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

[Michal Wajdeczko]

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

note that broken CTB descriptor is unrecoverable error, while on other
hand, in theory, we could recover from temporary non-moving CTB

> 
> 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 ?

> +
>  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

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