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

[Jani Nikula <jani.nikula@linux.intel.com>]

On Mon, 20 Sep 2021, Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com> 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.
>
>> 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?

So how about just

if (WARN_ON(...))
	return;

or whatever is needed to give both the user and the CI a better
opportunity to see the error.


BR,
Jani


>
>> 
>> 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.
>
> 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;
>> 

-- 
Jani Nikula, Intel Open Source Graphics Center