Re: [PATCH 7/7] drm/i915: GEM operations need to be done under the big lock

[Imre Deak <imre.deak@intel.com>]

On ke, 2016-01-13 at 14:32 +0000, Chris Wilson wrote:
> On Wed, Jan 13, 2016 at 02:11:42PM +0000, Tvrtko Ursulin wrote:
> > 
> > On 13/01/16 13:36, Imre Deak wrote:
> > > On ke, 2016-01-13 at 12:46 +0000, Tvrtko Ursulin wrote:
> > > > On 11/01/16 16:56, Chris Wilson wrote:
> > > > > On Mon, Jan 11, 2016 at 05:36:46PM +0200, Ville Syrjälä
> > > > > wrote:
> > > > > > On Mon, Jan 11, 2016 at 03:16:16PM +0000, Tvrtko Ursulin
> > > > > > wrote:
> > > > > > > Don't know, I leave this one to whoever grabbed the lock
> > > > > > > around
> > > > > > > intel_init_gt_powersave in the first place. Maybe there
> > > > > > > was a
> > > > > > > special
> > > > > > > reason.. after git blame od intel_display.c eventually
> > > > > > > completed, adding
> > > > > > > Imre and Ville to cc.
> > > > > > 
> > > > > > Hmm. I don't recall the details anymore, but looking at the
> > > > > > code
> > > > > > pushing
> > > > > > the locking down to valleyview_setup_pctx() looks entirely
> > > > > > reasonable to
> > > > > > me.
> > > > > 
> > > > > iirc, this locking only exists to keep the WARN() at bay. But
> > > > > it is
> > > > > pedagogical, I guess.
> > > > 
> > > > Don't really know this area, but what about the
> > > > intel_gen6_powersave_work->valleyview_enable_rps-
> > > > > valleyview_check_pctx
> > > > which dereferences the dev_priv->vlv_pctx, which is set/cleared
> > > > in
> > > > valleyview_setup_pctx/valleyview_cleanup_pctx, which would now
> > > > be
> > > > outside both struct_mutex and the rps lock?
> > > 
> > > dev_priv->vlv_pctx is not protected on the premise that the
> > > driver
> > > init/cleanup functions can't race and gen6_powersave_work() is
> > > scheduled only after intel_init_gt_powersave() and flushed
> > > before intel_cleanup_gt_powersave().
> > > 
> > > rps_lock protects the RPS HW accesses themselves and struct_mutex
> > > was
> > > taken for the GEM allocation. Taking it at high level around
> > > intel_init_gt_powersave() was kind of a copy&paste in the commit
> > > you
> > > found, there is more on that in 79f5b2c75992.
> > 
> > Thanks for digging this out.
> > 
> > It is more involved than what Chris pasted then, and while I hoped
> > to be able to quickly shove that into a patch, I cannot allow the
> > time for more the extensive analysis.
> 
> Imre just confirmed that struct_mutex is only for the GEM
> allocations in the vlv_setup_ctx, right Imre?

Yes, that's my understanding.

--Imre
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