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Re: [PATCH V8 16/23] coresight: etb10: implementing AUX API

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  Re: [PATCH V8 16/23] coresight: etb10: implementing AUX API Alexander Shishkin <alexander.shishkin@linux.intel.com> - 2016-01-26 17:00 +0100
    Re: [PATCH V8 16/23] coresight: etb10: implementing AUX API Mathieu Poirier <mathieu.poirier@linaro.org> - 2016-01-27 22:00 +0100

#1318089 — Re: [PATCH V8 16/23] coresight: etb10: implementing AUX API

Mathieu Poirier <mathieu.poirier@linaro.org> writes:

> Adding an ETB10 specific AUX area operations to be used
> by the perf framework when events are initialised.
>
> Part of this operation involves modeling the mmap'ed area
> based on the specific ways a sink buffer gathers information.

I don't mind being CC'd on the rest of the patches too, btw. :)

> +static unsigned long etb_reset_buffer(struct coresight_device *csdev,
> +				      struct perf_output_handle *handle,
> +				      void *sink_config, bool *lost)
> +{
> +	unsigned long size = 0;
> +	struct cs_buffers *buf = sink_config;
> +
> +	if (buf) {
> +		/*
> +		 * In snapshot mode ->data_size holds the new address of the
> +		 * ring buffer's head.  The size itself is the whole address
> +		 * range since we want the latest information.
> +		 */
> +		if (buf->snapshot)
> +			handle->head = local_xchg(&buf->data_size,
> +						  buf->nr_pages << PAGE_SHIFT);
> +
> +		/*
> +		 * Tell the tracer PMU how much we got in this run and if
> +		 * something went wrong along the way.  Nobody else can use
> +		 * this cs_buffers instance until we are done.  As such
> +		 * resetting parameters here and squaring off with the ring
> +		 * buffer API in the tracer PMU is fine.
> +		 */
> +		*lost = local_xchg(&buf->lost, 0);

This is a thin ice, you can't really make assumptions about bool's
storage size or even type, afaict.

> +		size = local_xchg(&buf->data_size, 0);
> +	}
> +
> +	return size;
> +}
> +
> +static void etb_update_buffer(struct coresight_device *csdev,
> +			      struct perf_output_handle *handle,
> +			      void *sink_config)
> +{
> +	int i, cur;
> +	u8 *buf_ptr;
> +	u32 read_ptr, write_ptr, capacity;
> +	u32 status, read_data, to_read;
> +	unsigned long flags, offset;
> +	struct cs_buffers *buf = sink_config;
> +	struct etb_drvdata *drvdata = dev_get_drvdata(csdev->dev.parent);
> +
> +	if (!buf)
> +		return;
> +
> +	capacity = drvdata->buffer_depth * ETB_FRAME_SIZE_WORDS;
> +
> +	spin_lock_irqsave(&drvdata->spinlock, flags);

This spinlock seems to be held over the entire readout operation,
however, I can't find clear rules wrt what structures etc are serialized
on it. Instead, the comment says "only one at a time pls". Same for
etm's big drvdata spinlock.

Regards,
--
Alex

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#1319806

On 26 January 2016 at 08:53, Alexander Shishkin
<alexander.shishkin@linux.intel.com> wrote:
> Mathieu Poirier <mathieu.poirier@linaro.org> writes:
>
>> Adding an ETB10 specific AUX area operations to be used
>> by the perf framework when events are initialised.
>>
>> Part of this operation involves modeling the mmap'ed area
>> based on the specific ways a sink buffer gathers information.
>
> I don't mind being CC'd on the rest of the patches too, btw. :)

Most definitely.

>
>> +static unsigned long etb_reset_buffer(struct coresight_device *csdev,
>> +                                   struct perf_output_handle *handle,
>> +                                   void *sink_config, bool *lost)
>> +{
>> +     unsigned long size = 0;
>> +     struct cs_buffers *buf = sink_config;
>> +
>> +     if (buf) {
>> +             /*
>> +              * In snapshot mode ->data_size holds the new address of the
>> +              * ring buffer's head.  The size itself is the whole address
>> +              * range since we want the latest information.
>> +              */
>> +             if (buf->snapshot)
>> +                     handle->head = local_xchg(&buf->data_size,
>> +                                               buf->nr_pages << PAGE_SHIFT);
>> +
>> +             /*
>> +              * Tell the tracer PMU how much we got in this run and if
>> +              * something went wrong along the way.  Nobody else can use
>> +              * this cs_buffers instance until we are done.  As such
>> +              * resetting parameters here and squaring off with the ring
>> +              * buffer API in the tracer PMU is fine.
>> +              */
>> +             *lost = local_xchg(&buf->lost, 0);
>
> This is a thin ice, you can't really make assumptions about bool's
> storage size or even type, afaict.

You are theoretically correct but I wonder if the value of &buf->lost
can get to a size where it won't fit in *lost... Nevertheless I'll fix
it with:

*lost = !!local_xchg(&buf->lost, 0);

>
>> +             size = local_xchg(&buf->data_size, 0);
>> +     }
>> +
>> +     return size;
>> +}
>> +
>> +static void etb_update_buffer(struct coresight_device *csdev,
>> +                           struct perf_output_handle *handle,
>> +                           void *sink_config)
>> +{
>> +     int i, cur;
>> +     u8 *buf_ptr;
>> +     u32 read_ptr, write_ptr, capacity;
>> +     u32 status, read_data, to_read;
>> +     unsigned long flags, offset;
>> +     struct cs_buffers *buf = sink_config;
>> +     struct etb_drvdata *drvdata = dev_get_drvdata(csdev->dev.parent);
>> +
>> +     if (!buf)
>> +             return;
>> +
>> +     capacity = drvdata->buffer_depth * ETB_FRAME_SIZE_WORDS;
>> +
>> +     spin_lock_irqsave(&drvdata->spinlock, flags);
>
> This spinlock seems to be held over the entire readout operation,
> however, I can't find clear rules wrt what structures etc are serialized
> on it. Instead, the comment says "only one at a time pls". Same for
> etm's big drvdata spinlock.

That spinlock is there to serialise actions coming from sysFS.  I
originally added the spinlock to 'etb_update_buffer()' to guard
against reading the internal RAM buffer from sysfs while a perf
session is active.  But after supplementing 'etb_dump()' with 'mode'
awareness this spinlock it is no longer required.

>
> Regards,
> --
> Alex

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