Re: [PATCH v9 5/6] Coresight: Add Coresight TMC Control Unit driver

On 1/28/2025 7:55 PM, James Clark wrote:

On 24/01/2025 7:25 am, Jie Gan wrote:

quoted

The Coresight TMC Control Unit hosts miscellaneous configuration 
registers
which control various features related to TMC ETR sink.

Based on the trace ID, which is programmed in the related CTCU ATID
register of a specific ETR, trace data with that trace ID gets into
the ETR buffer, while other trace data gets dropped.

Enabling source device sets one bit of the ATID register based on
source device's trace ID.
Disabling source device resets the bit according to the source
device's trace ID.

Signed-off-by: Jie Gan <redacted>
---
  drivers/hwtracing/coresight/Kconfig          |  12 +
  drivers/hwtracing/coresight/Makefile         |   1 +
  drivers/hwtracing/coresight/coresight-ctcu.c | 276 +++++++++++++++++++
  drivers/hwtracing/coresight/coresight-ctcu.h |  30 ++
  include/linux/coresight.h                    |   3 +-
  5 files changed, 321 insertions(+), 1 deletion(-)
  create mode 100644 drivers/hwtracing/coresight/coresight-ctcu.c
  create mode 100644 drivers/hwtracing/coresight/coresight-ctcu.h

 >

[...]

quoted

+/*
+ * ctcu_set_etr_traceid: Retrieve the ATID offset and trace ID.
+ *
+ * Returns 0 indicates success. None-zero result means failure.
+ */
+static int ctcu_set_etr_traceid(struct coresight_device *csdev, 
struct coresight_path *cs_path,
+                bool enable)
+{
+    struct coresight_device *sink = coresight_get_sink(cs_path->path);
+    struct ctcu_drvdata *drvdata = dev_get_drvdata(csdev->dev.parent);
+    u8 trace_id = cs_path->trace_id;
+    int port_num;
+
+    if ((sink == NULL) || !IS_VALID_CS_TRACE_ID(trace_id) || 
IS_ERR_OR_NULL(drvdata)) {
+        dev_err(&csdev->dev, "Invalid parameters\n");
+        return -EINVAL;
+    }
+
+    port_num = ctcu_get_active_port(sink, csdev);
+    if (port_num < 0)
+        return -EINVAL;
+
+    /*
+     * Skip the disable session if more than one TPDM device that
+     * connected to the same TPDA device has been enabled.
+     */
+    if (enable)
+        atomic_inc(&drvdata->traceid_refcnt[port_num][trace_id]);
+    else {
+        if (atomic_dec_return(&drvdata->traceid_refcnt[port_num] 
[trace_id]) > 0) {
+            dev_dbg(&csdev->dev, "Skip the disable session\n");
+            return 0;
+        }
+    }
+
+    dev_dbg(&csdev->dev, "traceid is %d\n", cs_path->trace_id);
+
+    return __ctcu_set_etr_traceid(csdev, trace_id, port_num, enable);

Hi Jie,

Using atomic_dec_return() here doesn't prevent __ctcu_set_etr_traceid() 
from running concurrent enable and disables. Once you pass the 
atomic_dec_return() a second call to enable it will mess it up.

I think you need a spinlock around the whole thing and then the 
refcounts don't need to be atomics.

Hi, James
Thanks for comment. I may not fully tested my codes here. What I was 
thinking is there's no way the refcnt could become a negative number 
under current framework. So I just added spinlock in 
__ctcu_set_etr_traceid() to ensure concurrent sessions correctly 
manipulate the register.

As the trace_id related to the bit of the ATID register, I think the 
concurrent processes are working fine with spinlock around read/write 
register.

I may not fully got your point here. Please help me to correct it.

Thanks,
Jie