[RFC] lib/st_ring: add single thread ring

[Mattias Rönnblom]

On 2023-08-22 07:43, Honnappa Nagarahalli wrote:
> 
> 
>> -----Original Message-----
>> From: Mattias Rönnblom <hofors at lysator.liu.se>
>> Sent: Monday, August 21, 2023 4:14 PM
>> To: Honnappa Nagarahalli <Honnappa.Nagarahalli at arm.com>;
>> jackmin at nvidia.com; konstantin.v.ananyev at yandex.ru
>> Cc: dev at dpdk.org; Ruifeng Wang <Ruifeng.Wang at arm.com>; Aditya
>> Ambadipudi <Aditya.Ambadipudi at arm.com>; Wathsala Wathawana
>> Vithanage <wathsala.vithanage at arm.com>; nd <nd at arm.com>
>> Subject: Re: [RFC] lib/st_ring: add single thread ring
>>
>> On 2023-08-21 08:04, Honnappa Nagarahalli wrote:
>>> Add a single thread safe and multi-thread unsafe ring data structure.
>>
>> One must have set the bar very low, if one needs to specify that an API is
>> single-thread safe.
>>
>>> This library provides an simple and efficient alternative to
>>> multi-thread safe ring when multi-thread safety is not required.
>>>
>>> Signed-off-by: Honnappa Nagarahalli <honnappa.nagarahalli at arm.com>
>>> ---
>>> v1:
>>> 1) The code is very prelimnary and is not even compiled
>>> 2) This is intended to show the APIs and some thoughts on
>>> implementation
>>
>> If you haven't done it already, maybe it might be worth looking around in the
>> code base for already-existing, more-or-less open-coded fifo/circular buffer
>> type data structures. Just to make sure those can be eliminated if this makes
>> it into DPDK.
>>
>> There's one in rte_event_eth_rx_adapter.c, and I think one in the SW
>> eventdev as well. Seems to be one in cmdline_cirbuf.h as well. I'm sure there
>> are many more.
> I knew there are some, but have not looked at them yet. I will look at them.
> 
>>
>> You could pick some other name for it, instead of the slightly awkward
>> "st_ring" (e.g., "fifo", "cbuf", "cbuffer", "circ_buffer"). That would also leave
>> you with more freedom to stray from the MT safe ring API without surprising
>> the user, if needed (and I think it is needed).
> The thought was to make it clear that this is for single-thread use (i.e.not even producer and consumer on different threads), may be I do not need to try hard.
> "fifo" might not be good option given that dequeue/enqueue at both ends of the ring are required/allowed.
> Wikipedia [1] and others [2], [3] indicates that this data structure should be called 'deque' (pronounced as deck). I would prefer to go with this (assuming this will be outside of 'rte_ring')
> 
> [1] https://en.wikipedia.org/wiki/Double-ended_queue
> [2] https://www.geeksforgeeks.org/deque-set-1-introduction-applications/
> [3] https://stackoverflow.com/questions/3880254/why-do-we-need-deque-data-structures-in-the-real-world#:~:text=A%20Deque%20is%20a%20double,thing%20on%20front%20of%20queue.
> 
>>
>> Hopefully you can reduce API complexity compared to the MT-safe version.
>> Having a name for these kinds of data structures doesn't make a lot of sense,
>> for example. Skip the dump function. Relax from always_inline to just regular
>> inline.
> Yes, plan is to reduce complexity (compared to rte_ring) and some APIs can be skipped until there is a need.
> 
>>
>> I'm not sure you need bulk/burst type operations. Without any memory
>> fences, an optimizing compiler should do a pretty good job of unrolling
>> multiple-element access type operations, assuming you leave the ST ring
>> code in the header files (otherwise LTO is needed).
> IMO, bulk/burst APIs are about the functionality rather than loop unrolling. APIs to work with single objects can be skipped (use bulk APIs with n=1).
> 

Given that this data structure will often be use in conjunction with 
other burst/bulk type operations, I agree.

What about peek? I guess you could have a burst/bulk peek as well, would 
that operation be needed. I think it will be needed, but the 
introduction of such API elements could always be deferred.

