On Mon, Aug 11, 2014 at 03:47:22PM +0300, Pekka Paalanen wrote: > > > What if I cannot even pick a maximum number of planes, but wanted to > > > (as the hardware allows) let the 2D compositing scale up basically > > > unlimited while becoming just slower and slower? > > > > > > I think at that point one would be looking at a rendering API really, > > > rather than a KMS API, so it's probably out of scope. Where is the line > > > between KMS 2D compositing with planes vs. 2D composite rendering? > > > > I think kms should still be real-time compositing - if you have to > > internally render to a buffer and then scan that one out due to lack of > > memory bandwidth or so that very much sounds like a rendering api. Ofc > > stuff like writeback buffers blurry that a bit. But hw writeback is still > > real-time. > > Agreed, that's a good and clear definition, even if it might make my > life harder. > > I'm still not completely sure, that using an intermediate buffer means > sacrificing real-time (i.e. being able to hit the next vblank the user > space is aiming for) performance, maybe the 2D engine output rate > fluctuates so that the scanout block would have problems but a buffer > can still be completed in time. Anyway, details. > > Would using an intermediate buffer be ok if we can still maintain > real-time? That is, say, if a compositor kicks the atomic update e.g. > 7 ms before vblank, we would still hit it even with the intermediate > buffer? If that is actually possible, I don't know yet.
I guess you could hide this in the kernel if you want. After all the entire point of kms is to shovel the memory management into the kernel driver's responsibility. But I agree with Rob that if there are intermediate buffers, it would be fairly neat to let userspace know about them. So I don't think the intermediate buffer thing would be a no-go for kms, but I suspect that will only happen when the videocore can't hit the next frame reliably. And that kind of stutter is imo not good for a kms driver. I guess you could forgo vblank timestamp support and just go with super-variable scanout times, but I guess that will make the video playback people unhappy - they already bitch about the sub 1% inaccuracy we have in our hdmi clocks. > > > Should I really be designing a driver-specific compositing API instead, > > > similar to what the Mesa OpenGL implementations use? Then have user > > > space maybe use the user space driver part via OpenWFC perhaps? > > > And when I mention OpenWFC, you probably notice, that I am not aware of > > > any standard user space API I could be implementing here. ;-) > > > > Personally I'd expose a bunch of planes with kms (enough so that you can > > reap the usual benefits planes bring wrt video-playback and stuff like > > that). So perhaps something in line with what current hw does in hw and > > then double it a bit or twice - 16 planes or so. Your driver would reject > > any requests that need intermediate buffers to store render results. I.e. > > everything that can't be scanned out directly in real-time at about 60fps. > > The fun with kms planes is also that right now we have 0 standards for > > z-ordering and blending. So would need to define that first. > > I do not yet know where that real-time limit is, but I'm guessing it > could be pretty low. If it is, we might start hitting software > compositing (like Pixman) very often, which is too slow to be usable. Well for other drivers/stacks we'd fall back to GL compositing. pixman would obviously be terribly. Curious question: Can you provoke the hw/firmware to render into abitrary buffers or does it only work together with real display outputs? So I guess the real question is: What kind of interface does videocore provide? Note that kms framebuffers are super-flexible and you're freee to add your own ioctl for special framebuffers which are rendered live by the vc. So that might be a possible way to expose this if you can't tell the vc which buffers to render into explicitly. > Defining z-order and blending sounds like peanuts compared to below. > > > Then expose everything else with a separate api. I guess you'll just end > > up with per-compositor userspace drivers due to the lack of a widespread > > 2d api. OpenVG is kinda dead, and cairo might not fit. > > Yeah, that is kind of the worst case, which also seems unavoidable. Yeah, there's no universal 2d accel standard at all. Which sucks for hw that can't do full gl. -Daniel -- Daniel Vetter Software Engineer, Intel Corporation +41 (0) 79 365 57 48 - http://blog.ffwll.ch
