On Dec 26, 2010, at 3:53 PM, Andrew Lutomirski wrote:
On Wed, Dec 22, 2010 at 4:06 PM, Mario Kleiner
<mario.kleiner@xxxxxxxxxxxxxxxx> wrote:
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.
That's disgusting. Does this affect many GPUs? (I can't imagine why
any sensible implementation wouldn't guarantee that the counter
increments just before the IRQ.)
;-). I don't know, but at least on the tested R500 and R600 class
Radeon's, this was the case, so i assume it's at least this way on
many radeon gpu's (probably all avivo parts?) out there. We don't
have any evergreen gpu's yet in our lab so i don't know how the more
recent parts behave. Also it doesn't matter
I guess it's also a matter of definition when a new video frame
starts? Leading edge / trailing edge of vblank? Start of vsync?
Something else?
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.
Maybe I'm missing something, but other than the race above (which
seems like it shouldn't exist on sane hardware), the new code seems
more complicated than necessary.
I don't think it's more complicated than necessary for what it tries
to achieve, but of course i'm a bit biased. It also started off more
simple and grew a bit when i found new issues with the tested gpu's.
The aim is to fix a couple of real races and to make vblank counts
and timestamps as trustworthy and oml_sync_control spec-conformant
(see http://www.opengl.org/registry/specs/OML/glx_sync_control.txt)
as possible. It only consumes a few additional bytes of memory
(approx. 40 bytes) per crtc, doesn't use excessive time inside the
irq handler and tries to avoid taking locks that are shared between
irq and non-irq context to avoid delays in irq execution, also if
used with a kernel with the preempt_rt patch applied (important for
my use case and other hard realtime apps). It's pretty self-contained
and because most of it is driver-independent it can handle similar
issues on different gpu's and kms drivers without the need for us to
check each single gpu + driver combo if it has such issues or not.
1. There's the hardware vblank counter race, and it's there on lots
of existing hardware, regardless if this is sane or not, so it needs
to be handled.
2. There are gpu's firing spurious vblank irq's as soon as you enable
irq's -- you need to filter those out, otherwise counts and
timestamps will be wrong for at least one refresh cycle after vblank
irq enable. You don't know when these redundant ones get delivered or
if they get delivered at all because a real vblank irq enable gets
triggered by some userspace calls, not locked to the video refresh
cycle and because the enable code itself holds spin_lock_irqsave
locks and may or may not (depending on number of cores and irq
routing) prevent delivery of the vblank irq's concurrent to its own
execution -> a possible race between the drm_handle_vblank() routine
and the drm_update_vblank_count() routine each time you call
drm_vblank_get().
3. There's gpu's sometimes, but not on other times, firing the irq
too early (1-3 scanlines observed on radeon's) so you get your irq
before the associated vblank interval and need to do at least all
timestamping as if you are already in that vblank. This may be
dependent on both video mode and on the dispatch delay of the vblank
irq handler (e.g., due to some other unrelated code holding off
irq's, e.g., by holding spin_lock_irqsave() locks).
4. In the old code it could happen that the vblank counter gets
incremented after it was "disabled", e.g., because vblank irq's were
turned off in the gpu, but there was still a vblank irq pending
(e.g., due to some spin_lock_irqsave on the relevant core),
incrementing the counter after it was supposedly frozen. With the new
oml_sync_control timestamping in place, such off-by-ones would be
worse as they would also corrupt timestamps.
There were some more issues which i can't remember from the top of my
head which get handled by the current code (Note to myself: Take more
notes!).
Why not do nothing at all on vblank disable and, on enable, do this:
Call get_vblank_counter and declare the return value to be correct.
Because declaring it to be correct isn't the same as it being
correct. Also the code needs to handle wraparound of the hardware
counter and for that it needs correct start values from the vblank
disable routine, which is why the disable routine needs to work as
race-free as possible. The vblank count for a frame also needs to be
consistent with the vblank timestamp for that frame at all times,
otherwise the oml_sync_control extension becomes too unreliable to be
trustworthy and useful for serious applications.
On each vblank IRQ (or maybe just the first one after enable), read
the counter again and if it matches the cached value, then assume we
raced on enable (i.e. we enabled right at the beginning of the vblank
interval, and this interrupt is redundant). In that case, do nothing.
Otherwise increment the cached value by one.
On hardware with the silly race where get_vblank_counter in the IRQ
can return a number one too low, use the scanout pos to fix it (i.e.
if the scanout position is at the end of the frame, assume we hit that
race and increment by 1).
See the other races above. Iirc i tried something similar already and
they made it fail/unreliable. The current patch uses the vblank
timestamp instead of the hardware vblank counter to detect and filter
redundant irq's, because with the timestamping patches at least on
intel and ati gpu's (and hopefully nouveau/nvidia and others asap)
these are well defined and accurate (to define the end of a vblank
interval) so they can serve as a reference point. The tbd fix for
race condition #1 will also use scanout position as a fixed reference.
The sample client code (below) for scheduling accurately timed
bufferswaps needs precise and trustworthy return values from
glXGetSyncValuesOML() for it to work. If vblank's are disabled at
time of invocation of glXGetSyncValuesOML() then that function will
trigger the real vblank irq enable path and use its return values for
swap scheduling - i.e. unless it is called within or close to a
vblank, it uses the vblank count and timestamp computed in
drm_update_vblank_count(), usually before the vblank irq had a chance
to run. For that reason it is important for my kind of applications
that it really delivers the right counts and timestamps especially in
the enable path.
This is the context in case you're interested why i'm so protective
of the current implementation: The toolkit i'm developing is probably
one of the currently most demanding clients of the new dri2 swap &
sync bits and it is used for neuroscience research. Many of the
experiments there require very precise visual timing and often sub-
millisecond accurate timestamping. Too many unhandled races in the
wrong places could really spoil the work of the scientists that are
my users. As long as we have a disable timeout of 5 seconds, vblank
irq's probably won't disable at all during most experiment sessions
and even if they do, the frequency of possible screwups is probably
small enough to be annoying but manageable with statistics (detecting/
discarding outliers in experimental results etc.). At a disable
timeout of around 50 msecs, the error rate would be unbearable.
That's why i would like to make sure that the implementation can
handle at least the already known quirks of a large number of the
gpu's out there if we go down to 50 msecs. But i assume races in that
code would affect the quality of "normal" media players as well, once
we choose such low timeouts.
Thanks and belated happy x-mas,
-mario
This means that it would be safe to disable vblanks from any context
at all, because it has no effect other than turning off the interupt.
--Andy
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)
*********************************************************************
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)
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