>>
>> I think you will want a peek-type operation on the reader side. That more for
>> convenience, rather than that I think the copies will actually be there in the
>> object code (such should be eliminated by the compiler, given that the
>> barriers are gone).
>>
>>> 3) More APIs and the rest of the implementation will come in subsequent
>>>      versions
>>>
>>>    lib/st_ring/rte_st_ring.h | 567
>> ++++++++++++++++++++++++++++++++++++++
>>>    1 file changed, 567 insertions(+)
>>>    create mode 100644 lib/st_ring/rte_st_ring.h
>>>
>>> diff --git a/lib/st_ring/rte_st_ring.h b/lib/st_ring/rte_st_ring.h new
>>> file mode 100644 index 0000000000..8cb8832591
>>> --- /dev/null
>>> +++ b/lib/st_ring/rte_st_ring.h
>>> @@ -0,0 +1,567 @@
>>> +/* SPDX-License-Identifier: BSD-3-Clause
>>> + * Copyright(c) 2023 Arm Limited
>>> + */
>>> +
>>> +#ifndef _RTE_ST_RING_H_
>>> +#define _RTE_ST_RING_H_
>>> +
>>> +/**
>>> + * @file
>>> + * RTE Signle Thread Ring (ST Ring)
>>> + *
>>> + * The ST Ring is a fixed-size queue intended to be accessed
>>> + * by one thread at a time. It does not provide concurrent access to
>>> + * multiple threads. If there are multiple threads accessing the ST
>>> +ring,
>>> + * then the threads have to use locks to protect the ring from
>>> + * getting corrupted.
>>
>> You are basically saying the same thing three times here.
>>
>>> + *
>>> + * - FIFO (First In First Out)
>>> + * - Maximum size is fixed; the pointers are stored in a table.
>>> + * - Consumer and producer part of same thread.
>>> + * - Multi-thread producers and consumers need locking.
>>
>> ...two more times here. One might get the impression you really don't trust
>> the reader.
>>
>>> + * - Single/Bulk/burst dequeue at Tail or Head
>>> + * - Single/Bulk/burst enqueue at Head or Tail
>>
>> Does this not sound more like a deque, than a FIFO/circular buffer? Are there
>> any examples where this functionality (the double-endedness) is needed in
>> the DPDK code base?
> I see, you are calling it 'deque' as well. Basically, this patch originated due to a requirement in MLX PMD [1]
> 
> [1] https://github.com/DPDK/dpdk/blob/main/drivers/net/mlx5/mlx5_hws_cnt.h#L381
> 
>>
>>> + *
>>> + */
>>> +
>>> +#ifdef __cplusplus
>>> +extern "C" {
>>> +#endif
>>> +
>>> +#include <rte_st_ring_core.h>
>>> +#include <rte_st_ring_elem.h>
>>
>> Is the intention to provide a ring with compile-time variable element size? In
>> other words, where the elements of a particular ring instance has the same
>> element size, but different rings may have different element sizes.
>>
>> Seems like a good idea to me, in that case. Although often you will have
>> pointers, it would be useful to store larger things like small structs, and
>> maybe smaller elements as well.
> Yes, the idea is to make the element size flexible and also compile-time constant.
> 
>>
>>> +
>>> +/**
>>> + * Calculate the memory size needed for a ST ring
>>> + *
>>> + * This function returns the number of bytes needed for a ST ring,
>>> +given
>>> + * the number of elements in it. This value is the sum of the size of
>>> + * the structure rte_st_ring and the size of the memory needed by the
>>> + * elements. The value is aligned to a cache line size.
>>> + *
>>> + * @param count
>>> + *   The number of elements in the ring (must be a power of 2).
>>> + * @return
>>> + *   - The memory size needed for the ST ring on success.
>>> + *   - -EINVAL if count is not a power of 2.
>>> + */
>>> +ssize_t rte_st_ring_get_memsize(unsigned int count);
>>> +
>>> +/**
>>> + * Initialize a ST ring structure.
