[Intel-gfx] [PATCH v5 01/10] drm/i915/perf: add missing delay for OA muxes configuration

[Chris Wilson]

Quoting Lionel Landwerlin (2019-06-27 09:00:36)
> Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin at intel.com>
> Fixes: 19f81df2859eb1 ("drm/i915/perf: Add OA unit support for Gen 8+")
> ---
>  drivers/gpu/drm/i915/i915_perf.c | 23 +++++++++++++++++++++++
>  1 file changed, 23 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/i915_perf.c b/drivers/gpu/drm/i915/i915_perf.c
> index d28a5bf80bd7..909e22835e84 100644
> --- a/drivers/gpu/drm/i915/i915_perf.c
> +++ b/drivers/gpu/drm/i915/i915_perf.c
> @@ -1838,6 +1838,29 @@ static int gen8_enable_metric_set(struct i915_perf_stream *stream)
>  
>         config_oa_regs(dev_priv, oa_config->mux_regs, oa_config->mux_regs_len);
>  
> +       /* It apparently takes a fairly long time for a new MUX
> +        * configuration to be be applied after these register writes.
> +        * This delay duration was derived empirically based on the
> +        * render_basic config but hopefully it covers the maximum
> +        * configuration latency.
> +        *
> +        * As a fallback, the checks in _append_oa_reports() to skip
> +        * invalid OA reports do also seem to work to discard reports
> +        * generated before this config has completed - albeit not
> +        * silently.

If you know the initial batch of reports is invalid after changing the
register, why not just silently discard them until you see X valid
reports? No gratuitous sleeps required, still a magic number (albeit it
may be a small number).
-Chris