[v2] net: page_pool: Add documentaion and examples on API usage

Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
---
 Documentation/networking/page_pool.rst | 148 +++++++++++++++++++++++++
 1 file changed, 148 insertions(+)
 create mode 100644 Documentation/networking/page_pool.rst

I accidentally send a wrong version.

Please igjnore this i'll send the proper one in a few minutes. 
Sorry for the noise.

On Tue, Feb 11, 2020 at 05:37:46PM +0200, Ilias Apalodimas wrote:
> Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
> ---
>  Documentation/networking/page_pool.rst | 148 +++++++++++++++++++++++++
>  1 file changed, 148 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..877fc5332983
> --- /dev/null
> +++ b/Documentation/networking/page_pool.rst
> @@ -0,0 +1,148 @@
> +=============
> +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 in-flight 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 in-flight 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   |
> +    +-----------------------+     +------------------------+
> +                                    ^                    ^
> +                                    |                    |
> +                                    | in-softirq         |
> +                                    |                    |
> +                                    v                    v
> +                          +-----------------+     +------------------+  
> +                          |     Fast cache  |     |  ptr-ring cache  | 
> +                          +-----------------+     +------------------+
> +
> +API interface
> +=============
> +Ideally the number of pools created should match the number of hardware queuesm
> +unless other hardware restriction make that impossible. 
> +
> +* page_pool_create(): Create a pool.
> +    * flags:      PP_FLAG_DMA_MAP, PP_FLAG_DMA_SYNC_DEV
> +    * 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 uses refcnt > 1 this will unmap the page. If the pool object is
> +  responsible for DMA operations and account for the in-flight counting. 
> +  If the refcnt is 1, the allocator owns the page and will try to recycle and 
> +  sync it to be re-used by the device using dma_sync_single_range_for_device().
> +
> +* 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.
> +
> +* page_pool_recycle_direct(): Recycle the page immediately. Must be used under
> +  NAPI context
> +
> +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_page(page_pool, page, false);
> +    xdp_rxq_info_unreg(&xdp_rxq);
> +    page_pool_destroy(page_pool);
> -- 
> 2.25.0
>