[PATCH v3 2/5] drm/xe/vf: Document SRIOV VF restore flow

[Michal Wajdeczko <michal.wajdeczko@intel.com>]

From: Tomasz Lis <tomasz.lis@intel.com>

This adds a documentation chapter, containing high level flow
of VF restore procedure.

v2: Better describe initial conditions, include GuC states on
  sequence diagram (Michal)
v3: moved DOC to .c (Michal)

Signed-off-by: Tomasz Lis <tomasz.lis@intel.com>
Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
---
 drivers/gpu/drm/xe/xe_sriov_vf.c | 107 +++++++++++++++++++++++++++++++
 1 file changed, 107 insertions(+)

diff --git a/drivers/gpu/drm/xe/xe_sriov_vf.c b/drivers/gpu/drm/xe/xe_sriov_vf.c
index 2fe49296a79d..ab334cea2a1c 100644
--- a/drivers/gpu/drm/xe/xe_sriov_vf.c
+++ b/drivers/gpu/drm/xe/xe_sriov_vf.c
@@ -12,6 +12,113 @@
 #include "xe_sriov_printk.h"
 #include "xe_sriov_vf.h"
 
+/**
+ * DOC: VF restore procedure in PF KMD and VF KMD
+ *
+ * Restoring previously saved state of a VF is one of core features of
+ * SR-IOV. All major VM Management applications allow saving and restoring
+ * the VM state, and doing that to a VM which uses SRIOV VF as one of
+ * the accessible devices requires support from KMD on both PF and VF side.
+ * VMM initiates all required operations through VFIO module, which then
+ * translates them into PF KMD calls. This description will focus on these
+ * calls, leaving out the module which initiates these steps (VFIO).
+ *
+ * In order to start the restore procedure, GuC needs to keep the VF in
+ * proper state. The PF driver can ensure GuC set it to VF_READY state
+ * by provisioning the VF, which in turn can be done after Function Level
+ * Reset of said VF (or after it was freshly created - in that case FLR
+ * is not needed). The FLR procedure ends with GuC sending message
+ * `GUC_PF_NOTIFY_VF_FLR_DONE`, and then provisioning data is sent to GuC.
+ * After the provisioning is completed, the VF needs to be paused, and
+ * at that point the actual restore can begin.
+ *
+ * During VF Restore, state of several resources is restored. These may
+ * include local memory content (system memory is restored by VMM itself),
+ * values of MMIO registers, stateless compression metadata and others.
+ * The final resource which also needs restoring is state of the VF
+ * submission maintained within GuC. For that, `GUC_PF_OPCODE_VF_RESTORE`
+ * message is used, with reference to the state blob to be consumed by
+ * GuC.
+ *
+ * Next, when VFIO is asked to set the VM into running state, the PF driver
+ * sends `GUC_PF_TRIGGER_VF_RESUME` to GuC. When sent after restore, this
+ * changes VF state within GuC to `VF_RESFIX_BLOCKED` rather than the
+ * usual `VF_RUNNING`. At this point GuC triggers an interrupt to inform
+ * the VF KMD within the VM that it was migrated.
+ *
+ * As soon as Virtual GPU of the VM starts, the VF driver within receives
+ * the MIGRATED interrupt and schedules post-migration recovery worker.
+ * That worker queries GuC for new provisioning (using MMIO communication),
+ * and applies fixups to any non-virtualized resources used by the VF.
+ *
+ * When the VF driver is ready to continue operation on the newly connected
+ * hardware, it sends `VF2GUC_NOTIFY_RESFIX_DONE` which causes it to
+ * enter the long awaited `VF_RUNNING` state, and therefore start handling
+ * CTB messages and scheduling workloads from the VF::
+ *
+ *      PF                             GuC                              VF
+ *     [ ]                              |                               |
+ *     [ ] PF2GUC_VF_CONTROL(pause)     |                               |
+ *     [ ]---------------------------> [ ]                              |
+ *     [ ]                             [ ]  GuC sets new VF state to    |
+ *     [ ]                             [ ]------- VF_READY_PAUSED       |
+ *     [ ]                             [ ]      |                       |
+ *     [ ]                             [ ] <-----                       |
+ *     [ ] success                     [ ]                              |
+ *     [ ] <---------------------------[ ]                              |
+ *     [ ]                              |                               |
+ *     [ ] PF loads resources from the  |                               |
+ *     [ ]------- saved image supplied  |                               |
+ *     [ ]      |                       |                               |
+ *     [ ] <-----                       |                               |
+ *     [ ]                              |                               |
+ *     [ ] GUC_PF_OPCODE_VF_RESTORE     |                               |
+ *     [ ]---------------------------> [ ]                              |
+ *     [ ]                             [ ]  GuC loads contexts and CTB  |
+ *     [ ]                             [ ]------- state from image      |
+ *     [ ]                             [ ]      |                       |
+ *     [ ]                             [ ] <-----                       |
+ *     [ ]                             [ ]                              |
+ *     [ ]                             [ ]  GuC sets new VF state to    |
+ *     [ ]                             [ ]------- VF_RESFIX_PAUSED      |
+ *     [ ]                             [ ]      |                       |
+ *     [ ] success                     [ ] <-----                       |
+ *     [ ] <---------------------------[ ]                              |
+ *     [ ]                              |                               |
+ *     [ ] GUC_PF_TRIGGER_VF_RESUME     |                               |
+ *     [ ]---------------------------> [ ]                              |
+ *     [ ]                             [ ]  GuC sets new VF state to    |
+ *     [ ]                             [ ]------- VF_RESFIX_BLOCKED     |
+ *     [ ]                             [ ]      |                       |
+ *     [ ]                             [ ] <-----                       |
+ *     [ ]                             [ ]                              |
+ *     [ ]                             [ ] GUC_INTR_SW_INT_0            |
+ *     [ ] success                     [ ]---------------------------> [ ]
+ *     [ ] <---------------------------[ ]                             [ ]
+ *      |                               |      VF2GUC_QUERY_SINGLE_KLV [ ]
+ *      |                              [ ] <---------------------------[ ]
+ *      |                              [ ]                             [ ]
+ *      |                              [ ]        new VF provisioning  [ ]
+ *      |                              [ ]---------------------------> [ ]
+ *      |                               |                              [ ]
+ *      |                               |       VF driver applies post [ ]
+ *      |                               |      migration fixups -------[ ]
+ *      |                               |                       |      [ ]
+ *      |                               |                       -----> [ ]
+ *      |                               |                              [ ]
+ *      |                               |    VF2GUC_NOTIFY_RESFIX_DONE [ ]
+ *      |                              [ ] <---------------------------[ ]
+ *      |                              [ ]                             [ ]
+ *      |                              [ ]  GuC sets new VF state to   [ ]
+ *      |                              [ ]------- VF_RUNNING           [ ]
+ *      |                              [ ]      |                      [ ]
+ *      |                              [ ] <-----                      [ ]
+ *      |                              [ ]                     success [ ]
+ *      |                              [ ]---------------------------> [ ]
+ *      |                               |                               |
+ *      |                               |                               |
+ */
+
 static void migration_worker_func(struct work_struct *w);
 
 /**
-- 
2.43.0