[PATCH v2 2/2] drm/i915/gt: document TLB cache invalidation functions

[Mauro Carvalho Chehab <mchehab@kernel.org>]

Add a description for the TLB cache invalidation algorithm and for
the related kAPI functions.

Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>
---

To avoid mailbombing on a large number of people, only mailing lists were C/C on the cover.
See [PATCH v2 0/2] at: https://lore.kernel.org/all/cover.1659077372.git.mchehab@kernel.org/

 Documentation/gpu/i915.rst          |   7 ++
 drivers/gpu/drm/i915/gt/intel_tlb.c |  25 +++++++
 drivers/gpu/drm/i915/gt/intel_tlb.h | 101 ++++++++++++++++++++++++++++
 3 files changed, 133 insertions(+)

diff --git a/Documentation/gpu/i915.rst b/Documentation/gpu/i915.rst
index 4e59db1cfb00..46911fdd79e8 100644
--- a/Documentation/gpu/i915.rst
+++ b/Documentation/gpu/i915.rst
@@ -58,6 +58,13 @@ Intel GVT-g Host Support(vGPU device model)
 .. kernel-doc:: drivers/gpu/drm/i915/intel_gvt.c
    :internal:
 
+TLB cache invalidation
+----------------------
+
+.. kernel-doc:: drivers/gpu/drm/i915/gt/intel_tlb.h
+
+.. kernel-doc:: drivers/gpu/drm/i915/gt/intel_tlb.c
+
 Workarounds
 -----------
 
diff --git a/drivers/gpu/drm/i915/gt/intel_tlb.c b/drivers/gpu/drm/i915/gt/intel_tlb.c
index af8cae979489..4873b7ecc015 100644
--- a/drivers/gpu/drm/i915/gt/intel_tlb.c
+++ b/drivers/gpu/drm/i915/gt/intel_tlb.c
@@ -145,6 +145,18 @@ static void mmio_invalidate_full(struct intel_gt *gt)
 	intel_uncore_forcewake_put_delayed(uncore, FORCEWAKE_ALL);
 }
 
+/**
+ * intel_gt_invalidate_tlb_full - do full TLB cache invalidation
+ * @gt: GT structure
+ * @seqno: sequence number
+ *
+ * Do a full TLB cache invalidation if the @seqno is bigger than the last
+ * full TLB cache invalidation.
+ *
+ * Note:
+ * The TLB cache invalidation logic depends on GEN-specific registers.
+ * It currently supports MMIO-based TLB flush for GEN8 to GEN12.
+ */
 void intel_gt_invalidate_tlb_full(struct intel_gt *gt, u32 seqno)
 {
 	intel_wakeref_t wakeref;
@@ -171,12 +183,25 @@ void intel_gt_invalidate_tlb_full(struct intel_gt *gt, u32 seqno)
 	}
 }
 
+/**
+ * intel_gt_init_tlb - initialize TLB-specific vars
+ * @gt: GT structure
+ *
+ * TLB cache invalidation logic internally uses some resources that require
+ * initialization. Should be called before doing any TLB cache invalidation.
+ */
 void intel_gt_init_tlb(struct intel_gt *gt)
 {
 	mutex_init(&gt->tlb.invalidate_lock);
 	seqcount_mutex_init(&gt->tlb.seqno, &gt->tlb.invalidate_lock);
 }
 
+/**
+ * intel_gt_fini_tlb - initialize TLB-specific vars
+ * @gt: GT structure
+ *
+ * Frees any resources needed by TLB cache invalidation logic.
+ */
 void intel_gt_fini_tlb(struct intel_gt *gt)
 {
 	mutex_destroy(&gt->tlb.invalidate_lock);
diff --git a/drivers/gpu/drm/i915/gt/intel_tlb.h b/drivers/gpu/drm/i915/gt/intel_tlb.h
index 46ce25bf5afe..dca70c33bd61 100644
--- a/drivers/gpu/drm/i915/gt/intel_tlb.h
+++ b/drivers/gpu/drm/i915/gt/intel_tlb.h
@@ -11,16 +11,117 @@
 
 #include "intel_gt_types.h"
 