>>> + *
>>> + * Initialize a ST ring structure in memory pointed by "r". The size
>>> +of the
>>> + * memory area must be large enough to store the ring structure and
>>> +the
>>> + * object table. It is advised to use rte_st_ring_get_memsize() to
>>> +get the
>>> + * appropriate size.
>>> + *
>>> + * The ST ring size is set to *count*, which must be a power of two.
>>> + * The real usable ring size is *count-1* instead of *count* to
>>> + * differentiate a full ring from an empty ring.
>>> + *
>>> + * The ring is not added in RTE_TAILQ_ST_RING global list. Indeed,
>>> +the
>>> + * memory given by the caller may not be shareable among dpdk
>>> + * processes.
>>> + *
>>> + * @param r
>>> + *   The pointer to the ring structure followed by the elements table.
>>> + * @param name
>>> + *   The name of the ring.
>>> + * @param count
>>> + *   The number of elements in the ring (must be a power of 2,
>>> + *   unless RTE_ST_RING_F_EXACT_SZ is set in flags).
>>> + * @param flags
>>> + *   An OR of the following:
>>> + *   - RTE_ST_RING_F_EXACT_SZ: If this flag is set, the ring will hold
>>> + *     exactly the requested number of entries, and the requested size
>>> + *     will be rounded up to the next power of two, but the usable space
>>> + *     will be exactly that requested. Worst case, if a power-of-2 size is
>>> + *     requested, half the ring space will be wasted.
>>> + *     Without this flag set, the ring size requested must be a power of 2,
>>> + *     and the usable space will be that size - 1.
>>> + * @return
>>> + *   0 on success, or a negative value on error.
>>> + */
>>> +int rte_st_ring_init(struct rte_st_ring *r, const char *name,
>>> +	unsigned int count, unsigned int flags);
>>> +
>>> +/**
>>> + * Create a new ST ring named *name* in memory.
>>> + *
>>> + * This function uses ``memzone_reserve()`` to allocate memory. Then
>>> +it
>>> + * calls rte_st_ring_init() to initialize an empty ring.
>>> + *
>>> + * The new ring size is set to *count*, which must be a power of two.
>>> + * The real usable ring size is *count-1* instead of *count* to
>>> + * differentiate a full ring from an empty ring.
>>> + *
>>> + * The ring is added in RTE_TAILQ_ST_RING list.
>>> + *
>>> + * @param name
>>> + *   The name of the ring.
>>> + * @param count
>>> + *   The size of the ring (must be a power of 2,
>>> + *   unless RTE_ST_RING_F_EXACT_SZ is set in flags).
>>> + * @param socket_id
>>> + *   The *socket_id* argument is the socket identifier in case of
>>> + *   NUMA. The value can be *SOCKET_ID_ANY* if there is no NUMA
>>> + *   constraint for the reserved zone.
>>> + * @param flags
>>> + *   - RTE_ST_RING_F_EXACT_SZ: If this flag is set, the ring will hold exactly
>> the
>>> + *     requested number of entries, and the requested size will be rounded
>> up
>>> + *     to the next power of two, but the usable space will be exactly that
>>> + *     requested. Worst case, if a power-of-2 size is requested, half the
>>> + *     ring space will be wasted.
>>> + *     Without this flag set, the ring size requested must be a power of 2,
>>> + *     and the usable space will be that size - 1.
>>> + * @return
>>> + *   On success, the pointer to the new allocated ring. NULL on error with
>>> + *    rte_errno set appropriately. Possible errno values include:
>>> + *    - E_RTE_NO_CONFIG - function could not get pointer to rte_config
>> structure
>>> + *    - EINVAL - count provided is not a power of 2
>>> + *    - ENOSPC - the maximum number of memzones has already been
>> allocated
>>> + *    - EEXIST - a memzone with the same name already exists
>>> + *    - ENOMEM - no appropriate memory area found in which to create
>> memzone
>>> + */
>>> +struct rte_st_ring *rte_st_ring_create(const char *name, unsigned int
>> count,
>>> +				 int socket_id, unsigned int flags);
>>> +
>>> +/**
>>> + * De-allocate all memory used by the ring.
