Re: [Intel-gfx] [PATCH v2 2/2] drm/i915/gt: document TLB cache invalidation functions

[Mauro Carvalho Chehab <mauro.chehab@linux.intel.com>]

On Tue, 2 Aug 2022 15:30:44 -0700
Niranjana Vishwanathapura <niranjana.vishwanathapura@intel.com> wrote:

> On Fri, Jul 29, 2022 at 09:03:55AM +0200, Mauro Carvalho Chehab wrote:
> >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  
> 
> Free TLB-specific vars

OK.

> 
> >+ * @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.  
> 
> I am trying to get my head around the below function.
> 
> void vma_invalidate_tlb(struct i915_address_space *vm, u32 tlb)
> {
>         WRITE_ONCE(tlb, intel_gt_next_invalidate_tlb_full(vm->gt));
> }
> 
> Though we pass obj->mm.tlb for 'tlb' while calling this function,
> aren't we writing to local 'tlb' variable here instead of obj->mm.tlb?

It should be passing a pointer. I wrote such fix after a review,
but somehow it ended getting lost. I'll send the fix at v3.

> >+ *
> >+ * 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,  
> 
> May be we can skip 'void' and just keep function name here.

Sure.

> >+ * 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
> >+ *  
> 
> Probably this empty comment line needs to be removed before the parameter
> description below?

Kernel-doc actually accepts both with or without a blank line. My
personal preference is to place a blank line, because sometimes the
function description plus function name is bigger than one line.
So, it is usually clearer when adding a blank line than doing
something like this (perfectly valid kerneldoc markup):

	/**
	 * long_function_name_foo - Lorem ipsum dolor sit amet, consectetur
	 * adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore.
	 * @bar: some parameter
	 *  ...

But yeah, kernel-doc documentation example doesn't have a blank
line. So, I'll drop it.

> 
> >+ * @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
> >+ *  
> 
> Same here.
> 
> -Niranjana
> 
> >+ * @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
> >  

Thanks!
Mauro