+/**
+ * DOC: TLB cache invalidation logic
+ *
+ * The way the current algorithm works is that a struct drm_i915_gem_object can
+ * be created on any order. At unbind/evict time, the object is warranted that
+ * it won't be used anymore. So, a sequence number provided by
+ * intel_gt_next_invalidate_tlb_full() is stored on it. This can happen either
+ * at __vma_put_pages() - for VMA sync unbind, or at ppgtt_unbind_vma() - for
+ * VMA async VMA bind.
+ *
+ * At __i915_gem_object_unset_pages(), intel_gt_invalidate_tlb_full() is called,
+ * where it checks if the sequence number of the object was already invalidated
+ * or not. If not, it flushes the TLB and increments the sequence number::
+ *
+ *   void intel_gt_invalidate_tlb_full(struct intel_gt *gt, u32 seqno)
+ *   {
+ *   ...
+ * 	with_intel_gt_pm_if_awake(gt, wakeref) {
+ * 		mutex_lock(&gt->tlb.invalidate_lock);
+ * 		if (tlb_seqno_passed(gt, seqno))
+ * 				goto unlock;
+ *
+ * 		// Some code to do TLB invalidation
+ *   ...
+ *
+ * 		write_seqcount_invalidate(&gt->tlb.seqno); // increment seqno
+ * 		mutex_lock(&gt->tlb.invalidate_lock);
+ *      }
+ *
+ * So, let's say the current seqno is 2 and 3 new objects were created,
+ * on this order::
+ *
+ * 	obj1
+ * 	obj2
+ * 	obj3
+ *
+ * They can be unbind/evict on a different order. At unbind/evict time,
+ * the mm.tlb will be stamped with the sequence number, using the number
+ * from the last TLB flush, plus 1.
+ *
+ * Different threads may be used on unbind/evict and/or unset pages.
+ * As the logic at void intel_gt_invalidate_tlb_full() is protected by a mutex,
+ * for simplicity, let's consider just two threads:
+ *
+ * +-------------------+-------------------------+---------------------------------+
+ * | sequence number   | Thread 0                | Thread 1                        +
+ * +===================+=========================+=================================+
+ * | seqno=2           |                         |                                 |
+ * |                   +-------------------------+---------------------------------+
+ * |                   | unbind/evict obj3.      |                                 |
+ * |                   |                         |                                 |
+ * |                   | obj3.mm.tlb = seqno | 1 |                                 |
+ * |                   | // obj3.mm.tlb = 3      |                                 |
+ * |                   +-------------------------+---------------------------------+
+ * |                   | unbind/evict obj1.      |                                 |
+ * |                   |                         |                                 |
+ * |                   | obj1.mm.tlb = seqno | 1 |                                 |
+ * |                   | // obj1.mm.tlb = 3      |                                 |
+ * |                   +-------------------------+---------------------------------+
+ * |                   |                         | __i915_gem_object_unset_pages() |
+ * |                   |                         | called for obj3 => TLB flush    |
+ * |                   |                         | invalidating both obj1 and obj2.|
+ * |                   |                         |                                 |
+ * |                   |                         | seqno += 2                      |
+ * +-------------------+-------------------------+---------------------------------+
+ * | seqno=4           |                         |                                 |
+ * |                   +-------------------------+---------------------------------+
+ * |                   | unbind/evict obj2.      |                                 |
+ * |                   |                         |                                 |
+ * |                   | obj2.mm.tlb = seqno | 1 |                                 |
+ * |                   | // obj2.mm.tlb = 5      |                                 |
+ * |                   +-------------------------+---------------------------------+
+ * |                   |                         | __i915_gem_object_unset_pages() |
+ * |                   |                         | called for obj1, don't flush    |
+ * |                   |                         | as past flush invalidated obj1. |
+ * |                   +-------------------------+---------------------------------+
+ * |                   |                         | __i915_gem_object_unset_pages() |
+ * |                   |                         | called for obj2 => TLB flush.   |
+ * |                   |                         | invalidating obj2.              |
+ * |                   |                         |                                 |
+ * |                   |                         | seqno += 2                      |
+ * +-------------------+-------------------------+---------------------------------+
+ * | seqno=6           |                         |                                 |
+ * +-------------------+-------------------------+---------------------------------+
+ */
+
 void intel_gt_invalidate_tlb_full(struct intel_gt *gt, u32 seqno);
 
 void intel_gt_init_tlb(struct intel_gt *gt);
 void intel_gt_fini_tlb(struct intel_gt *gt);
 
+/**
+ * intel_gt_tlb_seqno - Returns the current TLB invlidation sequence number
+ *
+ * @gt: GT structure
+ *
+ * There's no need to lock while calling it, as seqprop_sequence is thread-safe
+ */
 static inline u32 intel_gt_tlb_seqno(const struct intel_gt *gt)
 {
 	return seqprop_sequence(&gt->tlb.seqno);
 }
 
+/**
+ * intel_gt_next_invalidate_tlb_full - Returns the next TLB full invalidation
+ *	sequence number
+ *
+ * @gt: GT structure
+ *
+ * There's no need to lock while calling it, as seqprop_sequence is thread-safe
+ */
 static inline u32 intel_gt_next_invalidate_tlb_full(const struct intel_gt *gt)
 {
 	return intel_gt_tlb_seqno(gt) | 1;
-- 
2.36.1