DRM: Add the explanation about DRM debug level

Add the explanation about DRM debug level in the drmP header file. This is to
explain how/where to use the different DRM debug level.
Signed-off-by: Zhao Yakui <yakui.zhao@intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

include/drm/drmP.h

@@ -88,6 +88,37 @@ struct drm_device;
 #define DRM_UT_CORE 		0x01
 #define DRM_UT_DRIVER		0x02
 #define DRM_UT_KMS		0x04
+/*
+ * Three debug levels are defined.
+ * drm_core, drm_driver, drm_kms
+ * drm_core level can be used in the generic drm code. For example:
+ * 	drm_ioctl, drm_mm, drm_memory
+ * The macro definiton of DRM_DEBUG is used.
+ * 	DRM_DEBUG(fmt, args...)
+ * 	The debug info by using the DRM_DEBUG can be obtained by adding
+ * 	the boot option of "drm.debug=1".
+ *
+ * drm_driver level can be used in the specific drm driver. It is used
+ * to add the debug info related with the drm driver. For example:
+ * i915_drv, i915_dma, i915_gem, radeon_drv,
+ * 	The macro definition of DRM_DEBUG_DRIVER can be used.
+ * 	DRM_DEBUG_DRIVER(fmt, args...)
+ * 	The debug info by using the DRM_DEBUG_DRIVER can be obtained by
+ * 	adding the boot option of "drm.debug=0x02"
+ *
+ * drm_kms level can be used in the KMS code related with specific drm driver.
+ * It is used to add the debug info related with KMS mode. For example:
+ * the connector/crtc ,
+ * 	The macro definition of DRM_DEBUG_KMS can be used.
+ * 	DRM_DEBUG_KMS(fmt, args...)
+ * 	The debug info by using the DRM_DEBUG_KMS can be obtained by
+ * 	adding the boot option of "drm.debug=0x04"
+ *
+ * If we add the boot option of "drm.debug=0x06", we can get the debug info by
+ * using the DRM_DEBUG_KMS and DRM_DEBUG_DRIVER.
+ * If we add the boot option of "drm.debug=0x05", we can get the debug info by
+ * using the DRM_DEBUG_KMS and DRM_DEBUG.
+ */
 
 extern void drm_ut_debug_printk(unsigned int request_level,
 				const char *prefix,