screen go blank after 3 minutes (the default), 
so if I _knew_ what it was costing me in terms of battery time, then I 
could adjust that blanking interval accordingly.

> I suspect I should also investigate something that reports:
> "Interestingly, in our measurements we have found that, at least on
> newer laptops, the kernel sysfs method (which tends to use the BIOS on
> the machine) seems to not control the actual backlight, but rather
> change the colors of the pixels on the screen to appear darker. Using
> darker pixels obviously doesn't save as much power as reducing the
> intensity of the backlight."
> that I will at least poke at that.
> The backlight issue makes me also think about other non-obvious
> settings. For example, with framebuffer compression we use less power to
> clock out the image, so I'd like to measure a default desktop background
> vs a plain background vs a complex background image.
> And also I will be investigating the white vs black pixels setting when
> the screen is blanked - apparently white saves power.

Yeah, it would be nice to know what models take this route as I suspect 
it'll have an adverse impact on power consumption.

> If any one has any further ideas, let me know.
> Colin

Tim Gardner tim.gardner at

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