Chris Wilson <chris@xxxxxxxxxxxxxxxxxx> writes:
> On Tigerlake, we are seeing a repeat of commit d8f505311717 ("drm/i915/icl:
> Forcibly evict stale csb entries") where, presumably, due to a missing
> Global Observation Point synchronisation, the write pointer of the CSB
> ringbuffer is updated _prior_ to the contents of the ringbuffer. That is
> we see the GPU report more context-switch entries for us to parse, but
> those entries have not been written, leading us to process stale events,
> and eventually report a hung GPU.
>
> However, this effect appears to be much more severe than we previously
> saw on Icelake (though it might be best if we try the same approach
> there as well and measure), and Bruce suggested the good idea of resetting
> the CSB entry after use so that we can detect when it has been updated by
> the GPU. By instrumenting how long that may be, we can set a reliable
> upper bound for how long we should wait for:
>
> 513 late, avg of 61 retries (590 ns), max of 1061 retries (10099 ns)
>
> Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/2045
> References: d8f505311717 ("drm/i915/icl: Forcibly evict stale csb entries")
References: HSDES#1508287568
> Suggested-by: Bruce Chang <yu.bruce.chang@xxxxxxxxx>
> Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> Cc: Bruce Chang <yu.bruce.chang@xxxxxxxxx>
> Cc: Mika Kuoppala <mika.kuoppala@xxxxxxxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx # v5.4
> ---
> drivers/gpu/drm/i915/gt/intel_lrc.c | 21 ++++++++++++++++++---
> 1 file changed, 18 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/gt/intel_lrc.c b/drivers/gpu/drm/i915/gt/intel_lrc.c
> index d6e0f62337b4..d75712a503b7 100644
> --- a/drivers/gpu/drm/i915/gt/intel_lrc.c
> +++ b/drivers/gpu/drm/i915/gt/intel_lrc.c
> @@ -2498,9 +2498,22 @@ invalidate_csb_entries(const u64 *first, const u64 *last)
> */
> static inline bool gen12_csb_parse(const u64 *csb)
> {
> - u64 entry = READ_ONCE(*csb);
> - bool ctx_away_valid = GEN12_CSB_CTX_VALID(upper_32_bits(entry));
> - bool new_queue =
> + bool ctx_away_valid;
> + bool new_queue;
> + u64 entry;
> +
> + /* HSD#22011248461 */
s/220011248461/1508287568
> + entry = READ_ONCE(*csb);
> + if (unlikely(entry == -1)) {
> + preempt_disable();
As this seems to rather rare, consider falling back to mmio read
for this entry without the poll?
-Mika
> + if (wait_for_atomic_us((entry = READ_ONCE(*csb)) != -1, 50))
> + GEM_WARN_ON("50us CSB timeout");
> + preempt_enable();
> + }
> + WRITE_ONCE(*(u64 *)csb, -1);
> +
> + ctx_away_valid = GEN12_CSB_CTX_VALID(upper_32_bits(entry));
> + new_queue =
> lower_32_bits(entry) & GEN12_CTX_STATUS_SWITCHED_TO_NEW_QUEUE;
>
> /*
> @@ -4004,6 +4017,8 @@ static void reset_csb_pointers(struct intel_engine_cs *engine)
> WRITE_ONCE(*execlists->csb_write, reset_value);
> wmb(); /* Make sure this is visible to HW (paranoia?) */
>
> + /* Check that the GPU does indeed update the CSB entries! */
> + memset(execlists->csb_status, -1, (reset_value + 1) * sizeof(u64));
> invalidate_csb_entries(&execlists->csb_status[0],
> &execlists->csb_status[reset_value]);
>
> --
> 2.20.1
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