Understanding Panel Self Refresh
by Anand Lal Shimpi on August 7, 2013 1:47 PM EST- Posted in
- Smartphones
- Displays
- Mobile
- Panel Self Refresh
- Tablets
- SoCs
Earlier today Brian spent some time with the G2, LG's 5.2-inch flagship smartphone based on the Qualcomm Snapdragon 800 (MSM8974) SoC. I'd recommend reading his excellent piece in order to get all of the details on the new phone, but there's one disclosure I'd like to call out here: the G2 supports Panel Self Refresh.
To drive a 60Hz panel, your display controller must present the display with the contents of the frame buffer 60 times per second. Regardless of what's being displayed (static vs. active content), every second there are 60 updates pushed through the display pipeline to the display. When displaying fast moving content (e.g. video playback, games, scrolling), this update frequency is important and appreciated. When displaying static content however (E.g. staring at the home screen, reading a page of an eBook), the display pipeline and associated DRAM are consuming power sending display updates when it doesn't need to. Panel Self Refresh (PSR) is designed to address the latter case.
To be clear, PSR is an optimization to reduce SoC power, not to reduce display power. In the event that display content is static, the contents of the frame buffer (carved out of system RAM in the case of a smartphone) are copied to a small amount of memory tied to the display. In the case of LG's G2 we're likely looking at something around 8MB (1080p @ 32bpp). The refreshes then come from the panel's memory, allowing the display pipeline (and SoC) to drive down to an even lower power state. Chances are the panel's DRAM is also tied to a narrower bus and can be lower power than the system memory used by the SoC, making these refreshes even lower in power cost.
LG claims a 26% reduction in power when displaying a still image with PSR enabled. I'm curious to see the impact on overall battery life. There are elements of our WiFi web browsing test that could benefit from PSR but it's unclear how much of an improvement we'll see. The added cost of introducing additional memory into a device is something that panel vendors have been hesitant to do, but as companies look to continue to reduce platform power it's a vector worth considering. LG's dual-role as a component supplier and device maker likely made the decision to enable PSR a lot easier.
PSR potentially has bigger implications for notebook use where it's not uncommon to just stare at a desktop that's not animating at all. I feel like the more common use case in smartphones is to just lock your phone/display when you're not actively using it.
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MaxPowerTech - Wednesday, August 7, 2013 - link
Maybe this is a stupid question, but i always thought, that lcd-pixels don't need refreshing (except when they need to change color of course).And that is the reason why lcd-panels don't flicker, like crt and plasmas do.
So the only thing you need to do for PSR is not turning of the panel, when you don't get new data?
Death666Angel - Wednesday, August 7, 2013 - link
That is correct, but I'm pretty sure that they still expect to get the data, even when it is identical to what is already being displayed.XZerg - Wednesday, August 7, 2013 - link
you are correct and i believe there is some logic in the LCD to skip the "update" if there is no change to that pixel. this reduces power consumption.isnoop - Thursday, August 8, 2013 - link
If you've ever crashed an LCD smartphone or laptop hard enough, you've seen this in action: A ghost of the last image on the screen can be darkly imprinted (sometimes with horizontal lines through it) because the pixels were left in their last state.Ortanon - Thursday, August 8, 2013 - link
I think the reason they don't flicker is because the pixels don't provide light; there's nothing TO flicker, unless the backlight starts flickering for some reason lol. I don't think it's because they don't need refreshing. As far as I know, all common LCD technologies have an internal refresh that's necessary, which is probably why we need PSR instead of already having GPU drivers that know how to make a GPU wait when it can. The system is the thing that PSR is pausing, but only because the display was forcing it to keep working before.JDG1980 - Wednesday, August 7, 2013 - link
Can this technology be made to work with discrete GPUs or is it for integrated graphics only?zyankali - Wednesday, August 7, 2013 - link
PSR is part of the embedded Display Port (eDP) spec. eDP is only used in embedded displays because it requires a permanent physical connection. There is no reason why this wouldn't work on a discrete GPU, but the system must have the panel embedded. So computers using wired displays can't take advantage.zyankali - Wednesday, August 7, 2013 - link
You also have to remember this technology is aimed purely at power savings. It adds extra cost to a panel so will probably only be used in places where less power usage is really important. Which for now most likely means systems that already have integrated graphics.Laptops and AOIs that have discrete GPUs might have support in the future, but I can't see it being a top priority.
Krysto - Wednesday, August 7, 2013 - link
Nexus 10 had PSR too I think, or at least the SoC supported it.mort32 - Wednesday, August 7, 2013 - link
What's the technical reason to not have the GPU stop sending updates (like what it's doing with PSR) and have the LCD just not update the screen? Why have a memory buffer that holds the static image and still update the screen with it?