[Intel-gfx] [PATCH] drm/i915: fix SFC reset flow

[Tvrtko Ursulin]

On 16/09/2019 22:41, Daniele Ceraolo Spurio wrote:
> Our assumption that the we can ask the HW to lock the SFC even if not
> currently in use does not match the HW commitment. The expectation from
> the HW is that SW will not try to lock the SFC if the engine is not
> using it and if we do that the behavior is undefined; on ICL the HW
> ends up to returning the ack and ignoring our lock request, but this is
> not guaranteed and we shouldn't expect it going forward.
> 
> Reported-by: Owen Zhang <owen.zhang at intel.com>
> Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio at intel.com>
> Cc: Tvrtko Ursulin <tvrtko.ursulin at linux.intel.com>
> ---
>   drivers/gpu/drm/i915/gt/intel_reset.c | 25 +++++++++++++++++--------
>   1 file changed, 17 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/gt/intel_reset.c b/drivers/gpu/drm/i915/gt/intel_reset.c
> index 8327220ac558..900958804bd5 100644
> --- a/drivers/gpu/drm/i915/gt/intel_reset.c
> +++ b/drivers/gpu/drm/i915/gt/intel_reset.c
> @@ -352,13 +352,15 @@ static u32 gen11_lock_sfc(struct intel_engine_cs *engine)
>   	}
>   
>   	/*
> -	 * Tell the engine that a software reset is going to happen. The engine
> -	 * will then try to force lock the SFC (if currently locked, it will
> -	 * remain so until we tell the engine it is safe to unlock; if currently
> -	 * unlocked, it will ignore this and all new lock requests). If SFC
> -	 * ends up being locked to the engine we want to reset, we have to reset
> -	 * it as well (we will unlock it once the reset sequence is completed).
> +	 * If the engine is using a SFC, tell the engine that a software reset
> +	 * is going to happen. The engine will then try to force lock the SFC.
> +	 * If SFC ends up being locked to the engine we want to reset, we have
> +	 * to reset it as well (we will unlock it once the reset sequence is
> +	 * completed).
>   	 */
> +	if (!(intel_uncore_read_fw(uncore, sfc_usage) & sfc_usage_bit))
> +		return 0;
> +
>   	rmw_set_fw(uncore, sfc_forced_lock, sfc_forced_lock_bit);
>   
>   	if (__intel_wait_for_register_fw(uncore,
> @@ -366,10 +368,13 @@ static u32 gen11_lock_sfc(struct intel_engine_cs *engine)
>   					 sfc_forced_lock_ack_bit,
>   					 sfc_forced_lock_ack_bit,
>   					 1000, 0, NULL)) {
> -		DRM_DEBUG_DRIVER("Wait for SFC forced lock ack failed\n");
> +		/* did we race the unlock? */

How do we race here? Are we not in complete control of the engine at 
this point so the status of this engine using SFC or not should be 
static, no?

> +		if (intel_uncore_read_fw(uncore, sfc_usage) & sfc_usage_bit)
> +			DRM_ERROR("Wait for SFC forced lock ack failed\n");
>   		return 0;
>   	}
>   
> +	/* The HW could return the ack even if the sfc is not in use */

But the function checked whether SFC wasn't in use and bailed out early 
- so is this comment relevant? (I understand it is true against the 
specs just wondering about our exact code.)

>   	if (intel_uncore_read_fw(uncore, sfc_usage) & sfc_usage_bit)
>   		return sfc_reset_bit;
>   
> @@ -382,6 +387,7 @@ static void gen11_unlock_sfc(struct intel_engine_cs *engine)
>   	u8 vdbox_sfc_access = RUNTIME_INFO(engine->i915)->vdbox_sfc_access;
>   	i915_reg_t sfc_forced_lock;
>   	u32 sfc_forced_lock_bit;
> +	u32 lock;
>   
>   	switch (engine->class) {
>   	case VIDEO_DECODE_CLASS:
> @@ -401,7 +407,10 @@ static void gen11_unlock_sfc(struct intel_engine_cs *engine)
>   		return;
>   	}
>   
> -	rmw_clear_fw(uncore, sfc_forced_lock, sfc_forced_lock_bit);
> +	lock = intel_uncore_read_fw(uncore, sfc_forced_lock);
> +	if (lock & sfc_forced_lock_bit)
> +		intel_uncore_write_fw(uncore, sfc_forced_lock,
> +				      lock & ~sfc_forced_lock_bit);

Here we can't rely on the return code from gen11_lock_sfc and have to 
read the register ourselves? I guess it depends on my question about the 
race comment.

In addition to this I now see that gen11_reset_engines does not use the 
return value from gen11_lock_sfc when deciding which engines it needs to 
unlock. Should we change that as well?


>   }
>   
>   static int gen11_reset_engines(struct intel_gt *gt,
> 

Regards,

Tvrtko