Re: [RFC PATCH] drm/vc4: Add a load tracker to prevent HVS underflow errors

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Tue, Oct 16, 2018 at 03:12:54PM +0200, Daniel Vetter wrote:
> On Tue, Oct 16, 2018 at 3:10 PM Boris Brezillon
> <boris.brezillon@bootlin.com> wrote:
> >
> > Hi Daniel,
> >
> > On Tue, 16 Oct 2018 14:57:43 +0200
> > Daniel Vetter <daniel@ffwll.ch> wrote:
> >
> > > On Tue, Oct 16, 2018 at 11:40:45AM +0200, Boris Brezillon wrote:
> > > > The HVS block is supposed to fill the pixelvalve FIFOs fast enough to
> > > > meet the requested framerate. The problem is, the HVS and memory bus
> > > > bandwidths are limited, and if we don't take these limitations into
> > > > account we might end up with HVS underflow errors.
> > > >
> > > > This patch is trying to model the per-plane HVS and memory bus bandwidth
> > > > consumption and take a decision at atomic_check() time whether the
> > > > estimated load will fit in the HVS and membus budget.
> > > >
> > > > Note that we take an extra margin on the memory bus consumption to let
> > > > the system run smoothly when other blocks are doing heavy use of the
> > > > memory bus. Same goes for the HVS limit, except the margin is smaller in
> > > > this case, since the HVS is not used by external components.
> > > >
> > > > Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
> > > > ---
> > > > This logic has been validated using a simple shell script and
> > > > some instrumentation in the VC4 driver:
> > > >
> > > > - capture underflow errors at the HVS level and expose a debugfs file
> > > >   reporting those errors
> > > > - add debugfs files to expose when atomic_check fails because of the
> > > >   HVS or membus load limitation or when it fails for other reasons
> > > >
> > > > The branch containing those modification is available here [1], and the
> > > > script (which is internally using modetest) is here [2] (please note
> > > > that I'm bad at writing shell scripts :-)).
> > > >
> > > > Note that those modification tend to over-estimate the load, and thus
> > > > reject setups that might have previously worked, so we might want to
> > > > adjust the limits to avoid that.
> > > >
> > > > [1]https://github.com/bbrezillon/linux/tree/vc4/hvs-bandwidth-eval
> > > > [2]https://github.com/bbrezillon/vc4-hvs-bandwidth-test
> > >
> > > Any interest in using igt to test this stuff? We have at least a bunch of
> > > tests already in there that try all kinds of plane setups. And we use
> > > those to hunt for underruns on i915 hw.
> > >
> > > Wrt underrun reporting: On i915 we just dump them into dmesg at the error
> > > level, using DRM_ERROR, plus a tracepoint. See e.g.
> > > intel_pch_fifo_underrun_irq_handler(). If there's interest we could
> > > perhaps extract this into something common, similar to what was done with
> > > crc support already.
> >
> > Sounds like a good idea. I'll have a look at what's done in the intel
> > driver and will check how feasible this is to have a common
> > infrastructure to test that in igt.
> >
> > Thanks for the pointers.
> >
> >
> > > > +static int vc4_load_tracker_atomic_check(struct drm_atomic_state *state)
> > > > +{
> > > > +   struct drm_plane_state *old_plane_state, *new_plane_state;
> > > > +   struct vc4_dev *vc4 = to_vc4_dev(state->dev);
> > > > +   struct vc4_load_tracker_state *load_state;
> > > > +   struct drm_private_state *priv_state;
> > > > +   struct drm_plane *plane;
> > > > +   int ret, i;
> > > > +
> > >
> > > You're missing the modeset locking for vc4->load_tracker. See the
> > > kerneldoc for drm_atomic_get_private_obj_state().
> >
> > Hm, I missed this part of the doc, and I thought we were protected by
> > the modeset lock.
> >
> > > Probably a good time to
> > > implement the locking refactoring idea I have and just implement a per
> > > private_obj lock, and remove all the ad-hoc locking from all the callers?
> >
> > Let me see if I get it correctly. You want to add a lock to the
> > drm_private_obj struct, right?
> 
> Yup, plus automatically grab that lock in get_private_obj_state, and
> then drop the now superfluous locking from all the callers.

You mean this?
https://patchwork.freedesktop.org/patch/205943/

-- 
Ville Syrjälä
Intel
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