>>> + *
>>> + * @param r
>>> + *   Ring to free.
>>> + *   If NULL then, the function does nothing.
>>> + */
>>> +void rte_st_ring_free(struct rte_st_ring *r);
>>> +
>>> +/**
>>> + * Dump the status of the ring to a file.
>>> + *
>>> + * @param f
>>> + *   A pointer to a file for output
>>> + * @param r
>>> + *   A pointer to the ring structure.
>>> + */
>>> +void rte_st_ring_dump(FILE *f, const struct rte_st_ring *r);
>>> +
>>> +/**
>>> + * Enqueue fixed number of objects on a ST ring.
>>> + *
>>> + * This function copies the objects at the head of the ring and
>>> + * moves the head index.
>>> + *
>>> + * @param r
>>> + *   A pointer to the ring structure.
>>> + * @param obj_table
>>> + *   A pointer to a table of void * pointers (objects).
>>> + * @param n
>>> + *   The number of objects to add in the ring from the obj_table.
>>> + * @param free_space
>>> + *   if non-NULL, returns the amount of space in the ring after the
>>> + *   enqueue operation has finished.
>>> + * @return
>>> + *   The number of objects enqueued, either 0 or n
>>> + */
>>> +static __rte_always_inline unsigned int
>>> +rte_st_ring_enqueue_bulk(struct rte_st_ring *r, void * const *obj_table,
>>> +		      unsigned int n, unsigned int *free_space) {
>>> +	return rte_st_ring_enqueue_bulk_elem(r, obj_table, sizeof(void *),
>>> +			n, free_space);
>>> +}
>>> +
>>> +/**
>>> + * Enqueue upto a maximum number of objects on a ST ring.
>>> + *
>>> + * This function copies the objects at the head of the ring and
>>> + * moves the head index.
>>> + *
>>> + * @param r
>>> + *   A pointer to the ring structure.
>>> + * @param obj_table
>>> + *   A pointer to a table of void * pointers (objects).
>>> + * @param n
>>> + *   The number of objects to add in the ring from the obj_table.
>>> + * @param free_space
>>> + *   if non-NULL, returns the amount of space in the ring after the
>>> + *   enqueue operation has finished.
>>> + * @return
>>> + *   - n: Actual number of objects enqueued.
>>> + */
>>> +static __rte_always_inline unsigned int
>>> +rte_st_ring_enqueue_burst(struct rte_st_ring *r, void * const *obj_table,
>>> +		      unsigned int n, unsigned int *free_space) {
>>> +	return rte_st_ring_enqueue_burst_elem(r, obj_table, sizeof(void *),
>>> +			n, free_space);
>>> +}
>>> +
>>> +/**
>>> + * Enqueue one object on a ST ring.
>>> + *
>>> + * This function copies one object at the head of the ring and
>>> + * moves the head index.
>>> + *
>>> + * @param r
>>> + *   A pointer to the ring structure.
>>> + * @param obj
>>> + *   A pointer to the object to be added.
>>> + * @return
>>> + *   - 0: Success; objects enqueued.
>>> + *   - -ENOBUFS: Not enough room in the ring to enqueue; no object is
>> enqueued.
>>> + */
>>> +static __rte_always_inline int
>>> +rte_st_ring_enqueue(struct rte_st_ring *r, void *obj) {
>>> +	return rte_st_ring_enqueue_elem(r, &obj, sizeof(void *)); }
>>> +
>>> +/**
>>> + * Enqueue fixed number of objects on a ST ring at the tail.
>>> + *
>>> + * This function copies the objects at the tail of the ring and
>>> + * moves the tail index (backwards).
>>> + *
>>> + * @param r
>>> + *   A pointer to the ring structure.
>>> + * @param obj_table
>>> + *   A pointer to a table of void * pointers (objects).
>>> + * @param n
>>> + *   The number of objects to add in the ring from the obj_table.
>>> + * @param free_space
>>> + *   if non-NULL, returns the amount of space in the ring after the
>>> + *   enqueue operation has finished.
