Re: [RESEND 4/6] drm/i915/execlists: Disable submission tasklet upon wedging

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 16/07/2018 09:03, Chris Wilson wrote:
> If we declare the driver wedged before the GPU truly is, then we may see
> the GPU complete some CS events following our cancellation. This leaves
> us quite confused as we deleted all the bookkeeping and thus complain
> about the inconsistent state.
> 
> We can just ignore the remaining events and let the GPU idle by not
> feeding it, and so avoid trying to racily overwrite shared state. We
> rely on there being a full GPU reset before unwedging, giving us the
> opportunity to reset the shared state.
> 
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=107188
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> ---
>   drivers/gpu/drm/i915/intel_lrc.c | 8 ++++++++
>   1 file changed, 8 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
> index 4ef4439ff438..2c0050bab71e 100644
> --- a/drivers/gpu/drm/i915/intel_lrc.c
> +++ b/drivers/gpu/drm/i915/intel_lrc.c
> @@ -811,6 +811,11 @@ static void reset_csb_pointers(struct intel_engine_execlists *execlists)
>   	WRITE_ONCE(*execlists->csb_write, execlists->csb_write_reset);
>   }
>   
> +static void nop_submission_tasklet(unsigned long data)
> +{
> +	/* The driver is wedged; don't process any more events. */
> +}
> +
>   static void execlists_cancel_requests(struct intel_engine_cs *engine)
>   {
>   	struct intel_engine_execlists * const execlists = &engine->execlists;
> @@ -871,6 +876,9 @@ static void execlists_cancel_requests(struct intel_engine_cs *engine)
>   	execlists->queue = RB_ROOT_CACHED;
>   	GEM_BUG_ON(port_isset(execlists->port));
>   
> +	GEM_BUG_ON(__tasklet_is_enabled(&execlists->tasklet));
> +	execlists->tasklet.func = nop_submission_tasklet;
> +
>   	spin_unlock_irqrestore(&engine->timeline.lock, flags);
>   }
>   
> 

Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>

I am a bit uncertain whether we are theoretically safe without any 
memory barriers in or after intel_engines_reset_default_submission. 
Concern being that we reset and re-enable everything and someone still 
see the no_submission_tasklet as the set one. There's a lot of code in 
between so hopefully it is not possible.

Regards,

Tvrtko
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