[PATCH net-next v4 5/5] net: Document netmem driver support

[Mina Almasry <almasrymina@google.com>]

Document expectations from drivers looking to add support for device
memory tcp or other netmem based features.

Signed-off-by: Mina Almasry <almasrymina@google.com>

---

v4:
- Address comments from Randy.
- Change docs to netmem focus (Jakub).
- Address comments from Jakub.

---
 Documentation/networking/index.rst  |  1 +
 Documentation/networking/netmem.rst | 62 +++++++++++++++++++++++++++++
 2 files changed, 63 insertions(+)
 create mode 100644 Documentation/networking/netmem.rst

diff --git a/Documentation/networking/index.rst b/Documentation/networking/index.rst
index 46c178e564b3..058193ed2eeb 100644
--- a/Documentation/networking/index.rst
+++ b/Documentation/networking/index.rst
@@ -86,6 +86,7 @@ Contents:
    netdevices
    netfilter-sysctl
    netif-msg
+   netmem
    nexthop-group-resilient
    nf_conntrack-sysctl
    nf_flowtable
diff --git a/Documentation/networking/netmem.rst b/Documentation/networking/netmem.rst
new file mode 100644
index 000000000000..f9f03189c53c
--- /dev/null
+++ b/Documentation/networking/netmem.rst
@@ -0,0 +1,62 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+================
+Netmem
+================
+
+
+Introduction
+============
+
+Device memory TCP, and likely more upcoming features, are reliant on netmem
+support in the driver. This outlines what drivers need to do to support netmem.
+
+
+Driver support
+==============
+
+1. The driver must support page_pool. The driver must not do its own recycling
+   on top of page_pool.
+
+2. The driver must support the tcp-data-split ethtool option.
+
+3. The driver must use the page_pool netmem APIs. The netmem APIs are
+   currently 1-to-1 correspond with page APIs. Conversion to netmem should be
+   achievable by switching the page APIs to netmem APIs and tracking memory via
+   netmem_refs in the driver rather than struct page * :
+
+   - page_pool_alloc -> page_pool_alloc_netmem
+   - page_pool_get_dma_addr -> page_pool_get_dma_addr_netmem
+   - page_pool_put_page -> page_pool_put_netmem
+
+   Not all page APIs have netmem equivalents at the moment. If your driver
+   relies on a missing netmem API, feel free to add and propose to netdev@ or
+   reach out to almasrymina@google.com for help adding the netmem API.
+
+4. The driver must use the following PP_FLAGS:
+
+   - PP_FLAG_DMA_MAP: netmem is not dma-mappable by the driver. The driver
+     must delegate the dma mapping to the page_pool.
+   - PP_FLAG_DMA_SYNC_DEV: netmem dma addr is not necessarily dma-syncable
+     by the driver. The driver must delegate the dma syncing to the page_pool.
+   - PP_FLAG_ALLOW_UNREADABLE_NETMEM. The driver must specify this flag iff
+     tcp-data-split is enabled.
+
+5. The driver must not assume the netmem is readable and/or backed by pages.
+   The netmem returned by the page_pool may be unreadable, in which case
+   netmem_address() will return NULL. The driver must correctly handle
+   unreadable netmem, i.e. don't attempt to handle its contents when
+   netmem_address() is NULL.
+
+   Ideally, drivers should not have to check the underlying netmem type via
+   helpers like netmem_is_net_iov() or convert the netmem to any of its
+   underlying types via netmem_to_page() or netmem_to_net_iov(). In most cases,
+   netmem or page_pool helpers that abstract this complexity are provided
+   (and more can be added).
+
+6. The driver must use page_pool_dma_sync_netmem_for_cpu() in lieu of
+   dma_sync_single_range_for_cpu(). For some memory providers, dma_syncing for
+   CPU will be done by the page_pool, for others (particularly dmabuf memory
+   provider), dma syncing for CPU is the responsibility of the userspace using
+   dmabuf APIs. The driver must delegate the entire dma-syncing operation to
+   the page_pool which will do it correctly.
-- 
2.47.0.338.g60cca15819-goog