[PATCH 2/5] drm/amdgpu/sdma4: set wptr shadow atomically

[Alex Deucher]

On Thu, Jul 27, 2017 at 4:39 PM, Felix Kuehling <felix.kuehling at amd.com> wrote:
>
> On 17-07-27 03:46 PM, Alex Deucher wrote:
>> No functional change until wptr polling uses this
>> location (future patch).
>>
>> Cc: Frank Min <Frank.Min at amd.com>
>> Signed-off-by: Alex Deucher <alexander.deucher at amd.com>
>> ---
>>  drivers/gpu/drm/amd/amdgpu/sdma_v4_0.c | 4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/amd/amdgpu/sdma_v4_0.c b/drivers/gpu/drm/amd/amdgpu/sdma_v4_0.c
>> index 7cb5320..9392799 100644
>> --- a/drivers/gpu/drm/amd/amdgpu/sdma_v4_0.c
>> +++ b/drivers/gpu/drm/amd/amdgpu/sdma_v4_0.c
>> @@ -301,8 +301,8 @@ static void sdma_v4_0_ring_set_wptr(struct amdgpu_ring *ring)
>>                               lower_32_bits(ring->wptr << 2),
>>                               upper_32_bits(ring->wptr << 2));
>>               /* XXX check if swapping is necessary on BE */
>> -             adev->wb.wb[ring->wptr_offs] = lower_32_bits(ring->wptr << 2);
>> -             adev->wb.wb[ring->wptr_offs + 1] = upper_32_bits(ring->wptr << 2);
>> +             atomic64_set((atomic64_t *)&adev->wb.wb[ring->wptr_offs],
>> +                          (ring->wptr << 2));
>
> This looks a bit awkward. Would "writeq" do the job? That's what we use
> for updating kernel queue doorbells in KFD.

Probably.  We just want to make sure the whole 64 bit value is updated
before the GPU polls the value.

Alex

>
> The generic implementation of atomic64_set uses a spinlock. But that
> doesn't do anything for protecting against concurrent access by the GPU
> hardware.
>
> Regards,
>   Felix
>
>>               DRM_DEBUG("calling WDOORBELL64(0x%08x, 0x%016llx)\n",
>>                               ring->doorbell_index, ring->wptr << 2);
>>
>
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