drm/i915/guc: improve documentation

Add a short description of what we expect from GuC and some minor
improvements to existing documentation. Also remove a comment about a
difference between GuC and HuC that is not true anymore.

v2: add that the GuC is not mandatory (Martin)
v3: add extra newline for better text organization (Martin)
Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Martin Peres <martin.peres@linux.intel.com>
Acked-by: Anna Karas <anna.karas@intel.com>
Reviewed-by: Martin Peres <martin.peres@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191014183602.3643-2-daniele.ceraolospurio@intel.com

Documentation/gpu/i915.rst

@@ -436,12 +436,24 @@ WOPCM Layout
 GuC
 ---
 
-Firmware Layout
-~~~~~~~~~~~~~~~
+.. kernel-doc:: drivers/gpu/drm/i915/gt/uc/intel_guc.c
+   :doc: GuC
+
+GuC Firmware Layout
+~~~~~~~~~~~~~~~~~~~
 
 .. kernel-doc:: drivers/gpu/drm/i915/gt/uc/intel_uc_fw_abi.h
    :doc: Firmware Layout
 
+GuC Memory Management
+~~~~~~~~~~~~~~~~~~~~~
+
+.. kernel-doc:: drivers/gpu/drm/i915/gt/uc/intel_guc.c
+   :doc: GuC Memory Management
+.. kernel-doc:: drivers/gpu/drm/i915/gt/uc/intel_guc.c
+   :functions: intel_guc_allocate_vma
+
+
 GuC-specific firmware loader
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
@@ -457,12 +469,6 @@ GuC-based command submission
 .. kernel-doc:: drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
    :internal:
 
-GuC Address Space
-~~~~~~~~~~~~~~~~~
-
-.. kernel-doc:: drivers/gpu/drm/i915/gt/uc/intel_guc.c
-   :doc: GuC Address Space
-
 HuC
 ---
 .. kernel-doc:: drivers/gpu/drm/i915/gt/uc/intel_huc_fw.c

drivers/gpu/drm/i915/gt/uc/intel_guc.c

@@ -9,6 +9,27 @@
 #include "intel_guc_submission.h"
 #include "i915_drv.h"
 
+/**
+ * DOC: GuC
+ *
+ * The GuC is a microcontroller inside the GT HW, introduced in gen9. The GuC is
+ * designed to offload some of the functionality usually performed by the host
+ * driver; currently the main operations it can take care of are:
+ *
+ * - Authentication of the HuC, which is required to fully enable HuC usage.
+ * - Low latency graphics context scheduling (a.k.a. GuC submission).
+ * - GT Power management.
+ *
+ * The enable_guc module parameter can be used to select which of those
+ * operations to enable within GuC. Note that not all the operations are
+ * supported on all gen9+ platforms.
+ *
+ * Enabling the GuC is not mandatory and therefore the firmware is only loaded
+ * if at least one of the operations is selected. However, not loading the GuC
+ * might result in the loss of some features that do require the GuC (currently
+ * just the HuC, but more are expected to land in the future).
+ */
+
 static void gen8_guc_raise_irq(struct intel_guc *guc)
 {
 	struct intel_gt *gt = guc_to_gt(guc);
@@ -548,9 +569,15 @@ int intel_guc_resume(struct intel_guc *guc)
 }
 
 /**
- * DOC: GuC Address Space
+ * DOC: GuC Memory Management
  *
- * The layout of GuC address space is shown below:
+ * GuC can't allocate any memory for its own usage, so all the allocations must
+ * be handled by the host driver. GuC accesses the memory via the GGTT, with the
+ * exception of the top and bottom parts of the 4GB address space, which are
+ * instead re-mapped by the GuC HW to memory location of the FW itself (WOPCM)
+ * or other parts of the HW. The driver must take care not to place objects that
+ * the GuC is going to access in these reserved ranges. The layout of the GuC
+ * address space is shown below:
  *
  * ::
  *

drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c

@@ -29,6 +29,12 @@ enum {
 /**
  * DOC: GuC-based command submission
  *
+ * IMPORTANT NOTE: GuC submission is currently not supported in i915. The GuC
+ * firmware is moving to an updated submission interface and we plan to
+ * turn submission back on when that lands. The below documentation (and related
+ * code) matches the old submission model and will be updated as part of the
+ * upgrade to the new flow.
+ *
  * GuC client:
  * A intel_guc_client refers to a submission path through GuC. Currently, there
  * is only one client, which is charged with all submissions to the GuC. This

drivers/gpu/drm/i915/gt/uc/intel_uc_fw_abi.h

@@ -39,9 +39,6 @@
  * 3. Length info of each component can be found in header, in dwords.
  * 4. Modulus and exponent key are not required by driver. They may not appear
  *    in fw. So driver will load a truncated firmware in this case.
- *
- * The only difference between GuC and HuC firmwares is how the version
- * information is saved.
  */
 
 struct uc_css_header {