Am 09.08.2016 um 04:44 schrieb Michel Dänzer:
On 08/08/16 06:55 PM, Christian König wrote:
[SNIP]
Who said that games need to be able to enable/disable it in the
kernel side?
The assumption is that some games (and maybe other apps, e.g.
compositors) won't work correctly with variable refresh rate. So there
needs to be some way for the user to decide whether or not to use it
for a given app.
Completely agree on that, but this wasn't what I wanted to say.
My issue here is why does the kernel needs to know about this?
As far as I can see userspace should be able to handle this perfectly
fine on it's own.
I was basically thinking out loud that doing this via different modes
might be quite natural, *if* games allowed choosing a specific mode.
But unfortunately they don't. For the video playback case, how do you
envision the video player app communicating the refresh rate of the
currently playing video to the kernel?
Again the kernel doesn't need to know the refresh rate. All the kernel
needs to know is when to do the page flip.
So coming back to my example of a mode with 1920x1080 and 20-100Hz
refresh rate a classic modeline would then look something like this:
Modeline "1920x1080_dynamic" 302.50 1920 2072 2280 2640 1080 1083
1088 5735 -hsync +vsync
Note the high vertical total scan lines. Those basically reduce the
refresh rate from 100Hz (which this mode normally would have) down to
only 20Hz.
Now what userspace does on each page flip is specifying when this flip
should happen, e.g. when the frame should be started to be scanned out.
We can either specify this as frame counter + vertical line of the
previous frame or as something like CLOCK_MONOTONIC (I think I would
prefer CLOCK_MONOTONIC, but that's not a strong opinion).
In other words you put the whole concept upside down. It's no longer the
kernel which tells userspace when a vblank happened, but rather
userspace tells the kernel when it should happen (together with the
frame that should be displayed then).
Regards,
Christian.
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