Chris Wilson <chris@xxxxxxxxxxxxxxxxxx> writes:
> 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@xxxxxxxxxxxxxxxxxx>
> Cc: Mika Kuoppala <mika.kuoppala@xxxxxxxxxxxxxxx>
> Cc: Bruce Chang <yu.bruce.chang@xxxxxxxxx>
> Cc: Joonas Lahtinen <joonas.lahtinen@xxxxxxxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
Acked-by: Mika Kuoppala <mika.kuoppala@xxxxxxxxxxxxxxx>
> ---
> 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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