[RFC PATCH] drm: Add plane event

[Marcus Lorentzon]

On 04/18/2012 05:27 PM, Daniel Vetter wrote:
> On Wed, Apr 18, 2012 at 05:10:47PM +0200, Marcus Lorentzon wrote:
>> On 04/18/2012 04:36 PM, Daniel Vetter wrote:
>>> Last time around I've discussed with people we've ended up with 2 new
>>> ioctls:
>>>
>>> - atomic modeset, to configure the output state on more than one crtc at
>>>    the same time. This is useful to get pll assignement, memory bandwidth
>>>    constrains and similar stuff right. This ioctl is synchronous. A
>>>    testmode can be used to inquire the driver whether the proposed mode
>>>    actually works. This could be used for gui interfaces to automatically
>>>    grey out unsupportable configurations, e.g. if you have 3 crtc but on 2
>>>    pll and 2 modes need to have the same pixelclocks to drive all 3 crtcs.
>>>
>>> - an atomic pageflip. This one would be like the current pageflip ioclt,
>>>    i.e. run asynchronously and deliver a drm event upont completion. The
>>>    idea is that compositors can use this to make flicker-free compositition
>>>    with drm planes possible. I think we want drivers to be able to indicate
>>>    which crtc properties they can switch with this ioctl, e.g. I expect
>>>    some yuv->rbg color space properties might not work. All the changes
>>>    should be applied on the same vblank, obviously.
>> Why an atomic pagefilp? How is this different from an atomic modeset
>> where only fbs change? Can't drm frmwrk "optimize" this like SETCRTC
>> does today with base/full modeset helpers?
> The important difference is that the pageflip should happen vsynced on one
> single crtc. So it can't do anything which takes longer than a vsync
> (usually you need to wait a frame for the clocks to stabilize before
> switching on the next stage in the output pipeline), so any output
> configuration changes are pretty much out of the window. We also want
> pageflip to run asynchronously and return immediately after emitting all
> relevant commands. That's not really important for modeset.
>
> So yeah, you could smash all this into one giant ioctl, but I think the
> split is useful and would simplify things quite a bit on the
> implementation side. Otherwise you need to add funny queries so that
> userspace can figure out which modeset ops run asynchronous, which can be
> vblank synced. And it needs to tell the kernel for which it wants an
> event. Especially when you have multiple crtc and want to drive all of
> them with a compositor, this can get funny.
>
> Also, I'm toying around with ideas to split up the big modeset lock such
> that operations that only touch the crtc (like pageflip, plane changes and
> cursor changes) do not take the big modeset lock but only a per-crtc
> mutex. This way a compositor running on crtc A could run without hiccups
> while we do a modeset operation on crtc B, or while a hotplug detection is
> reading back the edid from a unused connector (which can easily take
> longer than a few frames). We have tons of bug reports from users that
> complain that every 10s their cursor stalls (due to hotplug detect).
>
> If you smash everything into one ioctl, I imagine you have plenty of fun
> implementing all this. Which is why I prefer to split this up.
> -Daniel

The async vs sync makes sense as reason for splitting them. My problem 
lies somewhere in between sync modeset and async flip - async 
crtc/plane/fbs modeset.
In Wayland and Android HW composer I need to asynchronously flip and do 
crtc/plane modeset, but no connector/crtc modeset (so it is a fast 
operation). Because I don't consider enable/disable/move a plane to be a 
full synchronous modeset (same mode different fbs/planes). But I still 
want the same async events to tell me the new plane setup is activated 
at vsync. But as you say, maybe the biggest issue here is the "big drm 
lock". So maybe user space would be ok to do this crtc-modeset in sync 
mode, if it doesn't block other operations on other crtcs. But I would 
prefer to be able to do the crtc modeset async so I don't have to have a 
thread per crtc. Or am I missing the obvious solution to this?

/BR
/Marcus