[Intel-gfx] drm/i915: Avoid accessing the stolen address when it is unavailable

[Chris Wilson]

On Fri, Oct 25, 2013 at 12:27:42AM +0000, Liu, Chuansheng wrote:
> Hello Chris and Ben,
> 
> > -----Original Message-----
> > From: Ben Widawsky [mailto:ben at bwidawsk.net]
> > Sent: Friday, October 25, 2013 4:57 AM
> > To: Chris Wilson; Liu, Chuansheng; daniel.vetter at ffwll.ch; airlied at linux.ie;
> > intel-gfx at lists.freedesktop.org; dri-devel at lists.freedesktop.org;
> > linux-kernel at vger.kernel.org; Li, Fei
> > Subject: Re: [Intel-gfx] drm/i915: Avoid accessing the stolen address when it is
> > unavailable
> > 
> > On Thu, Oct 24, 2013 at 01:17:06PM +0100, Chris Wilson wrote:
> > > On Fri, Oct 25, 2013 at 12:33:47AM +0800, Chuansheng Liu wrote:
> > > >
> > > > In our platform, we hit the the stolen region initialization failure case,
> > > > such as below log:
> > > > [drm:i915_stolen_to_physical] *ERROR* conflict detected with stolen
> > region: [0x7b000000]
> > > >
> > > > And it causes the dev_priv->mm.stolen_base is NULL, in this case, we
> > should
> > > > avoid accessing it any more.
> > > >
> > > > Here is possible call trace:
> > > > intel_enable_gt_powersave -- >
> > > > valleyview_enable_rps -- >
> > > > valleyview_setup_pctx
> > >
> > > The two create_stolen routines are no-ops in that case so all that
> > > happens instead is that we read VLV_PCBR. However, really if
> > > i915_gem_object_create_stolen_for_preallocated() fails we should abort
> > > loading the driver as it means we have a hardware conflict and undefined
> > > behaviour.
> In case of dev_priv->mm.stolen_base == NULL, and the valleyview_setup_pctx() is called
> at the first time, it will call i915_gem_object_create_stolen_for_preallocated(), which should
> should return NULL always due to (!drm_mm_initialized(&dev_priv->mm.stolen)).
> 
> After that, every time specially when doing pm operation, the above scenario will
> be called again and again.
> 
> Here this patch is to save some time for PM operation, we do not need to read
> VLV_PCBR and pcbr_offset calculation in case of stolen_base == NULL.
> 
> Is it making sense? Thanks.

I see. No, it is a pointless optimisation that leaks knowledge about
internals of another subsystem to paper over a kernel bug.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre