[Intel-gfx] [PATCH] RFC/RFT drm/i915/oa: Drop aging-tail

[Lionel Landwerlin]

On 19/02/2019 10:28, Chris Wilson wrote:
> Switch to using coherent reads that are serialised with the register
> read to avoid the memory latency problem in favour over an arbitrary
> delay. The only zeroes seen during testing on HSW+ have been from
> configuration changes that do not update (therefore were truly zero
> entries and should be skipped).
>
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: Lionel Landwerlin <lionel.g.landwerlin at intel.com>
> Cc: Matthew Auld <matthew.auld at intel.com>
> ---
>   drivers/gpu/drm/i915/i915_drv.h  |  59 ---
>   drivers/gpu/drm/i915/i915_perf.c | 625 +++++--------------------------
>   2 files changed, 87 insertions(+), 597 deletions(-)


I took the I915_READ_FW() changes + the i915_vma_(un)pin_iomap and I'm 
still seeing reads of the HW tail register pointing 2 reports behind the 
last one that actually has its reason & timestamp fields != 0.

That is within a run where at the timestamp register went from 
0xa21d5813 to 0xa3b3441a for example.

But the DRM_NOTE("Skipping spurious, invalid OA report\n"); didn't fire 
once, meaning the reports had their data landing some time after the 
oa_buffer_check() call.


To me this seems to show there is clearly an issue with the HW and that 
we need the workaround.


-Lionel