Re: [PATCH 5/5] drm/i915: Fix error capture on BYT/BDW

[Ben Widawsky <ben@bwidawsk.net>]

On Mon, Jan 27, 2014 at 01:45:22PM +0000, Chris Wilson wrote:
> On Sun, Jan 26, 2014 at 01:47:29PM -0800, Ben Widawsky wrote:
> > On Sun, Jan 26, 2014 at 07:55:59PM +0000, Chris Wilson wrote:
> > > On Sun, Jan 26, 2014 at 11:05:40AM -0800, Ben Widawsky wrote:
> > > > On Sun, Jan 26, 2014 at 11:47:40AM +0000, Chris Wilson wrote:
> > > > > On Fri, Jan 24, 2014 at 06:17:45PM -0800, Ben Widawsky wrote:
> > > > > > The previous check during error capture of whether or not the current VM
> > > > > > should be scanned used, gen < 7. That was more or less trying to
> > > > > > determine if there was a full PPGTT. At the time, this was sort of what
> > > > > > I meant to do because I was more interested in working backwards from
> > > > > > hardware state. However, this is incorrect because it will not include
> > > > > > platforms that are greater than gen7, and not having PPGTT.  Example
> > > > > > would be BYT which is gen7 but doesn't have PPGTT, BDW, or any platform
> > > > > > greater than gen7 with the PPGTT module parameter invoked.
> > > > > > 
> > > > > > I am /assuming/ BYT was broken, I have not actually checked.
> > > > > > 
> > > > > > While here, clean up the file a bit to avoid duplicate reads (now that
> > > > > > the PPGTT info is in the error state).
> > > > > > 
> > > > > > I think Mika/Chris may have been looking at this too.
> > > > > 
> > > > > Sure, we are looking (for identifying the guilty request/batch) by using
> > > > > the older, simpler mechanism of finding the first incomplete request. I
> > > > > think that search is now definite since we preallocate the request and no
> > > > > longer do request collascing if ENOMEM (i.e. there is a 1:1 relationship
> > > > > between seqno/batch/request).
> > > > > 
> > > > > That should also apply here and be much simpler.
> > > > 
> > > > How does that solve hangs which aren't caused by requests?
> > > 
> > > Was that an intentional rhetorical question?
> > > 
> > > The code you touch here only deals with requests - finding the current
> > > batchbuffer if any.
> > > -Chris
> > > 
> > 
> > It wasn't rhetorical. I temporarily ignored that all batches are tied to
> > a request.
> > 
> > So what's the plan now? Just looking at the callers, we seem to have a
> > couple of callers that can't easily identify the bad request.
> 
> I was thinking along the lines of:
> 
> @@ -737,31 +709,16 @@ i915_error_first_batchbuffer(struct drm_i915_private *dev_priv,
>         }
>  
>         seqno = ring->get_seqno(ring, false);
> -       list_for_each_entry(vm, &dev_priv->vm_list, global_link) {
> -               if (!is_active_vm(vm, ring))
> +       list_for_each_entry(request, &ring->request_list, list) {
> +               if (i915_seqno_passed(seqno, request->seqno))
>                         continue;
>  
> -               found_active = true;
> -
> -               list_for_each_entry(vma, &vm->active_list, mm_list) {
> -                       obj = vma->obj;
> -                       if (obj->ring != ring)
> -                               continue;
> -
> -                       if (i915_seqno_passed(seqno, obj->last_read_seqno))
> -                               continue;
> -
> -                       if ((obj->base.read_domains & I915_GEM_DOMAIN_COMMAND) == 0)
> -                               continue;
> -
> -                       /* We need to copy these to an anonymous buffer as the simplest
> -                        * method to avoid being overwritten by userspace.
> -                        */
> -                       return i915_error_object_create(dev_priv, obj, vm);
> -               }
> +               /* We need to copy these to an anonymous buffer as the simplest
> +                * method to avoid being overwritten by userspace.
> +                */
> +               return i915_error_object_create(dev_priv, request->batch_obj, request->ctx->vm);
>         }
>  
> -       WARN_ON(!found_active);
> 

The other issue is the existing method doesn't rely as much on proper
request handling, ie. this could be more resilient to driver bugs. I
kind of want to keep both...

-- 
Ben Widawsky, Intel Open Source Technology Center