[PATCH] drm/amdgpu: Attach exclusive fence to prime exported bo's. (v3)

[Christian König]

Am 07.11.2016 um 04:29 schrieb Michel Dänzer:
> On 07/11/16 11:47 AM, Mario Kleiner wrote:
>> External clients which import our bo's wait only
>> for exclusive dmabuf-fences, not on shared ones,
>> so attach fences on exported buffers as exclusive
>> ones, if the buffers get imported by an external
>> client.
>>
>> See discussion in thread:
>> https://lists.freedesktop.org/archives/dri-devel/2016-October/122370.html
>>
>> Tested on Intel iGPU + AMD Tonga dGPU as DRI3/Present
>> Prime render offload, and with the Tonga standalone as
>> primary gpu.
>>
>> v2: Add a wait for all shared fences before prime export,
>>      as suggested by Christian Koenig.
>>
>> v3: - Mark buffer prime_exported in amdgpu_gem_prime_pin,
>>      so we only use the exclusive fence when exporting a
>>      bo to external clients like a separate iGPU, but not
>>      when exporting/importing from/to ourselves as part of
>>      regular DRI3 fd passing.
>>
>>      - Propagate failure of reservation_object_wait_rcu back
>>      to caller.
>>
>> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=95472
>> (v1) Tested-by: Mike Lothian <mike at fireburn.co.uk>
> FWIW, I think the (v1) is usually added at the end of the line, not at
> the beginning.
>
> [...]
>
>> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_prime.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_prime.c
>> index 7700dc2..c4494d0 100644
>> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_prime.c
>> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_prime.c
>> @@ -81,6 +81,20 @@ int amdgpu_gem_prime_pin(struct drm_gem_object *obj)
>>   {
>>   	struct amdgpu_bo *bo = gem_to_amdgpu_bo(obj);
>>   	int ret = 0;
>> +	long lret;
>> +
>> +	/*
>> +	 * Wait for all shared fences to complete before we switch to future
>> +	 * use of exclusive fence on this prime_exported bo.
>> +	 */
>> +	lret = reservation_object_wait_timeout_rcu(bo->tbo.resv, true, false,
>> +						   MAX_SCHEDULE_TIMEOUT);
>> +	if (unlikely(lret < 0)) {
>> +		DRM_DEBUG_PRIME("Fence wait failed: %li\n", lret);
>> +		return lret;
>> +	}
>> +
>> +	bo->prime_exported = true;
> We should probably clear bo->prime_exported in amdgpu_gem_prime_unpin.
>
> Also, I think we should set bo->prime_exported (prime_shared?) in
> amdgpu_gem_prime_import_sg_table as well, so that we'll also set
> exclusive fences on BOs imported from other GPUs.
Yes, really good idea.

Additional to that are we sure that amdgpu_gem_prime_pin() is only 
called once for each GEM object? Could in theory be that somebody 
exports a BO to another GFX device as well as a V4L device for example.

In this case we would need a counter, but I need to double check that as 
well.

In general I would protected the flag/counter by the BO being reserved 
to avoid any races. In other word move those lines a bit down after the 
amdgpu_bo_reserve().

Regards,
Christian.