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

[Daniel, Thomas]

> -----Original Message-----
> From: Chris Wilson [mailto:chris at chris-wilson.co.uk]
> Sent: Thursday, November 20, 2014 11:41 AM
> To: Daniel, Thomas
> Cc: 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 11:25:57AM +0000, Daniel, Thomas wrote:
> > > -----Original Message-----
> > > From: Chris Wilson [mailto:chris at chris-wilson.co.uk]
> > > Sent: Thursday, November 20, 2014 11:16 AM
> > > To: Daniel, Thomas
> > > Cc: intel-gfx at lists.freedesktop.org; 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 11:12:05AM +0000, Thomas Daniel wrote:
> > > > 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.
> > I suppose I don't as this is protected by the struct mutex and unpin is called
> before returning.
> 
> The question is: do we need protection against kmalloc and a potential call
> into the shrinker who may steal the pages from underneath us. Here, we
> only do a seq_printf() under the lock after get_pages() and that uses a
> preallocated buffer.
I don't think so... If a context is in the context_list then the ctx_obj will have a nonzero refcount.  The struct mutex prevents the refcount from changing.

Can you identify any situation where the pages may go away?

Thomas.

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