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

["Dixit, Ashutosh" <ashutosh.dixit@intel.com>]

On Tue, 03 Mar 2020 14:19:05 -0800, Umesh Nerlige Ramappa wrote:
>
> From: Lionel Landwerlin <lionel.g.landwerlin@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@intel.com>
> Signed-off-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@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.
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