>>> + * @return
>>> + *   The number of objects enqueued, either 0 or n
>>> + */
>>> +static __rte_always_inline unsigned int
>>> +rte_st_ring_enqueue_at_tail_bulk(struct rte_st_ring *r,
>>> +				 void * const *obj_table, unsigned int n,
>>> +				 unsigned int *free_space)
>>> +{
>>> +	return rte_st_ring_enqueue_at_tail_bulk_elem(r, obj_table,
>>> +			sizeof(void *), n, free_space);
>>> +}
>>> +
>>> +/**
>>> + * Enqueue upto a maximum number of objects on a ST ring at the tail.
>>> + *
>>> + * This function copies the objects at the tail of the ring and
>>> + * moves the tail index (backwards).
>>> + *
>>> + * @param r
>>> + *   A pointer to the ring structure.
>>> + * @param obj_table
>>> + *   A pointer to a table of void * pointers (objects).
>>> + * @param n
>>> + *   The number of objects to add in the ring from the obj_table.
>>> + * @param free_space
>>> + *   if non-NULL, returns the amount of space in the ring after the
>>> + *   enqueue operation has finished.
>>> + * @return
>>> + *   - n: Actual number of objects enqueued.
>>> + */
>>> +static __rte_always_inline unsigned int
>>> +rte_st_ring_enqueue_at_tail_burst(struct rte_st_ring *r,
>>> +				  void * const *obj_table, unsigned int n,
>>> +				  unsigned int *free_space)
>>> +{
>>> +	return rte_st_ring_enqueue_at_tail_burst_elem(r, obj_table,
>>> +			sizeof(void *), n, free_space);
>>> +}
>>> +
>>> +/**
>>> + * Enqueue one object on a ST ring at tail.
>>> + *
>>> + * This function copies one object at the tail of the ring and
>>> + * moves the tail index (backwards).
>>> + *
>>> + * @param r
>>> + *   A pointer to the ring structure.
>>> + * @param obj
>>> + *   A pointer to the object to be added.
>>> + * @return
>>> + *   - 0: Success; objects enqueued.
>>> + *   - -ENOBUFS: Not enough room in the ring to enqueue; no object is
>> enqueued.
>>> + */
>>> +static __rte_always_inline int
>>> +rte_st_ring_enqueue_at_tail(struct rte_st_ring *r, void *obj) {
>>> +	return rte_st_ring_enqueue_at_tail_elem(r, &obj, sizeof(void *)); }
>>> +
>>> +/**
>>> + * Dequeue a fixed number of objects from a ST ring.
>>> + *
>>> + * This function copies the objects from the tail of the ring and
>>> + * moves the tail index.
>>> + *
>>> + * @param r
>>> + *   A pointer to the ring structure.
>>> + * @param obj_table
>>> + *   A pointer to a table of void * pointers (objects) that will be filled.
>>> + * @param n
>>> + *   The number of objects to dequeue from the ring to the obj_table.
>>> + * @param available
>>> + *   If non-NULL, returns the number of remaining ring entries after the
>>> + *   dequeue has finished.
>>> + * @return
>>> + *   The number of objects dequeued, either 0 or n
>>> + */
>>> +static __rte_always_inline unsigned int
>>> +rte_st_ring_dequeue_bulk(struct rte_st_ring *r, void **obj_table,
>> unsigned int n,
>>> +		unsigned int *available)
>>> +{
>>> +	return rte_st_ring_dequeue_bulk_elem(r, obj_table, sizeof(void *),
>>> +			n, available);
>>> +}
>>> +
>>> +/**
>>> + * Dequeue upto a maximum number of objects from a ST ring.
>>> + *
>>> + * This function copies the objects from the tail of the ring and
>>> + * moves the tail index.
>>> + *
>>> + * @param r
>>> + *   A pointer to the ring structure.
>>> + * @param obj_table
>>> + *   A pointer to a table of void * pointers (objects) that will be filled.
>>> + * @param n
>>> + *   The number of objects to dequeue from the ring to the obj_table.
>>> + * @param available
>>> + *   If non-NULL, returns the number of remaining ring entries after the
>>> + *   dequeue has finished.
