Re: [PATCH 02/12] drm/i915: Provide PDP updates via MMIO

[Ben Widawsky <ben@bwidawsk.net>]

On Mon, Nov 25, 2013 at 06:18:52PM +0000, Chris Wilson wrote:
> On Mon, Nov 25, 2013 at 09:54:33AM -0800, Ben Widawsky wrote:
> > The initial implementation of this function used MMIO to write the PDPs.
> > Upon review it was determined (correctly) that the docs say to use LRI.
> > The issue is there are times where we want to do a synchronous write
> > (GPU reset).
> > 
> > I've tested this, and it works. I've verified with as many people as
> > possible that it should work.
> > 
> > This should fix the failing reset problems.
> > 
> > Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
> > ---
> >  drivers/gpu/drm/i915/i915_gem_gtt.c | 12 ++++++++++--
> >  1 file changed, 10 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/i915_gem_gtt.c b/drivers/gpu/drm/i915/i915_gem_gtt.c
> > index 1a5272c..96dbf3d 100644
> > --- a/drivers/gpu/drm/i915/i915_gem_gtt.c
> > +++ b/drivers/gpu/drm/i915/i915_gem_gtt.c
> > @@ -197,12 +197,19 @@ static gen6_gtt_pte_t iris_pte_encode(dma_addr_t addr,
> >  
> >  /* Broadwell Page Directory Pointer Descriptors */
> >  static int gen8_write_pdp(struct intel_ring_buffer *ring, unsigned entry,
> > -			   uint64_t val)
> > +			   uint64_t val, bool synchronous)
> >  {
> > +	struct drm_i915_private *dev_priv = ring->dev->dev_private;
> >  	int ret;
> >  
> 
> i915_reset_in_progress(&dev_priv->gpu_error));
> doesn't actually mean that we are in the middle of a reset. Or does
> it?  Anyway intel_ring_begin() returns EIO/EAGAIN so we do not need
> to pass down the parameter. But the issue with mixing and matching LRI
> vs mmio is that if this was not a reset call, then we just upset the
> GPU even further.
> -Chris

/me sighs

The synchronous argument comes from the future. There are
three places where one could conceivably use it:
1. before rings are up
2. during hang
3. after rings are shut down

With the current code, only #2 is actually possible. I don't like
checking EIO/EAGAIN as there are cases where we expect failure, and
cases where we do not. Being able to strain out which one is which, is
helpful, and [in the future] the caller is the one that can take
appropriate action. Also I found also using the argument a bit nicer
since every gen would have to implement the same logic to determine if
the rings were usage.

You'll have to trust me that I won't use MMIO when I shouldn't be using
it.

I can understand your comment at this stage, but I hope my reasoning
makes sense. (Feel free to view my ppgtt branch if you'd like)

> 
> -- 
> Chris Wilson, Intel Open Source Technology Centre

-- 
Ben Widawsky, Intel Open Source Technology Center