[Bug 105359] kms_frontbuffer_tracking - FBC disabled

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=105359

--- Comment #4 from Marta Löfstedt <marta.lofstedt at intel.com> ---
https://patchwork.freedesktop.org/patch/208170/

This caused some discussion. I appears as if devs want a way for user space to
reset the "fbc state". I am assuming this also applies for FIFO underrun caused
FBC disabled.

I guess that is OK, the only one calling:
intel_fbc_handle_fifo_underrun_irq(dev_priv);

is: 
void intel_cpu_fifo_underrun_irq_handler(struct drm_i915_private *dev_priv,
                                         enum pipe pipe)

However, this comment in above function:

        /* There's no guarantee that underrun_detected won't be set to true
         * right after this check and before the work is scheduled, but that's
         * not a problem since we'll check it again under the work function
         * while FBC is locked. This check here is just to prevent us from
         * unnecessarily scheduling the work, and it relies on the fact that we
         * never switch underrun_detect back to false after it's true. */

so maybe not totally straight forward to implement.

Also, I believe that the massive hit from "FBC disabled" on kasan and shard
testlist on BAT is due to this FBC banned by FIFO underrun thing.

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