>>> + * @return
>>> + *   - Number of objects dequeued
>>> + */
>>> +static __rte_always_inline unsigned int
>>> +rte_st_ring_dequeue_burst(struct rte_st_ring *r, void **obj_table,
>>> +		unsigned int n, unsigned int *available) {
>>> +	return rte_st_ring_dequeue_burst_elem(r, obj_table, sizeof(void *),
>>> +			n, available);
>>> +}
>>> +
>>> +/**
>>> + * Dequeue one object from a ST ring.
>>> + *
>>> + * This function copies one object from the tail of the ring and
>>> + * moves the tail index.
>>> + *
>>> + * @param r
>>> + *   A pointer to the ring structure.
>>> + * @param obj_p
>>> + *   A pointer to a void * pointer (object) that will be filled.
>>> + * @return
>>> + *   - 0: Success, objects dequeued.
>>> + *   - -ENOENT: Not enough entries in the ring to dequeue, no object is
>>> + *     dequeued.
>>> + */
>>> +static __rte_always_inline int
>>> +rte_st_ring_dequeue(struct rte_st_ring *r, void **obj_p) {
>>> +	return rte_st_ring_dequeue_elem(r, obj_p, sizeof(void *)); }
>>> +
>>> +/**
>>> + * Dequeue a fixed number of objects from a ST ring from the head.
>>> + *
>>> + * This function copies the objects from the head of the ring and
>>> + * moves the head index (backwards).
>>> + *
>>> + * @param r
>>> + *   A pointer to the ring structure.
>>> + * @param obj_table
>>> + *   A pointer to a table of void * pointers (objects) that will be filled.
>>> + * @param n
>>> + *   The number of objects to dequeue from the ring to the obj_table.
>>> + * @param available
>>> + *   If non-NULL, returns the number of remaining ring entries after the
>>> + *   dequeue has finished.
>>> + * @return
>>> + *   The number of objects dequeued, either 0 or n
>>> + */
>>> +static __rte_always_inline unsigned int
>>> +rte_st_ring_dequeue_at_head_bulk(struct rte_st_ring *r, void
>> **obj_table, unsigned int n,
>>> +		unsigned int *available)
>>> +{
>>> +	return rte_st_ring_dequeue_bulk_elem(r, obj_table, sizeof(void *),
>>> +			n, available);
>>> +}
>>> +
>>> +/**
>>> + * Dequeue upto a maximum number of objects from a ST ring from the
>> head.
>>> + *
>>> + * This function copies the objects from the head of the ring and
>>> + * moves the head index (backwards).
>>> + *
>>> + * @param r
>>> + *   A pointer to the ring structure.
>>> + * @param obj_table
>>> + *   A pointer to a table of void * pointers (objects) that will be filled.
>>> + * @param n
>>> + *   The number of objects to dequeue from the ring to the obj_table.
>>> + * @param available
>>> + *   If non-NULL, returns the number of remaining ring entries after the
>>> + *   dequeue has finished.
>>> + * @return
>>> + *   - Number of objects dequeued
>>> + */
>>> +static __rte_always_inline unsigned int
>>> +rte_st_ring_dequeue_at_head_burst(struct rte_st_ring *r, void
>> **obj_table,
>>> +		unsigned int n, unsigned int *available) {
>>> +	return rte_st_ring_dequeue_burst_elem(r, obj_table, sizeof(void *),
>>> +			n, available);
>>> +}
>>> +
>>> +/**
>>> + * Dequeue one object from a ST ring from the head.
>>> + *
>>> + * This function copies the objects from the head of the ring and
>>> + * moves the head index (backwards).
>>> + *
>>> + * @param r
>>> + *   A pointer to the ring structure.
>>> + * @param obj_p
>>> + *   A pointer to a void * pointer (object) that will be filled.
>>> + * @return
>>> + *   - 0: Success, objects dequeued.
>>> + *   - -ENOENT: Not enough entries in the ring to dequeue, no object is
>>> + *     dequeued.
