1.. SPDX-License-Identifier: GPL-2.0
2
3=============
4Page Pool API
5=============
6
7The page_pool allocator is optimized for the XDP mode that uses one frame
8per-page, but it can fallback on the regular page allocator APIs.
9
10Basic use involves replacing alloc_pages() calls with the
11page_pool_alloc_pages() call.  Drivers should use page_pool_dev_alloc_pages()
12replacing dev_alloc_pages().
13
14API keeps track of inflight pages, in order to let API user know
15when it is safe to free a page_pool object.  Thus, API users
16must run page_pool_release_page() when a page is leaving the page_pool or
17call page_pool_put_page() where appropriate in order to maintain correct
18accounting.
19
20API user must call page_pool_put_page() once on a page, as it
21will either recycle the page, or in case of refcnt > 1, it will
22release the DMA mapping and inflight state accounting.
23
24Architecture overview
25=====================
26
27.. code-block:: none
28
29    +------------------+
30    |       Driver     |
31    +------------------+
32            ^
33            |
34            |
35            |
36            v
37    +--------------------------------------------+
38    |                request memory              |
39    +--------------------------------------------+
40        ^                                  ^
41        |                                  |
42        | Pool empty                       | Pool has entries
43        |                                  |
44        v                                  v
45    +-----------------------+     +------------------------+
46    | alloc (and map) pages |     |  get page from cache   |
47    +-----------------------+     +------------------------+
48                                    ^                    ^
49                                    |                    |
50                                    | cache available    | No entries, refill
51                                    |                    | from ptr-ring
52                                    |                    |
53                                    v                    v
54                          +-----------------+     +------------------+
55                          |   Fast cache    |     |  ptr-ring cache  |
56                          +-----------------+     +------------------+
57
58API interface
59=============
60The number of pools created **must** match the number of hardware queues
61unless hardware restrictions make that impossible. This would otherwise beat the
62purpose of page pool, which is allocate pages fast from cache without locking.
63This lockless guarantee naturally comes from running under a NAPI softirq.
64The protection doesn't strictly have to be NAPI, any guarantee that allocating
65a page will cause no race conditions is enough.
66
67* page_pool_create(): Create a pool.
68    * flags:      PP_FLAG_DMA_MAP, PP_FLAG_DMA_SYNC_DEV
69    * order:      2^order pages on allocation
70    * pool_size:  size of the ptr_ring
71    * nid:        preferred NUMA node for allocation
72    * dev:        struct device. Used on DMA operations
73    * dma_dir:    DMA direction
74    * max_len:    max DMA sync memory size
75    * offset:     DMA address offset
76
77* page_pool_put_page(): The outcome of this depends on the page refcnt. If the
78  driver bumps the refcnt > 1 this will unmap the page. If the page refcnt is 1
79  the allocator owns the page and will try to recycle it in one of the pool
80  caches. If PP_FLAG_DMA_SYNC_DEV is set, the page will be synced for_device
81  using dma_sync_single_range_for_device().
82
83* page_pool_put_full_page(): Similar to page_pool_put_page(), but will DMA sync
84  for the entire memory area configured in area pool->max_len.
85
86* page_pool_recycle_direct(): Similar to page_pool_put_full_page() but caller
87  must guarantee safe context (e.g NAPI), since it will recycle the page
88  directly into the pool fast cache.
89
90* page_pool_release_page(): Unmap the page (if mapped) and account for it on
91  inflight counters.
92
93* page_pool_dev_alloc_pages(): Get a page from the page allocator or page_pool
94  caches.
95
96* page_pool_get_dma_addr(): Retrieve the stored DMA address.
97
98* page_pool_get_dma_dir(): Retrieve the stored DMA direction.
99
100* page_pool_put_page_bulk(): Tries to refill a number of pages into the
101  ptr_ring cache holding ptr_ring producer lock. If the ptr_ring is full,
102  page_pool_put_page_bulk() will release leftover pages to the page allocator.
103  page_pool_put_page_bulk() is suitable to be run inside the driver NAPI tx
104  completion loop for the XDP_REDIRECT use case.
105  Please note the caller must not use data area after running
106  page_pool_put_page_bulk(), as this function overwrites it.
107
108Coding examples
109===============
110
111Registration
112------------
113
114.. code-block:: c
115
116    /* Page pool registration */
117    struct page_pool_params pp_params = { 0 };
118    struct xdp_rxq_info xdp_rxq;
119    int err;
120
121    pp_params.order = 0;
122    /* internal DMA mapping in page_pool */
123    pp_params.flags = PP_FLAG_DMA_MAP;
124    pp_params.pool_size = DESC_NUM;
125    pp_params.nid = NUMA_NO_NODE;
126    pp_params.dev = priv->dev;
127    pp_params.dma_dir = xdp_prog ? DMA_BIDIRECTIONAL : DMA_FROM_DEVICE;
128    page_pool = page_pool_create(&pp_params);
129
130    err = xdp_rxq_info_reg(&xdp_rxq, ndev, 0);
131    if (err)
132        goto err_out;
133
134    err = xdp_rxq_info_reg_mem_model(&xdp_rxq, MEM_TYPE_PAGE_POOL, page_pool);
135    if (err)
136        goto err_out;
137
138NAPI poller
139-----------
140
141
142.. code-block:: c
143
144    /* NAPI Rx poller */
145    enum dma_data_direction dma_dir;
146
147    dma_dir = page_pool_get_dma_dir(dring->page_pool);
148    while (done < budget) {
149        if (some error)
150            page_pool_recycle_direct(page_pool, page);
151        if (packet_is_xdp) {
152            if XDP_DROP:
153                page_pool_recycle_direct(page_pool, page);
154        } else (packet_is_skb) {
155            page_pool_release_page(page_pool, page);
156            new_page = page_pool_dev_alloc_pages(page_pool);
157        }
158    }
159
160Driver unload
161-------------
162
163.. code-block:: c
164
165    /* Driver unload */
166    page_pool_put_full_page(page_pool, page, false);
167    xdp_rxq_info_unreg(&xdp_rxq);
168