[Trac #1411] Video compression buffers may be needed.

[Zarro Boogs per Child]

#1411: Video compression buffers may be needed.
-------------------------+--------------------------------------------------
 Reporter:  jg           |        Owner:  rsmith 
     Type:  enhancement  |       Status:  new    
 Priority:  high         |    Milestone:  BTest-4
Component:  distro       |   Resolution:         
 Keywords:  power        |  
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Old description:

> The Geode has video compression buffers that can greatly reduce the
> memory bandwidth required to drive the display, but since we haven't seen
> any problems with bandwidth starvation on the mbus yet,it might be a non
> starter.
>
> Leaving it off saves us oodles of video space, and we just recently
> discovered that EXA doesn't like having its stride pulled out from
> underneath it, meaning that we would probably have to have the
> compression pitch enabled for the non-visible space too, which would be a
> total waste.
>
> On the other hand, memory references to the RAM chips take power, so the
> compression buffers may be needed to reduce power consumption when
> driving the display.  But the idle display may be able to be handled by
> the DCON, (if we can tolerate the latency effects), so this is not an
> obvious decision at all.
>
> It is something we need to investigate carefully as part of the power
> management work.

New description:

 The Geode has video compression buffers that can greatly reduce the memory
 bandwidth required to drive the display, but since we haven't seen any
 problems with bandwidth starvation on the mbus yet,it might be a non
 starter.

 Leaving it off saves us oodles of video space, and we just recently
 discovered that EXA doesn't like having its stride pulled out from
 underneath it, meaning that we would probably have to have the compression
 pitch enabled for the non-visible space too, which would be a total waste.

 On the other hand, memory references to the RAM chips take power, so the
 compression buffers may be needed to reduce power consumption when driving
 the display.  But the idle display may be able to be handled by the DCON,
 (if we can tolerate the latency effects, which are not great if the video
 drivers are left on in the Geode), so this is not an obvious decision at
 all.

 The compression buffers have been working in the past; but we need to make
 real measurements to decide the cost/benefit ratio of using them.

 It is something we need to investigate carefully as part of the power
 management work.

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Ticket URL: <http://dev.laptop.org/ticket/1411#comment:2>
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