[Intel-gfx] [PATCH v2 1/7] drm/i915/perf: rework aging tail workaround

[Chris Wilson]

Quoting Lionel Landwerlin (2019-02-19 10:31:52)
> On 19/02/2019 09:18, Chris Wilson wrote:
> > Quoting Lionel Landwerlin (2019-02-18 18:35:28)
> >> We're about to introduce an options to open the perf stream, giving
> >> the user ability to configure how often it wants the kernel to poll
> >> the OA registers for available data.
> >>
> >> Right now the workaround against the OA tail pointer race condition
> >> requires at least twice the internal kernel polling timer to make any
> >> data available.
> > I think this code is bogus. I do not think there is an aging problem on
> > hsw+, the only cause of zeroes being written that I've found is from
> > changing configs.
> > -Chris
> >
> 
> Hmm... I'm pretty sure there is an issue and it's actually worse on the 
> HSW machine I have than on later machines.
> 
> The DRM_NOTE in that function is appearing a lot more on HSW.

But how much of that is due to not actually using the correct mb
ourselves, and how much of that is the OA actually writing zero?
-Chris