Bill Fischofer(Bill-Fischofer-Linaro) replied on github web page:
platform/linux-generic/odp_queue.c
line 92
@@ -143,8 +150,10 @@ static int queue_capability(odp_queue_capability_t *capa)
capa->max_sched_groups = sched_fn->num_grps();
capa->sched_prios = odp_schedule_num_prio();
capa->plain.max_num = capa->max_queues;
+ capa->plain.max_size = CONFIG_QUEUE_SIZE;
Comment:
As noted earlier, due to "losing" one entry to distinguish queue empty/full,
this should be returned as `CONFIG_QUEUE_SIZE - 1`, and we also need to ensure
that `CONFIG_QUEUE_SIZE` is itself a power of 2.
> Bill Fischofer(Bill-Fischofer-Linaro) wrote:
> Since you're initializing `index.pool` and `index.buffer` there's no need to
> set `index.u32` here.
>> Bill Fischofer(Bill-Fischofer-Linaro) wrote:
>> We originally had this index partitioning based on `ODP_CONFIG_POOLS`. Do we
>> want to return to that here?
>>
>> If not, we at least need an `ODP_STATIC_ASSERT()` to ensure that
>> `ODP_CONFIG_POOLS < 256` or else bad things will happen here.
>>> Bill Fischofer(Bill-Fischofer-Linaro) wrote:
>>> This routine can be optimized to:
>>> ```
>>> return ring->head == ring->tail;
>>> ```
>>>> Bill Fischofer(Bill-Fischofer-Linaro) wrote:
>>>> Your invariant is the queue is empty when `head == tail` therefore the
>>>> queue is full when `abs(tail - head) == mask`, so the correct calculation
>>>> here is:
>>>>
>>>> `num = mask - abs(tail - head);`
>>>>
>>>> The effect is that a queue can only hold `size - 1` elements, otherwise
>>>> you cannot distinguish between a full and an empty queue without another
>>>> bit of metadata, which is a cost you're trying to avoid.
>>>>
>>>> This is somewhat problematic if the caller is trying to be "optimal" by
>>>> specifying a power of two in the `size` parameter of the
>>>> `odp_queue_param_t` passed to `odp_queue_create()`. For this reason we may
>>>> wish to return a `max_size` of a power of 2 - 1 in
>>>> `odp_queue_capability()` as part of this patch series.
>>>>> Bill Fischofer(Bill-Fischofer-Linaro) wrote:
>>>>> This only works if `size` is a power of 2. Should be documented as such,
>>>>> since this is an internal routine. In this case an `ODP_ASSERT(size ==
>>>>> ROUNDUP_POWER2_U32(size))` for this requirement would be a useful
>>>>> debugging aid.
>>>>>> Bill Fischofer(Bill-Fischofer-Linaro) wrote:
>>>>>> should be `num = abs(tail - head);` to deal with wrap arounds, otherwise
>>>>>> may be misinterpreted as overly large since it's `uint32_t`. Note that
>>>>>> GCC and clang recognize `abs()` and treat it as a builtin, so there's no
>>>>>> actual `stdlib.h` call here.
>>>>>>> Bill Fischofer(Bill-Fischofer-Linaro) wrote:
>>>>>>> Extra blank line should be removed (nit).
https://github.com/Linaro/odp/pull/492#discussion_r169821543
updated_at 2018-02-22 09:32:40
