On Tue, Jul 21, 2015 at 10:06:09AM +0300, Pekka Paalanen wrote:
On Mon, 20 Jul 2015 10:32:31 -0700 Stéphane Marchesin stephane.marchesin@gmail.com wrote:
On Mon, Jul 20, 2015 at 7:21 AM, Daniel Vetter daniel@ffwll.ch wrote:
On Mon, Jul 20, 2015 at 12:35:48PM +0300, Pekka Paalanen wrote:
On Mon, 20 Jul 2015 01:58:33 -0700 Stéphane Marchesin stephane.marchesin@gmail.com wrote:
On Mon, Jul 20, 2015 at 12:46 AM, Pekka Paalanen ppaalanen@gmail.com wrote:
On Sun, 19 Jul 2015 17:20:32 -0700 Stéphane Marchesin stephane.marchesin@gmail.com wrote:
> On Thu, Jul 16, 2015 at 11:08 PM, Pekka Paalanen ppaalanen@gmail.com wrote: > > > > On Thu, 16 Jul 2015 20:20:39 +0800 > > John Hunter zhjwpku@gmail.com wrote: > > > > > From: Zhao Junwang zhjwpku@gmail.com > > > > > > This supports the asynchronous commits, required for page-flipping > > > Since it's virtual hw it's ok to commit async stuff right away, we > > > never have to wait for vblank. > > > > Hi, > > > > in theory, yes. This is what a patch to bochs implemented not too long > > ago, so AFAIK you are only replicating the existing behaviour. > > > > However, if userspace doing an async commit (or sync, I suppose) does > > not incur any waits in the kernel in e.g. sending the page flip event, > > then flip driven programs (e.g. a Wayland compositor, say, Weston) > > will be running its rendering loop as a busy-loop, because the kernel > > does not throttle it to the (virtual) display refresh rate. > > > > This will cause maximal CPU usage and poor user experience as > > everything else needs to fight for CPU time and event dispatch to get > > through, like input. > > > > I would hope someone could do a follow-up to implement a refresh cycle > > emulation based on a clock. Userspace expects page flips to happen at > > most at refresh rate when asking for vblank-synced flips. It's only > > natural for userspace to drive its rendering loop based on the vblank > > cycle. > > > I've been asking myself the same question (for the UDL driver) and I'm > not sure if this policy should go in the kernel. After all, there > could be legitimate reasons for user space to render lots of frames > per second. It seems to me that if user space doesn't want too many > fps, it should just throttle itself.
If userspace wants to render lots of frames per second, IMO it should not be using vblank-synced operations in a way that may throttle it. The lots of frames use case is already non-working for the majority of the drivers without DRM_MODE_PAGE_FLIP_ASYNC, right?
The problem here I see is that one DRM driver decides to work different to other DRM drivers. All real-hardware DRM drivers, when asked to do vblank-synced update, actually do throttle to the vblank AFAIK.
udl is an exception here. It is (arguably) real hardware but doesn't throttle.
Is it too much to assume, that the video mode set in a driver (refresh rate) corresponds to the vblank rate which implicitly delays the completion of vblank-sync'd operations to at least the next vblank boundary?
I think it's wrong to make user space think that a vsynced display always matches the refresh rate in a world where:
- some displays have variable refresh rates (not just the fancy new
stuff like g-sync, look for lvds_downclock in the intel driver for example, also consider DSI displays)
- some displays have no refresh rate (the ones we are talking about
here: udl, bochs...)
Imo aiming for vrefresh to be accurate is good. For gsync and friends I think we should have an explicit range or flag to make userspace aware of what's going on.
I think the concept of vrefresh is flawed and not really future-proof (I gave a few examples in my previous email). I agree we should keep it as legacy, but we should add something else for the more advanced cases.
Right, so let's add something new for new hardware features and keep the existing behavior existing.
I suppose the problem is that the existing behavior is not really documented so we have to resort to screaming users?
If one does not ask for ASYNC with a page flip, does it mean flipping on the next vblank, or flipping such that it cannot tear but allowing techiques like scanline waits?
Since legacy page_flip is always for the full primary plane you can't do scanline waits - it covers everything anyway.
