[PATCH] drm/amdgpu:guarantee 128dws between vm flush and IB(v3)

[Grazvydas Ignotas]

On Thu, Jan 19, 2017 at 4:32 PM, Christian König
<deathsimple at vodafone.de> wrote:
> Am 19.01.2017 um 14:51 schrieb Grazvydas Ignotas:
>>
>> On Thu, Jan 19, 2017 at 11:10 AM, Christian König
>> <deathsimple at vodafone.de> wrote:
>>>
>>> Am 18.01.2017 um 12:42 schrieb Monk Liu:
>>>>
>>>> @@ -6743,6 +6741,15 @@ static void gfx_v8_ring_emit_cntxcntl(struct
>>>> amdgpu_ring *ring, uint32_t flags)
>>>>          if (amdgpu_sriov_vf(ring->adev))
>>>>                  gfx_v8_0_ring_emit_de_meta_init(ring,
>>>>                          (flags & AMDGPU_VM_DOMAIN) ? AMDGPU_CSA_VADDR :
>>>> ring->adev->virt.csa_vmid0_addr);
>>>> +
>>>> +       /* We need to pad some NOPs before emit_ib to prevent CE run
>>>> ahead
>>>> of
>>>> +        * vm_flush, which may trigger VM fault. */
>>>> +       if (ring->wptr > ring->last_vm_flush_pos) /* no wptr wrapping to
>>>> RB head */
>>>> +               amdgpu_ring_insert_nop(ring, 128 - (ring->wptr -
>>>> ring->last_vm_flush_pos));
>>>
>>>
>>> This can easily result in a negative number, couldn't it?
>>>
>>>> +       else
>>>> +               if (ring->ptr_mask + 1 - ring->last_vm_flush_pos +
>>>> ring->wptr < 128)
>>>> +                       amdgpu_ring_insert_nop(ring,
>>>> +                               128 - (ring->ptr_mask + 1 -
>>>> ring->last_vm_flush_pos + ring->wptr));
>>>
>>>
>>> I think it would be cleaner if you calculate the number of NOPs needed
>>> first
>>> for both cases and then check if the number isn't negative for both
>>> cases.
>>
>> What about this:
>> 128 - ((ring->wptr - ring->last_vm_flush_pos) & 127)
>
>
> That won't handle the case for negative nop count correctly either.
>
> See when we already emitted more than 128 dw we don't want to add some more.

Let me try again then:

count_added = (ring->wptr - ring->last_vm_flush_pos) & ring->ptr_mask;
if (count_added < 128)
    amdgpu_ring_insert_nop(ring, 128 - count_added);

Gražvydas