[PATCH 04/11] rvtrace: Add functions to start/stop tracing on a component path

[Bo Gan]

On 10/12/25 20:43, Anup Patel wrote:
> On Wed, Oct 8, 2025 at 2:45 PM Bo Gan <ganboing at gmail.com> wrote:
>>
>> On 10/1/25 23:07, Anup Patel wrote:
>>> From: Mayuresh Chitale <mchitale at ventanamicro.com>
>>>
>>> The perf driver framework needs to be able to start / stop all components
>>> in a trace component path during its operation. Add rvtrace_path_start()
>>> and rvtrace_path_stop() functions for this purpose.
>>>
>>> Co-developed-by: Anup Patel <apatel at ventanamicro.com>
>>> Signed-off-by: Anup Patel <apatel at ventanamicro.com>
>>> Signed-off-by: Mayuresh Chitale <mchitale at ventanamicro.com>
>>> ---
>>>    drivers/hwtracing/rvtrace/rvtrace-core.c | 44 ++++++++++++++++++++++++
>>>    include/linux/rvtrace.h                  |  6 ++++
>>>    2 files changed, 50 insertions(+)
>>>
>>> diff --git a/drivers/hwtracing/rvtrace/rvtrace-core.c b/drivers/hwtracing/rvtrace/rvtrace-core.c
>>> index 7013d50ca569..109be40d4b24 100644
>>> --- a/drivers/hwtracing/rvtrace/rvtrace-core.c
>>> +++ b/drivers/hwtracing/rvtrace/rvtrace-core.c
>>> @@ -614,6 +614,50 @@ static void rvtrace_release_path_nodes(struct rvtrace_path *path)
>>>        }
>>>    }
>>>
>>> +int rvtrace_path_start(struct rvtrace_path *path)
>>> +{
>>> +     const struct rvtrace_driver *rtdrv;
>>> +     struct rvtrace_component *comp;
>>> +     struct rvtrace_path_node *node;
>>> +     int ret;
>>> +
>>> +     list_for_each_entry(node, &path->comp_list, head) {
>>> +             comp = node->comp;
>>> +             rtdrv = to_rvtrace_driver(comp->dev.driver);
>>> +             if (!rtdrv->start)
>>> +                     continue;
>>> +
>>> +             ret = rtdrv->start(comp);
>>> +             if (ret)
>>> +                     return ret;
>>> +     }
>>> +
>>> +     return 0;
>>> +}
>>> +EXPORT_SYMBOL_GPL(rvtrace_path_start);
>>> +
>>> +int rvtrace_path_stop(struct rvtrace_path *path)
>>> +{
>>> +     const struct rvtrace_driver *rtdrv;
>>> +     struct rvtrace_component *comp;
>>> +     struct rvtrace_path_node *node;
>>> +     int ret;
>>> +
>>> +     list_for_each_entry(node, &path->comp_list, head) {
>>> +             comp = node->comp;
>>> +             rtdrv = to_rvtrace_driver(comp->dev.driver);
>>> +             if (!rtdrv->stop)
>>> +                     continue;
>>> +
>>> +             ret = rtdrv->stop(comp);
>>> +             if (ret)
>>> +                     return ret;
>>> +     }
>>> +
>>> +     return 0;
>>> +}
>>> +EXPORT_SYMBOL_GPL(rvtrace_path_stop);
>>> +
>>>    struct rvtrace_path *rvtrace_create_path(struct rvtrace_component *source,
>>>                                         struct rvtrace_component *sink,
>>>                                         enum rvtrace_component_mode mode)
>>> diff --git a/include/linux/rvtrace.h b/include/linux/rvtrace.h
>>> index f2174f463a69..e7bd335d388f 100644
>>> --- a/include/linux/rvtrace.h
>>> +++ b/include/linux/rvtrace.h
>>> @@ -273,10 +273,14 @@ struct rvtrace_path *rvtrace_create_path(struct rvtrace_component *source,
>>>                                         struct rvtrace_component *sink,
>>>                                         enum rvtrace_component_mode mode);
>>>    void rvtrace_destroy_path(struct rvtrace_path *path);
>>> +int rvtrace_path_start(struct rvtrace_path *path);
>>> +int rvtrace_path_stop(struct rvtrace_path *path);
>>>
>>>    /**
>>>     * struct rvtrace_driver - Representation of a RISC-V trace driver
>>>     * id_table: Table to match components handled by the driver
>>> + * start:        Callback to start tracing
>>> + * stop:         Callback to stop tracing
>>>     * probe:        Driver probe() function
>>>     * remove:       Driver remove() function
>>>     * get_trace_id: Get/allocate a trace ID
>>> @@ -285,6 +289,8 @@ void rvtrace_destroy_path(struct rvtrace_path *path);
>>>     */
>>>    struct rvtrace_driver {
>>>        const struct rvtrace_component_id *id_table;
>>> +     int                     (*start)(struct rvtrace_component *comp);
>>> +     int                     (*stop)(struct rvtrace_component *comp);
>>>        int                     (*probe)(struct rvtrace_component *comp);
>>>        void                    (*remove)(struct rvtrace_component *comp);
>>>        int                     (*get_trace_id)(struct rvtrace_component *comp,
>>
>> I'd suggest add another function (*quiesce) or something like that. Trace
>> components have a tr??Empty bit that indicates trace has been all flushed
>> out. Also along the path when you do rvtrace_path_stop, you need to ensure
>> the source has stopped and quiescent before beginning to stop the sink.
>> Otherwise you'll get partial or corrupted trace. In essence, follow Control
>> Interface Spec 11.3 Enabling and Disabling. FYI: my userspace driver:
>> https://github.com/ganboing/riscv-trace-umd/blob/master/rvtrace/funnel.py#L223
> 
> It's better to add functions on a need basis rather than adding
> it now without any potential user.
> 
> Regards,
> Anu

Hi Anup, my previous comment also applies to your current use case where you
have encoder->RAM sink directly connected together. Having a longer path,
e.g., funnels in between makes it worse. The driver needs to poll the empty
bit tr??Empty (bit 3) of the control register to check if trace has been
completely flushed. Otherwise, you get a partial trace, possibly with last
few messages missing or truncated. So, yes, there's really a need to do so.

Bo