Re: [Intel-gfx] [PATCH] drm/i915: fix blank screen booting crashes

[Matthew Brost <matthew.brost@intel.com>]

On Mon, Sep 20, 2021 at 08:28:13AM +0100, Tvrtko Ursulin wrote:
> 
> On 18/09/2021 00:38, Matthew Brost wrote:
> > From: Hugh Dickins <hughd@google.com>
> > 
> > 5.15-rc1 crashes with blank screen when booting up on two ThinkPads
> > using i915.  Bisections converge convincingly, but arrive at different
> > and surprising "culprits", none of them the actual culprit.
> 
> It is certainly surprising this patch crashed SNB and KBL.
> 
> How feasible would it be to make this code just not run when GuC is not
> used? Given the field it adds is called ce->guc_blocked it sounds like a
> natural and preferable thing to do... if possible.
> 

I can likely do this in a follow up patch.

> > netconsole (with init_netconsole() hacked to call i915_init() when
> > logging has started, instead of by module_init()) tells the story:
> > 
> > kernel BUG at drivers/gpu/drm/i915/i915_sw_fence.c:245!
> > with RSI: ffffffff814d408b pointing to sw_fence_dummy_notify().
> > I've been building with CONFIG_CC_OPTIMIZE_FOR_SIZE=y, and that
> > function needs to be 4-byte aligned.
> > 
> > v2:
> >   (Jani Nikula)
> >    - Change BUG_ON to WARN_ON
> 
> However in this case the code would then go on and call into a wrong
> function offset which may be worse than a BUG_ON, no?
>

Yea, I guess that would be bad too.
 
> > 
> > Fixes: 62eaf0ae217d ("drm/i915/guc: Support request cancellation")
> > Signed-off-by: Hugh Dickins <hughd@google.com>
> > Signed-off-by: Matthew Brost <matthew.brost@intel.com>
> > Reviewed-by: Matthew Brost <matthew.brost@intel.com>
> > ---
> >   drivers/gpu/drm/i915/gt/intel_context.c | 1 +
> >   drivers/gpu/drm/i915/i915_sw_fence.c    | 4 +++-
> >   2 files changed, 4 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/gt/intel_context.c b/drivers/gpu/drm/i915/gt/intel_context.c
> > index ff637147b1a9..f02c2202da9d 100644
> > --- a/drivers/gpu/drm/i915/gt/intel_context.c
> > +++ b/drivers/gpu/drm/i915/gt/intel_context.c
> > @@ -362,6 +362,7 @@ static int __intel_context_active(struct i915_active *active)
> >   	return 0;
> >   }
> > +__aligned(4)	/* Respect the I915_SW_FENCE_MASK */
> 
> Hugh suggested __i915_sw_fence_call which I think would be the right thing
> to do.
> 

Yep. Will do.

Matt

> Regards,
> 
> Tvrtko
> 
> >   static int sw_fence_dummy_notify(struct i915_sw_fence *sf,
> >   				 enum i915_sw_fence_notify state)
> >   {
> > diff --git a/drivers/gpu/drm/i915/i915_sw_fence.c b/drivers/gpu/drm/i915/i915_sw_fence.c
> > index c589a681da77..1217b124c1d0 100644
> > --- a/drivers/gpu/drm/i915/i915_sw_fence.c
> > +++ b/drivers/gpu/drm/i915/i915_sw_fence.c
> > @@ -14,8 +14,10 @@
> >   #if IS_ENABLED(CONFIG_DRM_I915_DEBUG)
> >   #define I915_SW_FENCE_BUG_ON(expr) BUG_ON(expr)
> > +#define I915_SW_FENCE_WARN_ON(expr) WARN_ON(expr)
> >   #else
> >   #define I915_SW_FENCE_BUG_ON(expr) BUILD_BUG_ON_INVALID(expr)
> > +#define I915_SW_FENCE_WARN_ON(expr) BUILD_BUG_ON_INVALID(expr)
> >   #endif
> >   static DEFINE_SPINLOCK(i915_sw_fence_lock);
> > @@ -242,7 +244,7 @@ void __i915_sw_fence_init(struct i915_sw_fence *fence,
> >   			  const char *name,
> >   			  struct lock_class_key *key)
> >   {
> > -	BUG_ON(!fn || (unsigned long)fn & ~I915_SW_FENCE_MASK);
> > +	I915_SW_FENCE_WARN_ON(!fn || (unsigned long)fn & ~I915_SW_FENCE_MASK);
> >   	__init_waitqueue_head(&fence->wait, name, key);
> >   	fence->flags = (unsigned long)fn;
> >