[Intel-gfx] [PATCH] drm: Return current vblank value for drmWaitVBlank queries

[Ville Syrjälä]

On Thu, Mar 19, 2015 at 03:33:11PM +0100, Daniel Vetter wrote:
> On Wed, Mar 18, 2015 at 03:52:56PM +0100, Mario Kleiner wrote:
> > On 03/18/2015 10:30 AM, Chris Wilson wrote:
> > >On Wed, Mar 18, 2015 at 11:53:16AM +0900, Michel Dänzer wrote:
> > >>drm_vblank_count_and_time() doesn't return the correct sequence number
> > >>while the vblank interrupt is disabled, does it? It returns the sequence
> > >>number from the last time vblank_disable_and_save() was called (when the
> > >>vblank interrupt was disabled). That's why drm_vblank_get() is needed here.
> > >
> > >Ville enlightened me as well. I thought the value was cooked so that
> > >time did not pass whilst the IRQ was disabled. Hopefully, I can impress
> > >upon the Intel folks, at least, that enabling/disabling the interrupts
> > >just to read the current hw counter is interesting to say the least and
> > >sits at the top of the profiles when benchmarking Present.
> > >-Chris
> > >
> > 
> > drm_wait_vblank() not only gets the counter but also the corresponding
> > vblank timestamp. Counters are recalculated in vblank_disable_and_save() for
> > irq off, then in the vblank irq on path, and every refresh in
> > drm_handle_vblank at vblank irq time.
> > 
> > The timestamps can be recalculated at any time iff the driver supports high
> > precision timestamping, which currently intel kms, radeon kms, and nouveau
> > kms do. But for other parts, like most SoC's, afaik you only get a valid
> > timestamp by sampling system time in the vblank irq handler, so there you'd
> > have a problem.
> > 
> > There are also some races around the enable/disable path which require a lot
> > of care and exact knowledge of when each hardware fires its vblanks, updates
> > its hardware counters etc. to get rid of them. Ville did that - successfully
> > as far as my tests go - for Intel kms, but other drivers would be less
> > forgiving.
> > 
> > Our current method is to:
> > 
> > a) Only disable vblank irqs after a default idle period of 5 seconds, so we
> > don't get races frequent/likely enough to cause problems for clients. And we
> > save the overhead for all the vblank irq on/off.
> > 
> > b) On drivers which have high precision timestamping and have been carefully
> > checked to be race free (== intel kms only atm.) we have instant disable, so
> > things like blinking cursors don't keep vblank irq on forever.
> > 
> > If b) causes so much overhead, maybe we could change the "instant disable"
> > into a "disable after a very short time", e.g., lowering the timeout from
> > 5000 msecs to 2-3 video refresh durations ~ 50 msecs? That would still
> > disable vblank irqs for power saving if the desktop is really idle, but
> > avoid on/off storms for the various drm_wait_vblank's that happen when
> > preparing a swap.
> 
> Yeah I think we could add code which only gets run for drivers which
> support instant disable (i915 doesn't do that on gen2 because the hw is
> lacking). There we should be able to update the vblank counter/timestamp
> correctly without enabling interrupts temporarily. Ofc we need to make
> sure we have enough nasty igt testcase to ensure there's not going to be
> jumps and missed frame numbers in that case.

Is enabling the interrupts the expensive part, or is it the actual
double timestamp read + scanout pos read? Or is it due to the several
spinlocks we have in this code?

Also why is userspace reading the vblank counter in the first place? Due
to the crazy OML_whatever stuff perhaps? In the simple swap interval case
you shouldn't really need to read it. And if we actually made the page
flip/atomic ioctl take a target vblank count and let the kernel deal
with it we wouldn't need to call the vblank ioctl at all.

As far as gen2 goes, I have some code somewhere that improved things a
bit. Essentially I just looked at the monotonic timestamps to figure out
if we missed any vblanks while interrupts were off. IIRC my current code
only added one to the cooked count in that case, but should be easy to
make it come up with a more realistic delta.

-- 
Ville Syrjälä
Intel OTC