Re: [PATCH 1/2] drm/i915: time out of load detect busy-waits

[Chris Wilson <chris@chris-wilson.co.uk>]

On Wed, 25 Apr 2012 21:26:50 +0200, Daniel Vetter <daniel@ffwll.ch> wrote:
> On Wed, Apr 25, 2012 at 06:14:37PM +0100, Chris Wilson wrote:
> > On Fri, 20 Apr 2012 21:03:35 +0200, Daniel Vetter <daniel.vetter@ffwll.ch> wrote:
> > > If we try to do that and the scanlines just wouldn't advance, we
> > > busy-hang the machine holding the modeset mutex. Not great for
> > > debugging.
> > > 
> > > References: https://bugzilla.kernel.org/show_bug.cgi?id=43020
> > > Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
> > 
> > Reviewer hangs head in shame:
> > 
> > > +		if (wait_for(I915_READ(pipe_dsl_reg) >= vactive, 1000))
> > > +			DRM_ERROR("timed out waiting for vactive in "
> > > +				  "load_detect, scanline: %u\n",
> > > +				  I915_READ(pipe_dsl_reg));
> > > +		if (wait_for((dsl = I915_READ(pipe_dsl_reg)) <= vsample, 1000))
> > > +			DRM_ERROR("timed out waiting for vsample in "
> > > +				  "load_detect, scanline: %u\n",
> > > +				  I915_READ(pipe_dsl_reg));
> > 
> > wait_for() catches us out everytime we convert and existing while(),
> > because the predicate is when it stops. Perhaps if we had a wait_until,
> > but anyway the fix here is:
> > 
> > if (wait_for(I915_READ(pipe_dsl_reg) < vactive, 1000))
> >   ...
> > if (wait_for((dsl = I915_READ(pipe_dsl_reg)) > vsample, 1000))
> >   ...
> dinq rectified, it never happened. Thanks for catching this.

wait_for() has even more subtleties in store for us, the unwary coder.
By default, it uses a 1ms sleep between polling the register, chosen to
be kind whilst waiting for panel bits to power up which do take a fair
amount of time. Here, that extra delay causes us to sample the vsync
rather than the border. The quirk of the [vh]sync is that the monitor bit
of ST00 is always true. And since we always seem to pick that row to read
we always think there is a CRT present.

The choice is either to use the busy-polling variant, wait_for_atomic,
or restructure the entire block to use a single timeout with direct
reads. And whilst you are modifying the code, convert the polling reads
to I915_READ_NOTRACE().
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre