drm/i915: Add a policy note for removing workarounds

Rodrigo gave a persuasive argument for keeping workarounds: that they
serve as a good guide for the bring up of the next generation. Not only
do workarounds persist into the early revisions, they show where the
workarounds were previously added to the code flow and sometimes the old
workarounds have an explanation that give insight into their wider
implications.

Based on his suggestion, document the policy that we want to keep the
workarounds from the current generation to guide the next. Older
preproduction workarounds we still want to remove to keep the code
clean.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20171117102635.8689-1-chris@chris-wilson.co.uk
Acked-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>

drivers/gpu/drm/i915/i915_drv.c

@@ -843,6 +843,11 @@ static void i915_workqueues_cleanup(struct drm_i915_private *dev_priv)
  * We don't keep the workarounds for pre-production hardware, so we expect our
  * driver to fail on these machines in one way or another. A little warning on
  * dmesg may help both the user and the bug triagers.
+ *
+ * Our policy for removing pre-production workarounds is to keep the
+ * current gen workarounds as a guide to the bring-up of the next gen
+ * (workarounds have a habit of persisting!). Anything older than that
+ * should be removed along with the complications they introduce.
  */
 static void intel_detect_preproduction_hw(struct drm_i915_private *dev_priv)
 {