[Intel-gfx] [PATCH] drm/i915/gt: Delay execlist processing for tgl

[Shi, Yang A]

Hi Chris:
	
	How to determine the length of the magic delay in here?


Best Regards.
Yang


> -----Original Message-----
> From: Intel-gfx <intel-gfx-bounces at lists.freedesktop.org> On Behalf Of Chris Wilson
> Sent: Friday, October 16, 2020 3:50 AM
> To: intel-gfx at lists.freedesktop.org
> Cc: stable at vger.kernel.org; Chris Wilson <chris at chris-wilson.co.uk>
> Subject: [Intel-gfx] [PATCH] drm/i915/gt: Delay execlist processing for tgl
> 
> When running gem_exec_nop, it floods the system with many requests (with the goal of
> userspace submitting faster than the HW can process a single empty batch). This causes
> the driver to continually resubmit new requests onto the end of an active context, a flood
> of lite-restore preemptions. If we time this just right, Tigerlake hangs.
> 
> Inserting a small delay between the processing of CS events and submitting the next
> context, prevents the hang. Naturally it does not occur with debugging enabled. The
> suspicion then is that this is related to the issues with the CS event buffer, and inserting
> an mmio read of the CS pointer status appears to be very successful in preventing the
> hang. Other registers, or uncached reads, or plain mb, do not prevent the hang, suggesting
> that register is key -- but that the hang can be prevented by a simple udelay, suggests it is
> just a timing issue like that encountered by commit 233c1ae3c83f ("drm/i915/gt: Wait for
> CSB entries on Tigerlake"). Also note that the hang is not prevented by applying
> CTX_DESC_FORCE_RESTORE, or by inserting a delay on the GPU between requests.
> 
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: Mika Kuoppala <mika.kuoppala at linux.intel.com>
> Cc: Bruce Chang <yu.bruce.chang at intel.com>
> Cc: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>
> Cc: stable at vger.kernel.org
> ---
>  drivers/gpu/drm/i915/gt/intel_lrc.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/gt/intel_lrc.c b/drivers/gpu/drm/i915/gt/intel_lrc.c
> index 6170f6874f52..d15d561152ba 100644
> --- a/drivers/gpu/drm/i915/gt/intel_lrc.c
> +++ b/drivers/gpu/drm/i915/gt/intel_lrc.c
> @@ -2711,6 +2711,9 @@ static void process_csb(struct intel_engine_cs *engine)
>  			smp_wmb(); /* complete the seqlock */
>  			WRITE_ONCE(execlists->active, execlists->inflight);
> 
> +			/* Magic delay for tgl */
> +			ENGINE_POSTING_READ(engine,
> RING_CONTEXT_STATUS_PTR);
> +
>  			WRITE_ONCE(execlists->pending[0], NULL);
>  		} else {
>  			if (GEM_WARN_ON(!*execlists->active)) {
> --
> 2.20.1
> 
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