Re: [PATCH 24/28] drm: use new iterator in drm_gem_plane_helper_prepare_fb

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 05/10/2021 11:27, Christian König wrote:
> Am 05.10.21 um 09:53 schrieb Tvrtko Ursulin:
>>
>> On 01/10/2021 11:06, Christian König wrote:
>>> Makes the handling a bit more complex, but avoids the use of
>>> dma_resv_get_excl_unlocked().
>>>
>>> Signed-off-by: Christian König <christian.koenig@amd.com>
>>> ---
>>>   drivers/gpu/drm/drm_gem_atomic_helper.c | 13 +++++++++++--
>>>   1 file changed, 11 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/drivers/gpu/drm/drm_gem_atomic_helper.c 
>>> b/drivers/gpu/drm/drm_gem_atomic_helper.c
>>> index e570398abd78..21ed930042b8 100644
>>> --- a/drivers/gpu/drm/drm_gem_atomic_helper.c
>>> +++ b/drivers/gpu/drm/drm_gem_atomic_helper.c
>>> @@ -143,6 +143,7 @@
>>>    */
>>>   int drm_gem_plane_helper_prepare_fb(struct drm_plane *plane, struct 
>>> drm_plane_state *state)
>>>   {
>>> +    struct dma_resv_iter cursor;
>>>       struct drm_gem_object *obj;
>>>       struct dma_fence *fence;
>>>   @@ -150,9 +151,17 @@ int drm_gem_plane_helper_prepare_fb(struct 
>>> drm_plane *plane, struct drm_plane_st
>>>           return 0;
>>>         obj = drm_gem_fb_get_obj(state->fb, 0);
>>> -    fence = dma_resv_get_excl_unlocked(obj->resv);
>>> -    drm_atomic_set_fence_for_plane(state, fence);
>>> +    dma_resv_iter_begin(&cursor, obj->resv, false);
>>> +    dma_resv_for_each_fence_unlocked(&cursor, fence) {
>>> +        dma_fence_get(fence);
>>> +        dma_resv_iter_end(&cursor);
>>> +        /* TODO: We only use the first write fence here */
>>
>> What is the TODO? NB instead?
> 
> The drm atomic API can unfortunately handle only one fence and we can 
> certainly have more than that.
> 
>>
>>> + drm_atomic_set_fence_for_plane(state, fence);
>>> +        return 0;
>>> +    }
>>> +    dma_resv_iter_end(&cursor);
>>>   +    drm_atomic_set_fence_for_plane(state, NULL);
>>
>>     dma_resv_iter_begin(&cursor, obj->resv, false);
>>     dma_resv_for_each_fence_unlocked(&cursor, fence) {
>>         dma_fence_get(fence);
>>         break;
>>     }
>>     dma_resv_iter_end(&cursor);
>>
>>     drm_atomic_set_fence_for_plane(state, fence);
>>
>> Does this work?
> 
> Yeah that should work as well.
> 
>>
>> But overall I am not sure this is nicer. Is the goal to remove 
>> dma_resv_get_excl_unlocked as API it just does not happen in this series?
> 
> Yes, the only user left is the i915_gem_object_last_write_engine() 
> function and that one you already removed in i915.

To me the above feels clumsier than dma_resv_get_excl_unlocked and you 
can even view it as open coding that helper. So don't know, someone else 
can have a casting vote.

I guess if support for more than one fence is coming soon(-ish) do drm 
atomic api then I could be convinced the iterator here makes sense today.

Regards,

Tvrtko

> Regards,
> Christian.
> 
>>
>> Regards,
>>
>> Tvrtko
>>
>>>       return 0;
>>>   }
>>>   EXPORT_SYMBOL_GPL(drm_gem_plane_helper_prepare_fb);
>>>
>