It used to be reasonable to assume a constant refresh rate apart from explicit mode changes. Should we keep this assumption the default and add API to say different?
I think, if the driver cannot implement proper semantics (which IMO includes the throttling) for vblank-sync'd operations and it does not want to fake them with a clock, it should just refuse vblank-synced operations.
Yes refusing vsynced flips for these drivers sounds reasonable. But please let's not bake in another assumption in the API (or rather, let's try to un-bake it).
Could you be more specific on everything, please?
What should drivers do in different situations, what guarantees we do have, and how does userspace predict the earliest possible flip time? How do you define flip time to begin with, if it's not tied to the scanout cycle (vblank)?
How should a compositor schedule eveything, and what can it tell to the clients about the timings in the immediate future?
You gave me the feeling that everything I thought I knew and relied on is wrong.
I guess we either kick out page_flip for all drivers who fake it. And if that's causing regressions then we probably want to fake it with a timer. Unpretty, but such is the game of backwards compat forever. But I'm not sure whether we established that we have a problem already, at least I'm missing users screaming about udl/bochs & friends.
I suppose not hearing users scream is that X.org is not affected, because it simply doesn't work in a way it would be affected? Or the various DDX'en. The combination of affected userspace with drivers like bochs is even more rare.
I first heard about Weston having an abysmal user experience on drm/bochs when a co-worker was looking at testing Weston in QEMU/stdvga, which according to others was supposed to be the best working QEMU output / DRM KMS driver combination. Might have something to do with needing it on a virtual ARM cpu.
Weston in a VM has not been too attractive before, because of the missing EGL platform support for swrast, but that has been fixed recently. I've also heard users (RebeccaBlackOS) to just revert to Weston's fbdev backend, when the DRM backend just doesn't work right in a VM.
So I would say it is a known problem with Weston, but users tend to just dismiss it rather than start pushing it forward. Whether you count Weston as an existing user in the first place is up to you, I suppose.
I asked Jasper about Mutter: < pq> Jasper, trying to find users of the KMS API who would work badly, if page flips were signalled always immediately < Jasper> pq, oh, god, us. < Jasper> pq, we'd spin ourselves in a paint loop to death
I think there might be two different problems here: a) signalling page flips immediately, and b) "variable refresh rate" systems / on-demand updates like g-sync, UDL, etc.
My immediate concern is to outlaw immediate signalling of operations that are intended to be vblank-synced (those that have traditionally taken time to complete on real hardware, if you don't like the definition of vblank-synced).
The problem with variable refresh rate systems is unpredictability, which is different. Submitting updates will take some time in any case, so you cannot really loop to death (I hope). There I would be happy to just know that predictions based on perpetual refresh rate are invalid. That would be enough for Wayland at first.
Stéphane also raised the concern that scanout downclocking etc. may cause flips to be quite late from predicted. This is less of a problem, rendering can also take arbitrary times and software is usually written to deal with missing deadlines. It's a problem that happens all the time anyway. As it is a problem with prediction accuracy, we could just wave it off by setting the imaginary "no constant refresh rate" flag. IMHO this is a problem that should be solved at another occasion, if necessary.
Downclocking should only be able to delay the very first frame in a sequence (video playback, animation). Imo we can shrug that off as "clock skew" ;-)
Imo once someone updates frames regularly vblank timestamps should be evenly spaced and pageflips not instant (at least by default).
Yeah I don't think I care about the old interface, it is what it is. But we should design something which works well for the future use cases.
Are you referring to the atomic API as the old thing?
In the end, the question is how paranoid should display servers be about the timings? Do we need to fix Weston, Mutter and likely others to deal with page flips being signalled immediately, or is it more appropriate to fix the DRM drivers to keep up the illusion of providing what the API appears to suggest?
I guess they can't assume too much about vblanks (too many drivers don't even bother with precise/irq-delay-correct timestamps), but I think assuming that doing a page_flip completion event based renderer shouldn't result in spinning is reasonable.
I guess for bochs/udl and others we could create a small drm driver which keeps track of the last vblank ts (we have those already) and suitable delays the even/timestamp to keep up the illusion. Or we just rip out pageflip support for those drivers. But if weston&co can't cope with that that would be worse. -Daniel