Re: [PATCH v2 22/23] coresight: trbe: Restart trace source for stopped buffers

[James Clark <james.clark@linaro.org>]

On 16/10/2026 17:55, Leo Yan wrote:
> When TRBSR reports a stopped buffer, the IRQ handler closes the current
> AUX buffer and programs TRBE for the next run. This creates a trace
> discontinuity. User tool has to reset the decoder when moving to the
> next AUX data block, which can reduce decoded trace coverage.
> 
> Stop and restart the trace source around TRBE reprogramming for stopped
> buffers. This emits ASYNC packets, allowing the decoder to recognize
> discontinuity without a full reset.
> 
> This duplicates part of the trace source enable/disable code in the
> ETMv4 driver. Reusing that code would require splitting the ETMv4
> callbacks into finer-grained helpers that TRBE can call, which would
> make the change much larger. Keep the TRBE change self-contained for
> now.
> 
> Signed-off-by: Leo Yan <leo.yan@arm.com>
> ---
>   drivers/hwtracing/coresight/coresight-trbe.c | 121 +++++++++++++++++++++++++--
>   1 file changed, 116 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/hwtracing/coresight/coresight-trbe.c b/drivers/hwtracing/coresight/coresight-trbe.c
> index 36f143a15e680b28aa6ca6a43ad7d3ee134693be..6bc82d5b55539795059da37039d2929b4ea504e3 100644
> --- a/drivers/hwtracing/coresight/coresight-trbe.c
> +++ b/drivers/hwtracing/coresight/coresight-trbe.c
> @@ -17,9 +17,11 @@
>   
>   #include <asm/barrier.h>
>   #include <asm/cpufeature.h>
> +#include <linux/iopoll.h>
>   #include <linux/kvm_host.h>
>   #include <linux/vmalloc.h>
>   
> +#include "coresight-etm4x.h"
>   #include "coresight-self-hosted-trace.h"
>   #include "coresight-trbe.h"
>   
> @@ -1132,14 +1134,106 @@ static void trbe_handle_spurious(struct trbe_cpudata *cpudata)
>   	set_trbe_enabled(cpudata, trblimitr);
>   }
>   
> +static u64 read_sys_trcstatr(void)
> +{
> +	/* Context synchronization for each polling */
> +	isb();
> +	return read_sysreg_s(SYS_TRCSTATR);
> +}
> +
> +static int arm_trbe_start_tracer(u64 trcprgctlr)
> +{
> +	u64 val;
> +
> +	/* A pending IRQ after AUX pause? */
> +	if (!(trcprgctlr & BIT(0)))
> +		return 0;
> +
> +	/* Restore the saved tracer state */
> +	write_sysreg_s(trcprgctlr, SYS_TRCPRGCTLR);
> +
> +	/*
> +	 * Context synchronization between writing to the TRCPRGCTLR and
> +	 * reading the TRCSTATR.
> +	 */
> +	isb();
> +
> +	/* Wait for TRCSTATR.IDLE to go down to 0 */
> +	if (read_poll_timeout_atomic(read_sys_trcstatr, val,
> +				     !(val & BIT(TRCSTATR_IDLE_BIT)),
> +				     1,		/* delay_us */
> +				     TIMEOUT_US, /* timeout_us */
> +				     false)) {	/* delay_before_read */
> +		pr_err("TRBE: timeout for TRCSTATR IDLE\n");
> +		return -ETIME;
> +	}
> +
> +	/* Context synchronization after programming the tracer */
> +	isb();
> +
> +	return 0;
> +}
> +
> +static int arm_trbe_stop_tracer(u64 *trcprgctlr)
> +{
> +	u64 val;
> +
> +	/* Disable tracer and save trcprgctlr */
> +	*trcprgctlr = read_sysreg_s(SYS_TRCPRGCTLR);
> +
> +	/* No need to stop tracer again */
> +	if (!(*trcprgctlr & BIT(0)))
> +		return 0;
> +
> +	write_sysreg_s(*trcprgctlr & ~BIT(0), SYS_TRCPRGCTLR);
> +
> +	/*
> +	 * Context synchronization between writing to the TRCPRGCTLR and
> +	 * reading the TRCSTATR.
> +	 */
> +	isb();
> +
> +	/*
> +	 * PMSTABLE alone does not permit re-enabling the tracer.
> +	 * Wait for TRCSTATR.IDLE to go to 1.
> +	 */
> +	if (read_poll_timeout_atomic(read_sys_trcstatr, val,
> +				     val & BIT(TRCSTATR_IDLE_BIT),
> +				     1,
> +				     TIMEOUT_US,
> +				     false)) {
> +		pr_err("TRBE: timeout for TRCSTATR IDLE=1\n");
> +		return -ETIME;
> +	}
> +
> +	/* Context synchronization after programming the tracer */
> +	isb();
> +
> +	return 0;
> +}
> +
>   static int trbe_handle_overflow(struct trbe_cpudata *cpudata,
> -				enum trbe_fault_action act)
> +				enum trbe_fault_action act,
> +				bool is_buffer_stopped)
>   {
>   	struct perf_output_handle *handle = cpudata->handle;
>   	struct perf_event *event = handle->event;
>   	struct trbe_buf *buf = etm_perf_sink_config(handle);
>   	unsigned long size;
>   	struct etm_event_data *event_data;
> +	u64 trcprgctlr = 0;
> +	int ret = 0;
> +
> +	/*
> +	 * Since the trace buffer has stopped, disable and re-enable the
> +	 * tracer to generate ASYNC packets. This allows the decoder to
> +	 * recognize the discontinuity in the trace stream.
> +	 */
> +	if (is_buffer_stopped) {
> +		ret = arm_trbe_stop_tracer(&trcprgctlr);
> +		if (ret)
> +			goto truncate_event;
> +	}
>   
>   	/*
>   	 * Clear the status. No context synchronization is required here,
> @@ -1172,11 +1266,27 @@ static int trbe_handle_overflow(struct trbe_cpudata *cpudata,
>   	 */
>   	buf = etm_perf_sink_config(handle);
>   	if (!buf || buf->cpudata != cpudata) {
> -		trbe_truncate_event(handle);
> -		return -EINVAL;
> +		ret = -EINVAL;
> +		goto truncate_event;
> +	}
> +
> +	ret = __arm_trbe_enable(buf, handle);
> +	if (ret)
> +		return ret;
> +
> +	if (is_buffer_stopped) {
> +		ret = arm_trbe_start_tracer(trcprgctlr);
> +		if (ret) {
> +			trbe_drain_and_disable_local(cpudata);
> +			goto truncate_event;
> +		}
>   	}
>   
> -	return __arm_trbe_enable(buf, handle);
> +	return 0;
> +
> +truncate_event:
> +	trbe_truncate_event(handle);
> +	return ret;
>   }
>   
>   static bool is_perf_trbe(struct perf_output_handle *handle)
> @@ -1267,7 +1377,8 @@ static irqreturn_t arm_trbe_irq_handler(int irq, void *dev)
>   	act = trbe_get_fault_act(handle, status);
>   	switch (act) {
>   	case TRBE_FAULT_ACT_WRAP:
> -		truncated = !!trbe_handle_overflow(cpudata, act);
> +		truncated = !!trbe_handle_overflow(cpudata, act,
> +						   !is_trbe_running(status));
>   		break;
>   	case TRBE_FAULT_ACT_SPURIOUS:
>   		trbe_handle_spurious(cpudata);
> 
Reviewed-by: James Clark <james.clark@linaro.org>