[Intel-gfx] [PATCH 7/7] drm/i915/perf: add flushing ioctl

[Lionel Landwerlin]

On 04/03/2020 07:48, Dixit, Ashutosh wrote:
> On Tue, 03 Mar 2020 14:19:05 -0800, Umesh Nerlige Ramappa wrote:
>> From: Lionel Landwerlin <lionel.g.landwerlin at intel.com>
>>
>> With the currently available parameters for the i915-perf stream,
>> there are still situations that are not well covered :
>>
>> If an application opens the stream with polling disable or at very low
>> frequency and OA interrupt enabled, no data will be available even
>> though somewhere between nothing and half of the OA buffer worth of
>> data might have landed in memory.
>>
>> To solve this issue we have a new flush ioctl on the perf stream that
>> forces the i915-perf driver to look at the state of the buffer when
>> called and makes any data available through both poll() & read() type
>> syscalls.
>>
>> v2: Version the ioctl (Joonas)
>> v3: Rebase (Umesh)
>>
>> Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin at intel.com>
>> Signed-off-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa at intel.com>
> [snip]
>
>> +/**
>> + * i915_perf_flush_data - handle `I915_PERF_IOCTL_FLUSH_DATA` ioctl
>> + * @stream: An enabled i915 perf stream
>> + *
>> + * The intention is to flush all the data available for reading from the OA
>> + * buffer
>> + */
>> +static void i915_perf_flush_data(struct i915_perf_stream *stream)
>> +{
>> +	stream->pollin = oa_buffer_check(stream, true);
>> +}
> Since this function doesn't actually wake up any thread (which anyway can
> be done by sending a signal to the blocked thread), is the only purpose of
> this function to update OA buffer head/tail? But in that it is not clear
> why a separate ioctl should be created for this, can't the read() call
> itself call oa_buffer_check() to update the OA buffer head/tail?
>
> Again just trying to minimize uapi changes if possible.

Most applications will call read() after being notified by 
poll()/select() that some data is available.

Changing that behavior will break some of the existing perf tests .


If any data is available, this new ioctl will wake up existing waiters 
on poll()/select().


-Lionel