drm/i915: Make default value for i915.mmio_debug a compile time option

The normal behaviour is to periodically check for a mmio access error,
and once detected enable mmio access checking. However this is useless
if the error only occurs once during module load, and so we may miss
such errors in CI. To allow ourselves to catch them, allow CI to opt into
always enabling mmio debugging.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190530121311.6794-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/Kconfig.debug

@@ -32,6 +32,7 @@ config DRM_I915_DEBUG
 	select DRM_I915_SW_FENCE_DEBUG_OBJECTS
 	select DRM_I915_SELFTEST
 	select DRM_I915_DEBUG_RUNTIME_PM
+	select DRM_I915_DEBUG_MMIO
         default n
         help
           Choose this option to turn on extra driver debugging that may affect
@@ -41,6 +42,19 @@ config DRM_I915_DEBUG
 
           If in doubt, say "N".
 
+config DRM_I915_DEBUG_MMIO
+	bool "Always insert extra checks around mmio access by default"
+	default n
+	help
+	  By default, always enables the extra sanity checks (extra register
+	  reads) around every mmio (register) access that will slow the system
+	  down. This sets the default value of i915.mmio_debug to -1 and can
+	  be overridden at module load.
+
+	  Recommended for driver developers only.
+
+	  If in doubt, say "N".
+
 config DRM_I915_DEBUG_GEM
         bool "Insert extra checks into the GEM internals"
         default n

drivers/gpu/drm/i915/i915_params.h

@@ -59,7 +59,7 @@ struct drm_printer;
 	param(char *, guc_firmware_path, NULL) \
 	param(char *, huc_firmware_path, NULL) \
 	param(char *, dmc_firmware_path, NULL) \
-	param(int, mmio_debug, 0) \
+	param(int, mmio_debug, -IS_ENABLED(CONFIG_DRM_I915_DEBUG_MMIO)) \
 	param(int, edp_vswing, 0) \
 	param(int, reset, 2) \
 	param(unsigned int, inject_load_failure, 0) \