On Dec 22, 2010, at 6:23 PM, Jesse Barnes wrote:
On Wed, 22 Dec 2010 05:36:13 +0100 Mario Kleiner <mario.kleiner@xxxxxxxxxxxxxxxx> wrote:-------------------------------------------------------------------- --Message: 1 Date: Mon, 20 Dec 2010 19:23:40 -0800 From: Keith Packard <keithp@xxxxxxxxxx> Subject: Re: [Intel-gfx] [PATCH] drm: Aggressively disable vblanks To: Andy Lutomirski <luto@xxxxxxx>, Jesse Barnes<jbarnes@xxxxxxxxxxxxxxxx>, Chris Wilson <chris@chris- wilson.co.uk>,David Airlie <airlied@xxxxxxxx> Cc: intel-gfx@xxxxxxxxxxxxxxxxxxxxx, dri-devel@xxxxxxxxxxxxxxxxxxxxx Message-ID: <yunfwtrrepv.fsf@xxxxxxxxxxxxxxx> Content-Type: text/plain; charset="us-ascii" On Mon, 20 Dec 2010 14:00:54 -0500, Andy Lutomirski <luto@xxxxxxx> wrote:Enabling and disabling the vblank interrupt (on devices that support it) is cheap. So disable it quickly after each interrupt.So, the concern (and reason for the original design) was that you might lose count of vblank interrupts as vblanks are counted differently whileoff than while on. If vblank interrupts get enabled near the interrupttime, is it possible that you'll end up off by one somehow? Leaving them enabled for 'a while' was supposed to reduce the impact of this potential error. -- keith.packard@xxxxxxxxx -------------- next part -------------- A non-text attachment was scrubbed... Name: not available Type: application/pgp-signature Size: 189 bytes Desc: not available URL: <http://lists.freedesktop.org/archives/dri-devel/attachments/ 20101220/105a9fb6/attachment-0001.pgp> ------------------------------ Message: 2 Date: Mon, 20 Dec 2010 22:34:12 -0500 From: Andrew Lutomirski <luto@xxxxxxx> Subject: Re: [Intel-gfx] [PATCH] drm: Aggressively disable vblanks To: Keith Packard <keithp@xxxxxxxxxx> Cc: intel-gfx@xxxxxxxxxxxxxxxxxxxxx, dri-devel@xxxxxxxxxxxxxxxxxxxxx Message-ID: <AANLkTik-1zni1DdS6i+1ARoenx6p=9jQAAiA6t5uGg_K@xxxxxxxxxxxxxx> Content-Type: text/plain; charset=ISO-8859-1 On Mon, Dec 20, 2010 at 10:23 PM, Keith Packard <keithp@xxxxxxxxxx> wrote:On Mon, 20 Dec 2010 14:00:54 -0500, Andy Lutomirski <luto@xxxxxxx> wrote:Enabling and disabling the vblank interrupt (on devices that support it) is cheap. ?So disable it quickly after each interrupt.So, the concern (and reason for the original design) was that you might lose count of vblank interrupts as vblanks are counted differently while off than while on. If vblank interrupts get enabled near the interrupt time, is it possible that you'll end up off by one somehow?So the race is: 1. Vblank happens.2. vblank_get runs, reads hardware counter, and enables the interrupt(and maybe not quite in that order) 3. Interrupt fires and increments the counter. Now the counter is one too high.What if we read the hardware counter from the IRQ the first time that it fires after being enabled? That way, if the hardware and softwarecounters match (mod 2^23 or whatever the magic number is), we don't increment the counter.Leaving them enabled for 'a while' was supposed to reduce the impact of this potential error.Fair enough,But five seconds is a *long* time, and anything short enough that the interrupt actually gets turned off in normal use risks the same race.--Andy ------------------------------ Message: 3 Date: Mon, 20 Dec 2010 19:47:11 -0800 From: Keith Packard <keithp@xxxxxxxxxx> Subject: Re: [Intel-gfx] [PATCH] drm: Aggressively disable vblanks To: Andrew Lutomirski <luto@xxxxxxx> Cc: intel-gfx@xxxxxxxxxxxxxxxxxxxxx, dri-devel@xxxxxxxxxxxxxxxxxxxxx Message-ID: <yunbp4frdmo.fsf@xxxxxxxxxxxxxxx> Content-Type: text/plain; charset="us-ascii" On Mon, 20 Dec 2010 22:34:12 -0500, Andrew Lutomirski <luto@xxxxxxx> wrote:But five seconds is a *long* time, and anything short enough that the interrupt actually gets turned off in normal use risks the same race.Right, so eliminating any race seems like the basic requirement. Can that be done? -- keith.packard@xxxxxxxxx -------------- next part -------------- A non-text attachment was scrubbed... Name: not available Type: application/pgp-signature Size: 189 bytes Desc: not available URL: <http://lists.freedesktop.org/archives/dri-devel/attachments/ 