drm: Refresh the explanation of debug categories

That comment wasn't super-readable, so I tried to improve it:

- Put the comment before the values it's documenting
- Add a mention to PRIME
- Reword things a bit to be a lighter read
- Add a note about the option to set the debug value at run-time

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

include/drm/drmP.h

@@ -88,41 +88,38 @@ struct videomode;
 #include <drm/drm_hashtab.h>
 #include <drm/drm_mm.h>
 
-#define DRM_UT_CORE 		0x01
-#define DRM_UT_DRIVER		0x02
-#define DRM_UT_KMS		0x04
-#define DRM_UT_PRIME		0x08
 /*
- * 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 definition 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".
+ * 4 debug categories are defined:
+ *
+ * CORE: Used in the generic drm code: drm_ioctl.c, drm_mm.c, drm_memory.c, ...
+ *	 This is the category used by the DRM_DEBUG() macro.
+ *
+ * DRIVER: Used in the vendor specific part of the driver: i915, radeon, ...
+ *	   This is the category used by the DRM_DEBUG_DRIVER() macro.
  *
- * 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"
+ * KMS: used in the modesetting code.
+ *	This is the category used by the DRM_DEBUG_KMS() macro.
  *
- * 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"
+ * PRIME: used in the prime code.
+ *	  This is the category used by the DRM_DEBUG_PRIME() macro.
  *
- * 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.
+ * Enabling verbose debug messages is done through the drm.debug parameter,
+ * each category being enabled by a bit.
+ *
+ * drm.debug=0x1 will enable CORE messages
+ * drm.debug=0x2 will enable DRIVER messages
+ * drm.debug=0x3 will enable CORE and DRIVER messages
+ * ...
+ * drm.debug=0xf will enable all messages
+ *
+ * An interesting feature is that it's possible to enable verbose logging at
+ * run-time by echoing the debug value in its sysfs node:
+ *   # echo 0xf > /sys/module/drm/parameters/debug
  */
+#define DRM_UT_CORE 		0x01
+#define DRM_UT_DRIVER		0x02
+#define DRM_UT_KMS		0x04
+#define DRM_UT_PRIME		0x08
 
 extern __printf(4, 5)
 void drm_ut_debug_printk(unsigned int request_level,