>>> + */
>>> +static __rte_always_inline int
>>> +rte_st_ring_at_head_dequeue(struct rte_st_ring *r, void **obj_p) {
>>> +	return rte_st_ring_dequeue_elem(r, obj_p, sizeof(void *)); }
>>> +
>>> +/**
>>> + * Flush a ST ring.
>>> + *
>>> + * This function flush all the elements in a ST ring
>>> + *
>>> + * @warning
>>> + * Make sure the ring is not in use while calling this function.
>>> + *
>>> + * @param r
>>> + *   A pointer to the ring structure.
>>> + */
>>> +void
>>> +rte_st_ring_reset(struct rte_st_ring *r);
>>> +
>>> +/**
>>> + * Return the number of entries in a ST ring.
>>> + *
>>> + * @param r
>>> + *   A pointer to the ring structure.
>>> + * @return
>>> + *   The number of entries in the ring.
>>> + */
>>> +static inline unsigned int
>>> +rte_st_ring_count(const struct rte_st_ring *r) {
>>> +	uint32_t count = (r->head - r->tail) & r->mask;
>>> +	return count;
>>> +}
>>> +
>>> +/**
>>> + * Return the number of free entries in a ST ring.
>>> + *
>>> + * @param r
>>> + *   A pointer to the ring structure.
>>> + * @return
>>> + *   The number of free entries in the ring.
>>> + */
>>> +static inline unsigned int
>>> +rte_st_ring_free_count(const struct rte_st_ring *r) {
>>> +	return r->capacity - rte_st_ring_count(r); }
>>> +
>>> +/**
>>> + * Test if a ST ring is full.
>>> + *
>>> + * @param r
>>> + *   A pointer to the ring structure.
>>> + * @return
>>> + *   - 1: The ring is full.
>>> + *   - 0: The ring is not full.
>>> + */
>>> +static inline int
>>> +rte_st_ring_full(const struct rte_st_ring *r) {
>>> +	return rte_st_ring_free_count(r) == 0; }
>>> +
>>> +/**
>>> + * Test if a ST ring is empty.
>>> + *
>>> + * @param r
>>> + *   A pointer to the ring structure.
>>> + * @return
>>> + *   - 1: The ring is empty.
>>> + *   - 0: The ring is not empty.
>>> + */
>>> +static inline int
>>> +rte_st_ring_empty(const struct rte_st_ring *r) {
>>> +	return r->tail == r->head;
>>> +}
>>> +
>>> +/**
>>> + * Return the size of the ring.
>>> + *
>>> + * @param r
>>> + *   A pointer to the ring structure.
>>> + * @return
>>> + *   The size of the data store used by the ring.
>>> + *   NOTE: this is not the same as the usable space in the ring. To query that
>>> + *   use ``rte_st_ring_get_capacity()``.
>>> + */
>>> +static inline unsigned int
>>> +rte_st_ring_get_size(const struct rte_st_ring *r) {
>>> +	return r->size;
>>> +}
>>> +
>>> +/**
>>> + * Return the number of elements which can be stored in the ring.
>>> + *
>>> + * @param r
>>> + *   A pointer to the ring structure.
>>> + * @return
>>> + *   The usable size of the ring.
>>> + */
>>> +static inline unsigned int
>>> +rte_st_ring_get_capacity(const struct rte_st_ring *r) {
>>> +	return r->capacity;
>>> +}
>>> +
>>> +/**
>>> + * Dump the status of all rings on the console
>>> + *
>>> + * @param f
>>> + *   A pointer to a file for output
>>> + */
>>> +void rte_st_ring_list_dump(FILE *f);
>>> +
>>> +/**
>>> + * Search a ST ring from its name
>>> + *
>>> + * @param name
>>> + *   The name of the ring.
>>> + * @return
>>> + *   The pointer to the ring matching the name, or NULL if not found,
>>> + *   with rte_errno set appropriately. Possible rte_errno values include:
>>> + *    - ENOENT - required entry not available to return.
>>> + */
>>> +struct rte_st_ring *rte_st_ring_lookup(const char *name);
>>> +
>>> +#ifdef __cplusplus
>>> +}
>>> +#endif
>>> +
>>> +#endif /* _RTE_ST_RING_H_ */