20101220/5ca3b674/attachment-0001.pgp> ------------------------------ Message: 4 Date: Mon, 20 Dec 2010 22:55:46 -0500 From: Andrew Lutomirski <luto@xxxxxxx> Subject: Re: [Intel-gfx] [PATCH] drm: Aggressively disable vblanks To: Keith Packard <keithp@xxxxxxxxxx> Cc: intel-gfx@xxxxxxxxxxxxxxxxxxxxx, dri-devel@xxxxxxxxxxxxxxxxxxxxx Message-ID: <AANLkTimk6RLkr8Lt76cR8ncLW_3kaX6Dqa+=id9_G-8C@xxxxxxxxxxxxxx> Content-Type: text/plain; charset=ISO-8859-1 On Mon, Dec 20, 2010 at 10:47 PM, Keith Packard <keithp@xxxxxxxxxx> wrote:On Mon, 20 Dec 2010 22:34:12 -0500, Andrew Lutomirski <luto@xxxxxxx> wrote:But five seconds is a *long* time, and anything short enough that the interrupt actually gets turned off in normal use risks the same race.Right, so eliminating any race seems like the basic requirement. Canthat be done?I'll give it a shot. Do you know if there's an existing tool to call drm_wait_vblank from userspace for testing? I know approximately nothing about libdrm or any userspace graphics stuff whatsoever. --Andy-- keith.packard@xxxxxxxxx------------------------------ Message: 5 Date: Mon, 20 Dec 2010 20:03:53 -0800 From: Jesse Barnes <jbarnes@xxxxxxxxxxxxxxxx> Subject: Re: [Intel-gfx] [PATCH] drm: Aggressively disable vblanks To: Andrew Lutomirski <luto@xxxxxxx> Cc: intel-gfx@xxxxxxxxxxxxxxxxxxxxx, dri-devel@xxxxxxxxxxxxxxxxxxxxx Message-ID: <20101220200353.12479178@jbarnes-desktop> Content-Type: text/plain; charset=US-ASCII On Mon, 20 Dec 2010 22:55:46 -0500 Andrew Lutomirski <luto@xxxxxxx> wrote:On Mon, Dec 20, 2010 at 10:47 PM, Keith Packard <keithp@xxxxxxxxxx> wrote:On Mon, 20 Dec 2010 22:34:12 -0500, Andrew Lutomirski <luto@xxxxxxx> wrote:But five seconds is a *long* time, and anything short enough that the interrupt actually gets turned off in normal use risks the same race.Right, so eliminating any race seems like the basic requirement. Canthat be done?I'll give it a shot.Do you know if there's an existing tool to call drm_wait_vblank from userspace for testing? I know approximately nothing about libdrm orany userspace graphics stuff whatsoever.Yeah, libdrm has a test program called vbltest; it should do what youneed.Not so fast please! After a lot of review by Jesse, Dave and Chris just merged a set of my patches into the drm-next (and the intel and radeon kms drivers) to implement precise timestamping of vblank's and pageflip completion and vblank counting for DRI2 and theOML_sync_control extension. It also fixes (hopefully) almost all race-conditions (that i could find or think of) related to vblank irq on/ off, a few of them surprising and due to "funny" behaviour of some gpu's when you enable/disable vblanks (e.g., radeon's spontaneously firing a vblank irq in the middle of a scanout cycle when vblank irq's get enabled, or firing the irq sometimes shortly before a vblank instead of in the vblank). There's one tiny race left in the vblank off path, which i wanted to address during the next weeks. Also i need to implement support for nouveau. After that we could simply reduce the vblank off timeout tosomething small like 50 msecs. Or use Andrew's heuristic on top of this.In any case, please check against the drm-next branch. I think your patches touch/conflict with most of the areas in drm that are modified in drm-next. At least my users need a very high level of precision and robustness in vblank counting and timestamping for neuro-science applications and similar stuff.To preserve all of that, the easiest way to reduce the amount of time the vblank remains enabled would probably be to reduce the vblank off timeout. Just make it 100ms or so since that will make sure it stays on for a few frames at least. Mario, for cases where you have very intermittent waits, I think you added a method to disable the timer altogether? That combined with a reduced timeout seems like it could make everyone happy... -- Jesse Barnes, Intel Open Source Technology Center
There's a new drm module parameter for selecting the timeout: echo 50 > /sys/module/drm/parameters/vblankoffdelay would set the timeout to 50 msecs. A setting of zero will disable the timer, so vblank irq's would stay on all the time.
