[PATCH v11 24/27] coresight: trbe: Save and restore state across CPU low power state

Leo Yan
2026-05-01T16:49:32Z

From: Yabin Cui <yabinc@google.com>

TRBE context can be lost when a CPU enters low power states. If a trace
source is restored while TRBE is not, tracing may run without an active
sink, which can lead to hangs on some devices (e.g., Pixel 9).

The save and restore flows are described in the section K5.5 "Context
switching" of Arm ARM (ARM DDI 0487 L.a). This commit adds save and
restore callbacks with following the software usages defined in the
architecture manual.

During the restore flow, since TRBLIMITR_EL1.E resets to 0 on a warm
reset, the trace buffer unit is disabled when idle resume, it is safe to
restore base/pointer/status registers first and program TRBLIMITR_EL1
last.

Signed-off-by: Yabin Cui <yabinc@google.com>
Tested-by: James Clark <james.clark@linaro.org>
Reviewed-by: Yeoreum Yun <yeoreum.yun@arm.com>
Reviewed-by: James Clark <james.clark@linaro.org>
Co-developed-by: Leo Yan <leo.yan@arm.com>
Tested-by: Jie Gan <jie.gan@oss.qualcomm.com>
Signed-off-by: Leo Yan <leo.yan@arm.com>
---
drivers/hwtracing/coresight/coresight-trbe.c | 59 +++++++++++++++++++++++++++-
1 file changed, 58 insertions(+), 1 deletion(-)

diff --git a/drivers/hwtracing/coresight/coresight-trbe.c b/drivers/hwtracing/coresight/coresight-trbe.c
index 14e35b9660d76e47619cc6026b94929b3bb3e02b..c7cbca45f2debd4047b93283ea9fe5dd9e1f2ebf 100644
--- a/drivers/hwtracing/coresight/coresight-trbe.c
+++ b/drivers/hwtracing/coresight/coresight-trbe.c
@@ -116,6 +116,20 @@ static int trbe_errata_cpucaps[] = {
*/
#define TRBE_WORKAROUND_OVERWRITE_FILL_MODE_SKIP_BYTES 256

+/*
+ * struct trbe_save_state: Register values representing TRBE state
+ * @trblimitr - Trace Buffer Limit Address Register value
+ * @trbbaser - Trace Buffer Base Register value
+ * @trbptr - Trace Buffer Write Pointer Register value
+ * @trbsr - Trace Buffer Status Register value
+ */
+struct trbe_save_state {
+ u64 trblimitr;
+ u64 trbbaser;
+ u64 trbptr;
+ u64 trbsr;
+};
+
/*
* struct trbe_cpudata: TRBE instance specific data
* @trbe_flag - TRBE dirty/access flag support
@@ -134,6 +148,7 @@ struct trbe_cpudata {
enum cs_mode mode;
struct trbe_buf *buf;
struct trbe_drvdata *drvdata;
+ struct trbe_save_state save_state;
DECLARE_BITMAP(errata, TRBE_ERRATA_MAX);
};

@@ -1189,6 +1204,46 @@ static irqreturn_t arm_trbe_irq_handler(int irq, void *dev)
return IRQ_HANDLED;
}

+static int arm_trbe_save(struct coresight_device *csdev)
+{
+ struct trbe_cpudata *cpudata = dev_get_drvdata(&csdev->dev);
+ struct trbe_save_state *state = &cpudata->save_state;
+
+ state->trblimitr = read_sysreg_s(SYS_TRBLIMITR_EL1);
+
+ /* Disable the unit, ensure the writes to memory are complete */
+ if (state->trblimitr & TRBLIMITR_EL1_E)
+ trbe_drain_and_disable_local(cpudata);
+
+ state->trbbaser = read_sysreg_s(SYS_TRBBASER_EL1);
+ state->trbptr = read_sysreg_s(SYS_TRBPTR_EL1);
+ state->trbsr = read_sysreg_s(SYS_TRBSR_EL1);
+ return 0;
+}
+
+static void arm_trbe_restore(struct coresight_device *csdev)
+{
+ struct trbe_cpudata *cpudata = dev_get_drvdata(&csdev->dev);
+ struct trbe_save_state *state = &cpudata->save_state;
+
+ write_sysreg_s(state->trbbaser, SYS_TRBBASER_EL1);
+ write_sysreg_s(state->trbptr, SYS_TRBPTR_EL1);
+ write_sysreg_s(state->trbsr, SYS_TRBSR_EL1);
+
+ if (!(state->trblimitr & TRBLIMITR_EL1_E)) {
+ write_sysreg_s(state->trblimitr, SYS_TRBLIMITR_EL1);
+ } else {
+ /*
+ * The section K5.5 Context switching, Arm ARM (ARM DDI 0487
+ * L.a), S_PKLXF requires a Context synchronization event to
+ * guarantee the Trace Buffer Unit will observe the new values
+ * of the system registers.
+ */
+ isb();
+ set_trbe_enabled(cpudata, state->trblimitr);
+ }
+}
+
static const struct coresight_ops_sink arm_trbe_sink_ops = {
.enable = arm_trbe_enable,
.disable = arm_trbe_disable,
@@ -1198,7 +1253,9 @@ static const struct coresight_ops_sink arm_trbe_sink_ops = {
};

static const struct coresight_ops arm_trbe_cs_ops = {
- .sink_ops = &arm_trbe_sink_ops,
+ .pm_save_disable = arm_trbe_save,
+ .pm_restore_enable = arm_trbe_restore,
+ .sink_ops = &arm_trbe_sink_ops,
};

static ssize_t align_show(struct device *dev, struct device_attribute *attr, char *buf)

--
2.34.1