Re: [PATCH v2] drm/i915/tgl: Magic interrupt shadow to relieve some random lockups

[Mika Kuoppala <mika.kuoppala@linux.intel.com>]

Chris Wilson <chris@chris-wilson.co.uk> writes:

> My current theory is that this masks interrupt delivery to the local CPU
> during a critical phase. Purely papering over the symptoms with a delay
> plucked out of thin air from testing on tgl1-gem, refined slightly by
> just waiting for the next ack (though technically the next CS event may
> not be the corresponding event for this submit, but an intermediate
> completion).
>
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
> Cc: Andi Shyti <andi.shyti@intel.com>
> ---
>  drivers/gpu/drm/i915/gt/intel_lrc.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
>
> diff --git a/drivers/gpu/drm/i915/gt/intel_lrc.c b/drivers/gpu/drm/i915/gt/intel_lrc.c
> index ab725a6ca0ac..35410d647b52 100644
> --- a/drivers/gpu/drm/i915/gt/intel_lrc.c
> +++ b/drivers/gpu/drm/i915/gt/intel_lrc.c
> @@ -1155,6 +1155,7 @@ assert_pending_valid(const struct intel_engine_execlists *execlists,
>  static void execlists_submit_ports(struct intel_engine_cs *engine)
>  {
>  	struct intel_engine_execlists *execlists = &engine->execlists;
> +	unsigned int tail = READ_ONCE(*execlists->csb_write);
>  	unsigned int n;
>  
>  	GEM_BUG_ON(!assert_pending_valid(execlists, "submit"));
> @@ -1186,6 +1187,14 @@ static void execlists_submit_ports(struct intel_engine_cs *engine)
>  	/* we need to manually load the submit queue */
>  	if (execlists->ctrl_reg)
>  		writel(EL_CTRL_LOAD, execlists->ctrl_reg);
> +

I don't doubt we forget this but a comment would still be
good that we limit the intr generation for reasons still
unknown.

> +	if (IS_TIGERLAKE(engine->i915)) {
> +		u64 start = local_clock();
> +		do
> +			cpu_relax();
> +		while (tail == READ_ONCE(*execlists->csb_write) &&
> +		       (local_clock() - start) >> 20 == 0);

1ms..well, if it works :O

Need to track this closely and pls consider limiting this to current revid
just so that we would notice the diff immediately when we get updates.

Acked-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>

> +	}
>  }
>  
>  static bool ctx_single_port_submission(const struct intel_context *ce)
> -- 
> 2.23.0
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