The default setting is still 5000 msecs as before, but reducing this to 100 msecs wouldn't be a real problem imho. At least i didn't observe any miscounting during extensive testing with 100 msecs.
The patches in drm-next fix a couple of races that i observed on intel and radeon during testing and a few that i didn't see but that i could imagine happening. It tries to make sure that the saved final count at vblank irq disable of the software vblank_count and the gpu counter are consistent - no off by one errors. They also try to detect and filter out spurious vblank interrupts at vblank enable time, e.g., on the radeon.
There's still one possible race in the disable path which i will try to fix: We don't know when exactly the hardware counter increments wrt. the processing of the vblank interrupt - it could increment a few (dozen/hundred) microseconds before or after the irq handler runs, so if you happen to query the hardware counter while the gpu is inside the vblank you can't be sure if you picked up the old count or the new count for that vblank.
This only matters during vblank disable. For that reason it's not such a good idea to disable vblank irq's from within the vblank irq handler. I tried that and it didn't work well --> When doing it from within irq you basically maximize the chance of hitting the time window when the race can happen. Delaying within the irq handler by a millisecond would fix that, but that's not what we want.
Having the disable in a function triggered by a timer like now is the most simple solution i could come up with. There we can burn a few dozen microseconds if neccessary to remove this race.
There could be other races that i couldn't think of and that i didn't see during my testing with my 2 radeons and 1 intel gpu. Therefore i think we should keep vblank irq's enabled for at least 2 or maybe 3 refresh cycles if they aren't used by anyone. Apps that want to schedule swaps very precisely and the ddx/drm/kms-driver itself do multiple queries in quick succession for a typical swapbuffers call, i.e., drm_vblank_get() -> query -> drm_vblank_put(), so on an otherwise idle graphics system the refcount will toggle between zero and one multiple times during a swap, usually within a few milliseconds. If the timeout is big enough so that irq's don't get disabled within such a sequence of toggles, even if there's a bit of scheduling delay for the x-server or client, then a client will see at least consistent vblank count during a swap, even if there are still some races hiding somewhere that we don't handle properly. That should be good enough, and paranoid clients can always increase the timeout value or set it to infinite.
E.g., my toolkit schedules a swap for a specific system time like this: 1. glXGetSyncValuesOML(... &base_msc, &base_ust);2. calculate target_msc based on user provided swap deadline t and (base_msc, base_ust) as a baseline.
3. glXSwapBuffersMscOML(...., target_msc,...);4. glXWaitForSbcOML() or use Intel_swap_events for retrieving the true msc and ust of swap completion.
=> Doesn't matter if there would be an off-by-one error in vblank counting due to an unknown race, as long as it doesn't happen between 1. and 4. As long as there aren't any client/x-server scheduling delays between step 1 and 3 of more than /sys/module/drm/parameters/ vblankoffdelay msecs, nothing can go wrong even if there are race conditions left in that area.
=> 50-100 msecs as new default would be probably good enough and at the same time prevent the "blinking cursor keeps vblank irq's on all the time" problem.
I didn't reduce the timeout in the current patches because the filtering for race-conditions and other gpu funkyness relies on somewhat precise vblank timestamps and the timestamping hooks aren't yet implemented in the nouveau kms. Maybe i manage to get this working over christmas. Patches to nouveau would be simple, i just don't know the mmio register addresses for crtc scanout position on nvidia gpu's.
-mario ********************************************************************* Mario Kleiner Max Planck Institute for Biological Cybernetics Spemannstr. 38 72076 Tuebingen Germany e-mail: mario.kleiner@xxxxxxxxxxxxxxxx office: +49 (0)7071/601-1623 fax: +49 (0)7071/601-616 www: http://www.kyb.tuebingen.mpg.de/~kleinerm ********************************************************************* "For a successful technology, reality must take precedence over public relations, for Nature cannot be fooled." (Richard Feynman) _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx http://lists.freedesktop.org/mailman/listinfo/dri-devel
