Re: [PATCH 2/2] drm/i915: Reorder hw init to avoid executing with invalid context/mm state

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

On Fri, Dec 05, 2014 at 02:38:46PM +0000, Chris Wilson wrote:
> On Fri, Dec 05, 2014 at 04:31:35PM +0200, Ville Syrjälä wrote:
> > On Fri, Dec 05, 2014 at 02:15:22PM +0000, Chris Wilson wrote:
> > > Currently we initialise the rings, add the first context switch to the
> > > ring and execute our golden state then enable (aliasing or full) ppgtt.
> > > However, as we enable ppgtt using direct MMIO but load the PD using
> > > MI_LRI, we end up executing the context switch and golden render state
> > > with an invalid PD generating page faults. To solve this issue, first do
> > > the ppgtt PD setup, then set the default context and write the commands
> > > to run the render state into the ring, before we activate the ring. This
> > > allows us to be sure that the register state is valid before we begin
> > > execution.
> > > 
> > > This was spotted when writing the seqno/request conversion, but only with
> > > the ERROR capture did I realise that it was a necessity now.
> > > 
> > > RFC: cleanup the error handling in i915_gem_init_hw.
> > > 
> > > Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> > > ---
> > >  drivers/gpu/drm/i915/i915_gem.c         | 20 ++++++++++----------
> > >  drivers/gpu/drm/i915/intel_ringbuffer.c |  9 ++++++---
> > >  2 files changed, 16 insertions(+), 13 deletions(-)
> > > 
> > > diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> > > index c1c11418231b..c13842d3cbc9 100644
> > > --- a/drivers/gpu/drm/i915/i915_gem.c
> > > +++ b/drivers/gpu/drm/i915/i915_gem.c
> > > @@ -4796,15 +4796,15 @@ i915_gem_init_hw(struct drm_device *dev)
> > >  	 */
> > >  	init_unused_rings(dev);
> > >  
> > > -	for_each_ring(ring, dev_priv, i) {
> > > -		ret = ring->init_hw(ring);
> > > -		if (ret)
> > > -			return ret;
> > > -	}
> > > -
> > >  	for (i = 0; i < NUM_L3_SLICES(dev); i++)
> > >  		i915_gem_l3_remap(&dev_priv->ring[RCS], i);
> > 
> > This is going to assume ring->head/tail are already valid?
> 
> We write into the ring obj, not the ring itself, which should be setup
> during the various intel_init_engine, i.e. the backing storage is
> independent of the actual registers.

I mean the software shadows, not the registers themselves. When the GPU
hangs I expect rign->head != ring->tail. So what makes those two identical
again after the GPU reset?

> 
> If you take it to the logical conclusion, each operation on the ring is
> just a transaction that is buffered until it is commited to the
> hardware, or rolled back in its entirety if aborted. We can then queue
> up multiple transactions before first enabling the hardware, or equally
> preserve such transactions across a GPU reset.
> -Chris
> 
> -- 
> Chris Wilson, Intel Open Source Technology Centre

-- 
Ville Syrjälä
Intel OTC
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