Re: [PATCH v2 06/10] coresight: trbe: Fix handling of spurious interrupts

[Anshuman Khandual <anshuman.khandual@arm.com>]

On 7/23/21 6:16 PM, Suzuki K Poulose wrote:
> On a spurious IRQ, right now we disable the TRBE and then re-enable
> it back, resetting the "buffer" pointers(i.e BASE, LIMIT and more
> importantly WRITE) to the original pointers from the AUX handle.
> This implies that we overwrite any trace that was written so far,
> (by overwriting TRBPTR) while we should have ignored the IRQ.

The ideas was that a state (pointers) reset would improve the chances
of not getting the spurious IRQ once again. This is assuming that some
thing during this current state machine, had caused the spurious IRQ.
Hence just restart it back from the beginning. Yes, it does lose some
trace data but whats the real possibility of such spurious IRQs in the
first place ?

> 
> This patch cleans the behavior, by only stopping the TRBE if the
> IRQ was indeed raised, as we can read the TRBSR without stopping
> the TRBE (Only writes to the TRBSR requires the TRBE disabled).
> And also, on detecting a spurious IRQ after examining the TRBSR,
> we simply re-enable the TRBE without touching the other parameters.

This makes sense. I was not sure if TRBSR could be safely read without
actually stopping the TRBE.

> 
> Cc: Anshuman Khandual <anshuman.khandual@arm.com>
> Cc: Mathieu Poirier <mathieu.poirier@linaro.org>
> Cc: Mike Leach <mike.leach@linaro.org>
> Cc: Leo Yan <leo.yan@linaro.org>
> Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
> ---
>  drivers/hwtracing/coresight/coresight-trbe.c | 29 ++++++++++----------
>  1 file changed, 15 insertions(+), 14 deletions(-)
> 
> diff --git a/drivers/hwtracing/coresight/coresight-trbe.c b/drivers/hwtracing/coresight/coresight-trbe.c
> index 62e1a08f73ff..503bea0137ae 100644
> --- a/drivers/hwtracing/coresight/coresight-trbe.c
> +++ b/drivers/hwtracing/coresight/coresight-trbe.c
> @@ -679,15 +679,16 @@ static int arm_trbe_disable(struct coresight_device *csdev)
>  
>  static void trbe_handle_spurious(struct perf_output_handle *handle)
>  {
> -	struct trbe_buf *buf = etm_perf_sink_config(handle);
> +	u64 limitr = read_sysreg_s(SYS_TRBLIMITR_EL1);
>  
> -	buf->trbe_limit = compute_trbe_buffer_limit(handle);
> -	buf->trbe_write = buf->trbe_base + PERF_IDX2OFF(handle->head, buf);
> -	if (buf->trbe_limit == buf->trbe_base) {
> -		trbe_drain_and_disable_local();
> -		return;
> -	}
> -	trbe_enable_hw(buf);
> +	/*
> +	 * If the IRQ was spurious, simply re-enable the TRBE
> +	 * back without modifiying the buffer parameters to

Typo here 		^^^^^^ s/modifiying/modifying

> +	 * retain the trace collected so far.
> +	 */
> +	limitr |= TRBLIMITR_ENABLE;
> +	write_sysreg_s(limitr, SYS_TRBLIMITR_EL1);
> +	isb();
>  }
>  
>  static void trbe_handle_overflow(struct perf_output_handle *handle)
> @@ -760,12 +761,7 @@ static irqreturn_t arm_trbe_irq_handler(int irq, void *dev)
>  	enum trbe_fault_action act;
>  	u64 status;
>  
> -	/*
> -	 * Ensure the trace is visible to the CPUs and
> -	 * any external aborts have been resolved.
> -	 */
> -	trbe_drain_and_disable_local();
> -
> +	/* Reads to TRBSR_EL1 is fine when TRBE is active */
>  	status = read_sysreg_s(SYS_TRBSR_EL1);
>  	/*
>  	 * If the pending IRQ was handled by update_buffer callback
> @@ -774,6 +770,11 @@ static irqreturn_t arm_trbe_irq_handler(int irq, void *dev)
>  	if (!is_trbe_irq(status))

Warn here that a non-related IRQ has been delivered to this handler ?
But moving the trbe_drain_and_disable_local() later, enables it to
return back immediately after detecting an unrelated IRQ.

>  		return IRQ_NONE;
>  
> +	/*
> +	 * Ensure the trace is visible to the CPUs and
> +	 * any external aborts have been resolved.
> +	 */
> +	trbe_drain_and_disable_local();
>  	clr_trbe_irq();
>  	isb();
>  
> 

Actually there are two types of spurious interrupts here.

1. Non-TRBE spurious interrupt

Fails is_trbe_irq() test and needs to be returned immediately from
arm_trbe_irq_handler(), after an warning for the platform IRQ
delivery wiring.

2. TRBE spurious interrupt

Clears is_trbe_irq() and get handled in trbe_handle_spurious(). I
still think leaving this unchanged might be better as it reduces
the chance of getting further spurious TRBE interrupts.