[Intel-gfx] [PATCH v2] drm/i915/dpt: Use shmem for dpt objects

[Sripada, Radhakrishna]

Hi Tvrtko,

> -----Original Message-----
> From: Tvrtko Ursulin <tvrtko.ursulin at linux.intel.com>
> Sent: Thursday, July 20, 2023 2:17 AM
> To: Yang, Fei <fei.yang at intel.com>; Sripada, Radhakrishna
> <radhakrishna.sripada at intel.com>; intel-gfx at lists.freedesktop.org
> Cc: stable at vger.kernel.org; Ville Syrjälä <ville.syrjala at linux.intel.com>; Wilson,
> Chris P <chris.p.wilson at intel.com>
> Subject: Re: [PATCH v2] drm/i915/dpt: Use shmem for dpt objects
> 
> 
> On 19/07/2023 21:53, Yang, Fei wrote:
> >> On 18/07/2023 23:51, Radhakrishna Sripada wrote:
> >>> Dpt objects that are created from internal get evicted when there is
> >>> memory pressure and do not get restored when pinned during scanout.
> >>> The pinned page table entries look corrupted and programming the
> >>> display engine with the incorrect pte's result in DE throwing pipe faults.
> >>>
> >>> Create DPT objects from shmem and mark the object as dirty when
> >>> pinning so that the object is restored when shrinker evicts an unpinned
> buffer object.
> >>>
> >>> v2: Unconditionally mark the dpt objects dirty during pinning(Chris).
> >>>
> >>> Fixes: 0dc987b699ce ("drm/i915/display: Add smem fallback allocation
> >>> for dpt")
> >>> Cc: <stable at vger.kernel.org> # v6.0+
> >>> Cc: Ville Syrjälä <ville.syrjala at linux.intel.com>
> >>> Cc: Tvrtko Ursulin <tvrtko.ursulin at linux.intel.com>
> >>> Suggested-by: Chris Wilson <chris.p.wilson at intel.com>
> >>> Signed-off-by: Fei Yang <fei.yang at intel.com>
> >>> Signed-off-by: Radhakrishna Sripada <radhakrishna.sripada at intel.com>
> >>> ---
> >>>    drivers/gpu/drm/i915/display/intel_dpt.c | 4 +++-
> >>>    1 file changed, 3 insertions(+), 1 deletion(-)
> >>>
> >>> diff --git a/drivers/gpu/drm/i915/display/intel_dpt.c
> >>> b/drivers/gpu/drm/i915/display/intel_dpt.c
> >>> index 7c5fddb203ba..fbfd8f959f17 100644
> >>> --- a/drivers/gpu/drm/i915/display/intel_dpt.c
> >>> +++ b/drivers/gpu/drm/i915/display/intel_dpt.c
> >>> @@ -166,6 +166,8 @@ struct i915_vma *intel_dpt_pin(struct
> i915_address_space *vm)
> >>>               i915_vma_get(vma);
> >>>       }
> >>>
> >>> +    dpt->obj->mm.dirty = true;
> >>> +
> >>>       atomic_dec(&i915->gpu_error.pending_fb_pin);
> >>>       intel_runtime_pm_put(&i915->runtime_pm, wakeref);
> >>>
> >>> @@ -261,7 +263,7 @@ intel_dpt_create(struct intel_framebuffer *fb)
> >>>               dpt_obj = i915_gem_object_create_stolen(i915, size);
> >>>       if (IS_ERR(dpt_obj) && !HAS_LMEM(i915)) {
> >>>               drm_dbg_kms(&i915->drm, "Allocating dpt from smem\n");
> >>> -            dpt_obj = i915_gem_object_create_internal(i915, size);
> >>> +            dpt_obj = i915_gem_object_create_shmem(i915, size);
> >>>       }
> >>>       if (IS_ERR(dpt_obj))
> >>>               return ERR_CAST(dpt_obj);
> >>
> >> Okay I think I get it after some more looking at the DPT code paths.
> >> Problem seems pretty clear - page tables are stored in dpt_obj and so
> >> are lost when backing store is discarded.
> >>
> >> Changing to shmem object indeed looks the easiest option.
> >>
> >> Some related thoughts:
> >>
> >> 1)
> >> I wonder if intel_dpt_suspend/resume remain needed after this patch.
> >> Could you investigate please? On a glance their job was to restore the
> >> PTEs which would be lost from internal objects backing storage. With
> >> shmem objects that content should be preserved.
> >
> > intel_dpt_suspend is "suspending" the whole VM where, not only the dpt
> > objects are mapped into, but also the framebuffer objects. I don't have
> > much knowledge on how the framebuffer objects are managed, but the
> suspend
> > resume path still look necessary to me, unless the content of these
> > framebuffer objects are also preserved.
> 
> I don't think it has anything to do with fb content, but you are correct
> it is still needed. Because 9755f055f512 ("drm/i915: Restore memory
> mapping for DPT FBs across system suspend/resume") reminds me backing
> store for DPT PTEs can be either lmem, stolen or internal (now shmem).
> Even though with this patch internal is out of the picture, stolen
> remains and so the issue of losing the page table content remains.
> Perhaps resume could be optimised to only restore PTEs when VM page
> tables are backed by stolen which may win some suspend/resume speed on
> some platforms.

I will have to look into how suspend resume will change with the current flow
as you said it can be looked in a later patch.

> 
> Regards,
> 
> Tvrtko
> 
> >
> >> 2)
> >> I wonder if i915_vma_flush_writes should be used (as a companion of
> >> i915_vma_pin_iomap) from DPT dpt_bind_vma, dpt_insert_entries, etc. But
> >> then I am also not sure if it does the right thing for the
> >> i915_gem_object_pin_map path of i915_vma_pin_iomap. Perhaps it should
> >> call __i915_gem_object_flush_map itself for that mapping flavour and
> >> not do the ggtt flushing in that case.
I am not sure if dpt_bind_vma will be called each time during pinning. IMO it gets called
Only when the fb object needs to be bind after and unbind(triggered during obj destroy).
Do you think if i915_vma_flush_writes should not be used if dpt objects are created from internal?
Or should we have a different flavor of i915_vm_pin_iomap that skips i915_vma_set_ggtt_write
so that we can drop i915_vma_flush_writes during unpinning and move i915_vma_set_ggtt_write
to dpt_insert_entires and do i915_vma_flush during clear range? Then I guess __i915_gem_object_flush_map
called during vma bind and not object pinning. In either case I believe it is a larger cleanup
which requires more extensive validation and analysis.

> >>
> >> In summary I think the fix is safe and correct but at least point 1) I
> >> think needs looking into. It can be a follow up work too.

If you think this fix can work then I will look into the suspend/resume as a follow up and will appreciate
an r-b for this change.  I believe 2) is a larger cleanup that may not be immediately required. I will have
to dig more into the ramifications of the changes proposed above.

Thoughts ?

--Radhakrishna(RK) Sripada
> >>
> >> Regards,
> >>
> >> Tvrtko
> >>