mirror of
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git
synced 2024-08-28 20:02:17 +00:00
![Ilias Apalodimas](/assets/img/avatar_default.png)
Add documentation explaining the basic functionality and design principles of the API Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org> Acked-by: Randy Dunlap <rdunlap@infradead.org> Signed-off-by: David S. Miller <davem@davemloft.net>
159 lines
5.4 KiB
ReStructuredText
159 lines
5.4 KiB
ReStructuredText
.. 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 involves 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: 2^order 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);
|