Re: [Intel-gfx] [PATCH 13/26] drm/i915: use the new iterator in i915_gem_busy_ioctl

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

On 20/09/2021 11:13, Christian König wrote:
> Am 20.09.21 um 10:45 schrieb Tvrtko Ursulin:
>>
>> On 17/09/2021 13:35, Christian König wrote:
>>> This makes the function much simpler since the complex
>>> retry logic is now handled else where.
>>>
>>> Signed-off-by: Christian König <christian.koenig@amd.com>
>>> ---
>>>   drivers/gpu/drm/i915/gem/i915_gem_busy.c | 32 ++++++++----------------
>>>   1 file changed, 11 insertions(+), 21 deletions(-)
>>>
>>> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_busy.c 
>>> b/drivers/gpu/drm/i915/gem/i915_gem_busy.c
>>> index 6234e17259c1..b1cb7ba688da 100644
>>> --- a/drivers/gpu/drm/i915/gem/i915_gem_busy.c
>>> +++ b/drivers/gpu/drm/i915/gem/i915_gem_busy.c
>>> @@ -82,8 +82,8 @@ i915_gem_busy_ioctl(struct drm_device *dev, void 
>>> *data,
>>>   {
>>>       struct drm_i915_gem_busy *args = data;
>>>       struct drm_i915_gem_object *obj;
>>> -    struct dma_resv_list *list;
>>> -    unsigned int seq;
>>> +    struct dma_resv_iter cursor;
>>> +    struct dma_fence *fence;
>>>       int err;
>>>         err = -ENOENT;
>>> @@ -109,27 +109,17 @@ i915_gem_busy_ioctl(struct drm_device *dev, 
>>> void *data,
>>>        * to report the overall busyness. This is what the wait-ioctl 
>>> does.
>>>        *
>>>        */
>>> -retry:
>>> -    seq = raw_read_seqcount(&obj->base.resv->seq);
>>> -
>>> -    /* Translate the exclusive fence to the READ *and* WRITE engine */
>>> -    args->busy = 
>>> busy_check_writer(dma_resv_excl_fence(obj->base.resv));
>>> -
>>> -    /* Translate shared fences to READ set of engines */
>>> -    list = dma_resv_shared_list(obj->base.resv);
>>> -    if (list) {
>>> -        unsigned int shared_count = list->shared_count, i;
>>> -
>>> -        for (i = 0; i < shared_count; ++i) {
>>> -            struct dma_fence *fence =
>>> -                rcu_dereference(list->shared[i]);
>>> -
>>> +    args->busy = false;
>>> +    dma_resv_iter_begin(&cursor, obj->base.resv, true);
>>> +    dma_resv_for_each_fence_unlocked(&cursor, fence) {
>>
>> You did not agree with my suggestion to reset args->busy on restart 
>> and so preserve current behaviour?
> 
> No, I want to keep the restart behavior internally to the dma_resv 
> object and as far as I can see it should not make a difference here.

To be clear, on paper difference between old and new implementation is 
if the restart happens while processing the shared fences.

Old implementation unconditionally goes to "args->busy =
 >>> busy_check_writer(dma_resv_excl_fence(obj->base.resv));" and so 
overwrites the set of flags returned to userspace.

New implementation can merge new read flags to the old set of flags and 
so return a composition of past and current fences.

Maybe it does not matter hugely in this case, depends if userspace 
typically just restarts until flags are clear. But I am not sure.

On the higher level - what do you mean with wanting to keep the restart 
behaviour internal? Not providing iterators users means of detecting it? 
I think it has to be provided.

Regards,

Tvrtko

> Regards,
> Christian.
> 
>>
>> Regards,
>>
>> Tvrtko
>>
>>> +        if (dma_resv_iter_is_exclusive(&cursor))
>>> +            /* Translate the exclusive fence to the READ *and* WRITE 
>>> engine */
>>> +            args->busy = busy_check_writer(fence);
>>> +        else
>>> +            /* Translate shared fences to READ set of engines */
>>>               args->busy |= busy_check_reader(fence);
>>> -        }
>>>       }
>>> -
>>> -    if (args->busy && read_seqcount_retry(&obj->base.resv->seq, seq))
>>> -        goto retry;
>>> +    dma_resv_iter_end(&cursor);
>>>         err = 0;
>>>   out:
>>>
>