[RFC PATCH 37/97] drm/i915/guc: Add stall timer to non blocking CTB send function

[Matthew Brost]

On Mon, May 24, 2021 at 02:58:12PM +0200, Michal Wajdeczko wrote:
> 
> 
> On 06.05.2021 21:13, Matthew Brost wrote:
> > Implement a stall timer which fails H2G CTBs once a period of time
> > with no forward progress is reached to prevent deadlock.
> > 
> > Also update to ct_write to return -EDEADLK rather than -EPIPE on a
> > corrupted descriptor.
> 
> broken descriptor is really separate issue compared to no progress from
> GuC side, I would really like to keep old error code
>

I know you do as you have brought it up several times. Again to the rest
of the stack these two things mean the exact same thing.
 
> note that broken CTB descriptor is unrecoverable error, while on other
> hand, in theory, we could recover from temporary non-moving CTB
> 

Yea but we don't, in both cases we disable submission which in turn
triggers a full GPU reset.

> > 
> > Signed-off-by: John Harrison <John.C.Harrison at Intel.com>
> > Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio at intel.com>
> > Signed-off-by: Matthew Brost <matthew.brost at intel.com>
> > ---
> >  drivers/gpu/drm/i915/gt/uc/intel_guc_ct.c | 48 +++++++++++++++++++++--
> >  1 file changed, 45 insertions(+), 3 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/gt/uc/intel_guc_ct.c b/drivers/gpu/drm/i915/gt/uc/intel_guc_ct.c
> > index af7314d45a78..4eab319d61be 100644
> > --- a/drivers/gpu/drm/i915/gt/uc/intel_guc_ct.c
> > +++ b/drivers/gpu/drm/i915/gt/uc/intel_guc_ct.c
> > @@ -69,6 +69,8 @@ static inline struct drm_device *ct_to_drm(struct intel_guc_ct *ct)
> >  #define CTB_H2G_BUFFER_SIZE	(SZ_4K)
> >  #define CTB_G2H_BUFFER_SIZE	(SZ_4K)
> >  
> > +#define MAX_US_STALL_CTB	1000000
> 
> nit: maybe we should make it a CONFIG value ?
> 

Sure.

> > +
> >  struct ct_request {
> >  	struct list_head link;
> >  	u32 fence;
> > @@ -315,6 +317,7 @@ int intel_guc_ct_enable(struct intel_guc_ct *ct)
> >  
> >  	ct->requests.last_fence = 1;
> >  	ct->enabled = true;
> > +	ct->stall_time = KTIME_MAX;
> >  
> >  	return 0;
> >  
> > @@ -378,7 +381,7 @@ static int ct_write(struct intel_guc_ct *ct,
> >  	unsigned int i;
> >  
> >  	if (unlikely(ctb->broken))
> > -		return -EPIPE;
> > +		return -EDEADLK;
> >  
> >  	if (unlikely(desc->status))
> >  		goto corrupted;
> > @@ -449,7 +452,7 @@ static int ct_write(struct intel_guc_ct *ct,
> >  	CT_ERROR(ct, "Corrupted descriptor head=%u tail=%u status=%#x\n",
> >  		 desc->head, desc->tail, desc->status);
> >  	ctb->broken = true;
> > -	return -EPIPE;
> > +	return -EDEADLK;
> >  }
> >  
> >  /**
> > @@ -494,6 +497,17 @@ static int wait_for_ct_request_update(struct ct_request *req, u32 *status)
> >  	return err;
> >  }
> >  
> > +static inline bool ct_deadlocked(struct intel_guc_ct *ct)
> > +{
> > +	bool ret = ktime_us_delta(ktime_get(), ct->stall_time) >
> > +		MAX_US_STALL_CTB;
> > +
> > +	if (unlikely(ret))
> > +		CT_ERROR(ct, "CT deadlocked\n");
> > +
> > +	return ret;
> > +}
> > +
> >  static inline bool ctb_has_room(struct intel_guc_ct_buffer *ctb, u32 len_dw)
> >  {
> >  	struct guc_ct_buffer_desc *desc = ctb->desc;
> > @@ -505,6 +519,26 @@ static inline bool ctb_has_room(struct intel_guc_ct_buffer *ctb, u32 len_dw)
> >  	return space >= len_dw;
> >  }
> >  
> > +static int has_room_nb(struct intel_guc_ct *ct, u32 len_dw)
> > +{
> > +	struct intel_guc_ct_buffer *ctb = &ct->ctbs.send;
> > +
> > +	lockdep_assert_held(&ct->ctbs.send.lock);
> > +
> > +	if (unlikely(!ctb_has_room(ctb, len_dw))) {
> > +		if (ct->stall_time == KTIME_MAX)
> > +			ct->stall_time = ktime_get();
> > +
> > +		if (unlikely(ct_deadlocked(ct)))
> > +			return -EDEADLK;
> > +		else
> > +			return -EBUSY;
> > +	}
> > +
> > +	ct->stall_time = KTIME_MAX;
> > +	return 0;
> > +}
> > +
> >  static int ct_send_nb(struct intel_guc_ct *ct,
> >  		      const u32 *action,
> >  		      u32 len,
> > @@ -517,7 +551,7 @@ static int ct_send_nb(struct intel_guc_ct *ct,
> >  
> >  	spin_lock_irqsave(&ctb->lock, spin_flags);
> >  
> > -	ret = ctb_has_room(ctb, len + 1);
> > +	ret = has_room_nb(ct, len + 1);
> >  	if (unlikely(ret))
> >  		goto out;
> >  
> > @@ -561,11 +595,19 @@ static int ct_send(struct intel_guc_ct *ct,
> >  retry:
> >  	spin_lock_irqsave(&ct->ctbs.send.lock, flags);
> >  	if (unlikely(!ctb_has_room(ctb, len + 1))) {
> > +		if (ct->stall_time == KTIME_MAX)
> > +			ct->stall_time = ktime_get();
> >  		spin_unlock_irqrestore(&ct->ctbs.send.lock, flags);
> > +
> > +		if (unlikely(ct_deadlocked(ct)))
> > +			return -EDEADLK;
> > +
> 
> likely, instead of duplicating code, you can reuse has_room_nb here
>

In this patch yes, in the following patch no as this check changes
between non-blockig and blocking once we introduce G2H credits. I'd
rather just leave it as is than churning on the patches.

Matt 
 
> >  		cond_resched();
> >  		goto retry;
> >  	}
> >  
> > +	ct->stall_time = KTIME_MAX;
> > +
> >  	fence = ct_get_next_fence(ct);
> >  	request.fence = fence;
> >  	request.status = 0;
> >