On 19/10/17 18:15, Suzuki K Poulose wrote:
The TMC-ETR supports routing the Coresight trace data to the System memory. It supports two different modes in which the memory could be used.
- Contiguous memory - The memory is assumed to be physically
contiguous.
- Scatter Gather list - The memory can be chunks of 4K pages,
which are specified in a table of pointers which itself could be multiple 4K size pages.
To avoid the complications of the managing the buffer, this series adds a layer for managing the ETR buffer, which makes the best possibly choice based on what is available. The allocation can be tuned by passing in flags, existing pages (e.g, perf ring buffer) etc.
Towards supporting ETR Scatter Gather mode, we introduce a generic TMC scatter-gather table which can be used to manage the data and table pages. The table can be filled in the format expected by the Scatter-Gather mode.
The TMC ETR-SG mechanism doesn't allow starting the trace at non-zero offset (required by perf). So we make some tricky changes to the table at run time to allow starting at any "Page aligned" offset and then wrap around to the beginning of the buffer with very less overhead. See patches for more description.
The series also improves the way the ETR is controlled by different modes (sysfs vs. perf) by keeping mode specific data. This allows access to the trace data collected in sysfs mode, even when the ETR is operated in perf mode. Also with the transparent management of the buffer and scatter-gather mechanism, we can allow the user to request for larger trace buffers for sysfs mode. This is supported by providing a sysfs file, "buffer_size" which accepts a page aligned size, which will be used by the ETR when allocating a buffer.
Finally, it cleans up the etm perf sink callbacks a little bit and then adds the support for ETR sink. For the ETR, we try our best to use the perf ring buffer as the target hardware buffer, provided :
- The ETR is dma coherent (since the pages will be shared with userspace perf tool).
- The perf is used in snapshot mode (The ETR cannot be stopped based on the size of the data written hence we could easily overwrite the buffer. We may be able to fix this in the future)
- The ETR supports the Scatter-Gather mode.
If we can't use the perf buffers directly, we fallback to using software buffering where we have to copy the trace data back to the perf ring buffer.
Just to be clear :
The perf tool doesn't support the perf AUX api for coresight. I have used the perf tool from perf-OpenCSD [1] project to control the tracing.
[1] https://git.linaro.org/people/mathieu.poirier/coresight.git perf-opencsd-4.14-rc1
Suzuki