[Intel-gfx] proposal for DVFS (Dynamic Voltage Frequency Scaling)

[Daniel Vetter]

You realize that Ville has implemented all this already and his
patches are only waiting for review?
-Daniel

On Thu, Jan 15, 2015 at 2:50 PM, Thulasimani, Sivakumar
<sivakumar.thulasimani at intel.com> wrote:
> Hi,
>
>                 I’ll be working on implementing Dynamic Voltage Frequency
> Scaling in i915, whose rough proposal is provided below. Please go through
> the options and provide your feedback.
>
>
>
> What is DVFS ?
>
>                 Any SKU is capable of running at more than one display CD
> clock value but is configured to a default value during boot and is left
> untouched afterwards. Lowering this Display CD clock  will result in better
> power savings while raising this will result in capability to support larger
> resolution displays. So the best scenario is to always detect the attached
> displays and adjust the CD Clock to the minimum required by the displays.
> DVFS is the process of dynamically changing the display CD clock based on
> attached displays.
>
>
>
> Usecases: ( considers low resolution LFP is used with 4K HDMI panel)
>
> Ø  EDP (19x10) LFP panel alone is present at boot and can be driven with
> lower CD clock, thus saving power.
>
> Ø  EDP (19x10) LFP panel was present at boot and 4K HDMI display is hot
> plugged in to the system. CD clock can be increased to the required value
> and then we can drive 4K panel. Thus save power while only LFP is in use but
> also provide the ability to support 4K displays.
>
> Ø  4K HDMI display is unplugged from the system, CD clock can now be lowered
> to the value required to drive LFP alone to save power.
>
> Ø  Boot with LFP and 4K HDMI connected, the CD clock will be programmed to
> the value required to drive 4K HDMI.
>
>
>
> Technical Restrictions:
>
> Ø  DVFS can be performed only when all displays are turned off
> (pipe/port/plane,etc)
>
>
>
> Ux Restrictions:
>
> Ø  Flash/flicker should be avoided as much as possible( i.e during unplug of
> HDMI avoid immediately lowering the CD clock after hotunplug since it will
> result in flicker on LFP )
>
>
>
> Possible Solutions :
>
> Ø  Policy + implementation in driver
>
> o   Driver will set the cd clock during boot based on attached panels
>
> o   Post boot driver has to track each modeset for CD clock and resolution
> support.
>
> o   If any change in CD clock is needed, driver has to disable all displays
> , change the cd clock and enable back all displays
>
> o   Pros:
>
> ·         Changes are contained within driver
>
> o   Cons:
>
> ·         Complexity increases within driver
>
> ·         User land/ HWC will be blind to internal operations
>
> Ø  Policy in HWC and implementation in driver
>
> o   Driver will set the cd clock during boot based on attached panels
>
> o   Post boot, driver will fail any modeset that cannot be supported
>
> o   HWC will be responsible for disabling all displays, issuing IOCTL to
> change CD clock and enabling required displays
>
> o   Pros:
>
> ·         Driver implementation will be simple and clean
>
> ·         User land/HWC will be aware of operations and handle any special
> cases such as video playback
>
> o   Cons:
>
> ·         Solution requires HWC & Driver changes.
>
>
>
> Our recommendation is for HWC + Driver changes considering our primary
> target of Android and the pros/cons mentioned there.
>
>
>
> Sorry for the long mail J, Please provide your feedback or questions on
> this.
>
>
>
> regards,
>
> Sivakumar
>
>
>
>
> _______________________________________________
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> Intel-gfx at lists.freedesktop.org
> http://lists.freedesktop.org/mailman/listinfo/intel-gfx
>



-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch