Hi Mike,

 

Interesting idea. Could you explain how you deal with the situation where

paths for different sources need different configuration of the downstream

CoreSight? The situation I’m thinking of is (ignoring funnels):

 

  ETM#1

      \

        ETF -> ETR

      /

  ETM#2

 

A path with source ETM#1 to sink ETF can’t be active at the same time as a

path from source ETM#2 to sink ETR, because the former will need the ETF

to be in buffer mode while the latter will need the ETF to be in FIFO mode.

 

I’d expect you could build these two incompatible paths, but not

simultaneously enable them? So coresight_enable_path would check

that any other paths using the same ETF were using it in the same mode,

and if it was idle, it would switch it into the right mode.

 

Also how is the trace source id handled? As we have only about 120 possible

trace source ids and we have chips with 128 cores funnelling into one sink,

we can’t have a fixed allocation of trace sources to trace source ids (i.e. we

can’t fix it in the device tree or anything like that). So we need to be able to

dynamically allocate trace source ids. Could that be done in

coresight_enable_path? So all enabled paths would have distinct

trace source ids.

 

Al

 

 

From: CoreSight <coresight-bounces@lists.linaro.org> On Behalf Of Mike Bazov
Sent: 14 August 2018 14:01
To: Mathieu Poirier <mathieu.poirier@linaro.org>
Cc: coresight@lists.linaro.org
Subject: Re: Enabling Coresight in atomic context.

 

Hello,

 

> Patches are always welcomed and I don't think there is an "easy" way

> to get out of this one.  What you want to do will probably end up

> being fairly complex.  I would start by closely understanding how

> operation of the CS infrastructure is done from the perf interface

> you should be find just sticking to the kernel part.  There

> reservation of a "path" and memory for the sink is done in preparatory

> steps where it is permitted to sleep (non-atomic).  After that

> components can be enabled from an atomic context, i.e when the process

> of interest is installed on a processor.  Currently things are woven

> with the perf_aux_output_[begin|end]() interface but that could easily

> be decoupled.

 

> On the aspect of trace collection, did you envision using the entries 

> in devFS?  If that is the case a mechanism to correlate tracer

> configuration and trace data will need to be developed, just like what

> we did for perf.

 

> Taking a step back, tracers can also be found on X86 and MIPs (if I'm

> not mistaking) architectures.  As such the new kernel API would have

> to be usable by those as well, which complicates the task even

> further.

> So all that being said I think it is feasible, but be prepared to

> invest a significant amount of time and effort.

 

 The "generic" tracing kernel API is a different thing. In it's Coresight implementation it will use the kernel API I need.

 After taking a few days to understand how the infrastructure works,  to make the API as flexible as it can be, I thought about this:

 Just like there's a perf implementation and a sysfs implementation, the "api" implementation(coresight-api) will be introduced, which will also be

 a new mode(CS_MODE_API).

 

 I propose these APIs(some of them exist, but need to be exported and changed a little):

 

 * coresight_build_path(struct coresight_device *source, struct coresight_device *sink): 

    Create a coresight path from the provided source and sink,.

 

 * coresight_enable_path(struct coresight_path *path):
    Enable a Coresight path except the source. This will

    also glue a source to a specific path. You cannot assign a different path to  this source until the path is destroyed.

 

 * coresight_disable_path(struct coresight_path *path)

    Disable the path to the sink, including the sink.(if there is more than 1 path to the same sink, does not disable the sink until a refcount reaches 0).

 

 * coresight_destroy_path(struct coresight_path *path):

   Frees the path, releases the source from that path. The source device can be assigned to a different path.

 

 * coresight_enable_source(struct coresight_device *source);

   Enables the source. This will actually make the source device play the actual trace data in to the sink(i.e. etm4_enable_hw(), or, increase a refcount if

   the source is already playing). Uses the path assigned in "coresight_enable_sink()".

 

 * coresight_disable_source(struct coresight_device *source);

   Disables the source. This will stop the source from playing trace data(or, if the refcount > 0, decrease the refcount).

   Uses the path assigned in "coresight_enable_sink()".

 

 * coresight_read_sink(struct coresight_device *sink, void *buf, size_t size);

   Read trace data from the sink(advance the read pointer).

 

 * coresight_setup_sink_buffer(struct coresight_device *sink, void *pages, int nr_pages);

   Allocate a sink buffer(similar to the perf functionality)

 

The sysfs and api modes will use different buffers to avoid collision.

 

I realize most of the API is actually making the internal coresight implementation "public", but I really think this is necessary. Building a path to a specific sink

is something a user would want to do, as well as disabling and enabling the path whenever he wishes(this is something I actually need). 

 

In order to use this API, the user needs a method of getting the actual (struct coresight_device *). There will be a list of coresight devices

exported in the "coresight.h" header, which can be iterated using a macro "foreach_coresight_device()". The user will be able to extract a specific 

sink and source for his needs. 

 

I think this API is powerful, and will give the user full Coresight functionality. From diving into the code, this seems very possible,

and will not require major infrastructure changes.

I will appreciate your thoughts, tips, and hints.

 

Thanks, Mike.

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