[Intel-gfx] [PATCH] drm/i915: Don't pin LRC in GGTT when dumping in debugfs

[Chris Wilson]

On Tue, Nov 25, 2014 at 02:30:55PM +0000, Daniel, Thomas wrote:
> > -----Original Message-----
> > From: daniel.vetter at ffwll.ch [mailto:daniel.vetter at ffwll.ch] On Behalf Of
> > Daniel Vetter
> > Sent: Thursday, November 20, 2014 12:51 PM
> > To: Chris Wilson; Daniel, Thomas; intel-gfx at lists.freedesktop.org; akash goel
> > (akash.goels at gmail.com)
> > Subject: Re: [Intel-gfx] [PATCH] drm/i915: Don't pin LRC in GGTT when
> > dumping in debugfs
> > 
> > On Thu, Nov 20, 2014 at 1:28 PM, Chris Wilson <chris at chris-wilson.co.uk>
> > wrote:
> > >> Can you identify any situation where the pages may go away?
> > >
> > > Anytime you trigger an allocation, the system may reap any objects
> > > pages. It will even steal the dev->struct_mutex. To protect against
> > > the shrinker you have to call pin_pages(). Here, there are no
> > > allocations inside the loop and so you don't need to worry about the
> > > shrinker stealing your pages.
> > 
> > Hm actually I think better safe than sorry here. At least I have
> > (again) completely forgotten about our dear shrinker ...
> > -Daniel
> 
> Does this discussion count as a review?  What was the conclusion - do I need to make a version without pinning or is it better safe than sorry?

To bring it full circle:

>> LRC object does not need to be mapped into the GGTT when dumping. Just use
>> pin_pages. A side-effect of this patch is that a compiler warning goes away
>> (not checking return value of i915_gem_obj_ggtt_pin).

> Please explain why you need to pin the pages.

In particular, explain why just calling pin_pages() doesn't do what you
want. And then afterwards you can leave a note in the commitlog why you
use pin_pages() as overkill.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre