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Message-Id: <20200220182521.859730-1-ilias.apalodimas@linaro.org>
Date: Thu, 20 Feb 2020 20:25:21 +0200
From: Ilias Apalodimas <ilias.apalodimas@...aro.org>
To: brouer@...hat.com, davem@...emloft.net, netdev@...r.kernel.org
Cc: lorenzo@...nel.org, rdunlap@...radead.org, toke@...hat.com,
Ilias Apalodimas <ilias.apalodimas@...aro.org>
Subject: [PATCH net-next] net: page_pool: Add documentation for page_pool API
Add documentation explaining the basic functionality and design
principles of the API
Signed-off-by: Ilias Apalodimas <ilias.apalodimas@...aro.org>
---
Documentation/networking/page_pool.rst | 159 +++++++++++++++++++++++++
1 file changed, 159 insertions(+)
create mode 100644 Documentation/networking/page_pool.rst
diff --git a/Documentation/networking/page_pool.rst b/Documentation/networking/page_pool.rst
new file mode 100644
index 000000000000..098d339ef272
--- /dev/null
+++ b/Documentation/networking/page_pool.rst
@@ -0,0 +1,159 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+=============
+Page Pool API
+=============
+
+The page_pool allocator is optimized for the XDP mode that uses one frame
+per-page, but it can fallback on the regular page allocator APIs.
+
+Basic use involve replacing alloc_pages() calls with the
+page_pool_alloc_pages() call. Drivers should use page_pool_dev_alloc_pages()
+replacing dev_alloc_pages().
+
+API keeps track of inflight pages, in order to let API user know
+when it is safe to free a page_pool object. Thus, API users
+must run page_pool_release_page() when a page is leaving the page_pool or
+call page_pool_put_page() where appropriate in order to maintain correct
+accounting.
+
+API user must call page_pool_put_page() once on a page, as it
+will either recycle the page, or in case of refcnt > 1, it will
+release the DMA mapping and inflight state accounting.
+
+Architecture overview
+=====================
+
+.. code-block:: none
+
+ +------------------+
+ | Driver |
+ +------------------+
+ ^
+ |
+ |
+ |
+ v
+ +--------------------------------------------+
+ | request memory |
+ +--------------------------------------------+
+ ^ ^
+ | |
+ | Pool empty | Pool has entries
+ | |
+ v v
+ +-----------------------+ +------------------------+
+ | alloc (and map) pages | | get page from cache |
+ +-----------------------+ +------------------------+
+ ^ ^
+ | |
+ | cache available | No entries, refill
+ | | from ptr-ring
+ | |
+ v v
+ +-----------------+ +------------------+
+ | Fast cache | | ptr-ring cache |
+ +-----------------+ +------------------+
+
+API interface
+=============
+The number of pools created **must** match the number of hardware queues
+unless hardware restrictions make that impossible. This would otherwise beat the
+purpose of page pool, which is allocate pages fast from cache without locking.
+This lockless guarantee naturally comes from running under a NAPI softirq.
+The protection doesn't strictly have to be NAPI, any guarantee that allocating
+a page will cause no race conditions is enough.
+
+* page_pool_create(): Create a pool.
+ * flags: PP_FLAG_DMA_MAP, PP_FLAG_DMA_SYNC_DEV
+ * order: order^n pages on allocation
+ * pool_size: size of the ptr_ring
+ * nid: preferred NUMA node for allocation
+ * dev: struct device. Used on DMA operations
+ * dma_dir: DMA direction
+ * max_len: max DMA sync memory size
+ * offset: DMA address offset
+
+* page_pool_put_page(): The outcome of this depends on the page refcnt. If the
+ driver bumps the refcnt > 1 this will unmap the page. If the page refcnt is 1
+ the allocator owns the page and will try to recycle it in one of the pool
+ caches. If PP_FLAG_DMA_SYNC_DEV is set, the page will be synced for_device
+ using dma_sync_single_range_for_device().
+
+* page_pool_put_full_page(): Similar to page_pool_put_page(), but will DMA sync
+ for the entire memory area configured in area pool->max_len.
+
+* page_pool_recycle_direct(): Similar to page_pool_put_full_page() but caller
+ must guarantee safe context (e.g NAPI), since it will recycle the page
+ directly into the pool fast cache.
+
+* page_pool_release_page(): Unmap the page (if mapped) and account for it on
+ inflight counters.
+
+* page_pool_dev_alloc_pages(): Get a page from the page allocator or page_pool
+ caches.
+
+* page_pool_get_dma_addr(): Retrieve the stored DMA address.
+
+* page_pool_get_dma_dir(): Retrieve the stored DMA direction.
+
+Coding examples
+===============
+
+Registration
+------------
+
+.. code-block:: c
+
+ /* Page pool registration */
+ struct page_pool_params pp_params = { 0 };
+ struct xdp_rxq_info xdp_rxq;
+ int err;
+
+ pp_params.order = 0;
+ /* internal DMA mapping in page_pool */
+ pp_params.flags = PP_FLAG_DMA_MAP;
+ pp_params.pool_size = DESC_NUM;
+ pp_params.nid = NUMA_NO_NODE;
+ pp_params.dev = priv->dev;
+ pp_params.dma_dir = xdp_prog ? DMA_BIDIRECTIONAL : DMA_FROM_DEVICE;
+ page_pool = page_pool_create(&pp_params);
+
+ err = xdp_rxq_info_reg(&xdp_rxq, ndev, 0);
+ if (err)
+ goto err_out;
+
+ err = xdp_rxq_info_reg_mem_model(&xdp_rxq, MEM_TYPE_PAGE_POOL, page_pool);
+ if (err)
+ goto err_out;
+
+NAPI poller
+-----------
+
+
+.. code-block:: c
+
+ /* NAPI Rx poller */
+ enum dma_data_direction dma_dir;
+
+ dma_dir = page_pool_get_dma_dir(dring->page_pool);
+ while (done < budget) {
+ if (some error)
+ page_pool_recycle_direct(page_pool, page);
+ if (packet_is_xdp) {
+ if XDP_DROP:
+ page_pool_recycle_direct(page_pool, page);
+ } else (packet_is_skb) {
+ page_pool_release_page(page_pool, page);
+ new_page = page_pool_dev_alloc_pages(page_pool);
+ }
+ }
+
+Driver unload
+-------------
+
+.. code-block:: c
+
+ /* Driver unload */
+ page_pool_put_full_page(page_pool, page, false);
+ xdp_rxq_info_unreg(&xdp_rxq);
--
2.25.1
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