x86/amd_node: Fix PCI device reference counting in amd_smn_init()grafted

The local "root" pointer is a temporary variable used during the device
search. Therefore, refcount related to the search iterators should be cleaned
up after the search is complete.

Use the __free() cleanup macro to ensure the refcount is decremented when the
temporary pointer goes out of scope.

Additionally, increment the refcount when caching a root pointer. This ensures
the in-use refcount is separate from the temporary search refcounting.

Finally, drop the redundant "root = NULL" before the second search loop. The
pci_get_class() iterator always decrements the refcount of its "from"
argument, so the first loop can only fall through with "root" already NULL.

Fixes: 0a4b61d9c2e4 ("x86/amd_node: Fix AMD root device caching")
Closes: https://sashiko.dev/#/patchset/20260806160159.230453-1-jason.andryuk%40amd.com
Reported-by: Sashiko <sashiko-bot@kernel.org>
Assisted-by: LLM
Signed-off-by: Yazen Ghannam <yazen.ghannam@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Mario Limonciello (AMD) <superm1@kernel.org>
Cc: <stable@kernel.org>
Link: https://patch.msgid.link/20260903154325.74343-1-yazen.ghannam@amd.com

1 files changed, 104 insertions, 0 deletions

diff --git a/Documentation/networking/netmem.rst b/Documentation/networking/netmem.rst
new file mode 100644
index 000000000..217869d11
--- /dev/null
+++ b/Documentation/networking/netmem.rst
@@ -0,0 +1,104 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+==================================
+Netmem Support for Network Drivers
+==================================
+
+This document outlines the requirements for network drivers to support netmem,
+an abstract memory type that enables features like device memory TCP. By
+supporting netmem, drivers can work with various underlying memory types
+with little to no modification.
+
+Benefits of Netmem :
+
+* Flexibility: Netmem can be backed by different memory types (e.g., struct
+  page, DMA-buf), allowing drivers to support various use cases such as device
+  memory TCP.
+* Future-proof: Drivers with netmem support are ready for upcoming
+  features that rely on it.
+* Simplified Development: Drivers interact with a consistent API,
+  regardless of the underlying memory implementation.
+
+Driver RX Requirements
+======================
+
+1. The driver must support page_pool.
+
+2. The driver must support the tcp-data-split ethtool option.
+
+3. The driver must use the page_pool netmem APIs for payload memory. The netmem
+   APIs 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 the maintainers and/or 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, which knows when
+     dma-mapping is (or is not) appropriate.
+   - 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,
+     which knows when dma-syncing is (or is not) appropriate.
+   - 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.
+
+7. Avoid implementing driver-specific recycling on top of the page_pool. Drivers
+   cannot hold onto a struct page to do their own recycling as the netmem may
+   not be backed by a struct page. However, you may hold onto a page_pool
+   reference with page_pool_fragment_netmem() or page_pool_ref_netmem() for
+   that purpose, but be mindful that some netmem types might have longer
+   circulation times, such as when userspace holds a reference in zerocopy
+   scenarios.
+
+Driver TX Requirements
+======================
+
+1. The Driver must not pass the netmem dma_addr to any of the dma-mapping APIs
+   directly. This is because netmem dma_addrs may come from a source like
+   dma-buf that is not compatible with the dma-mapping APIs.
+
+   Helpers like netmem_dma_unmap_page_attrs() & netmem_dma_unmap_addr_set()
+   should be used in lieu of dma_unmap_page[_attrs](), dma_unmap_addr_set().
+   The netmem variants will handle netmem dma_addrs correctly regardless of the
+   source, delegating to the dma-mapping APIs when appropriate.
+
+   Not all dma-mapping 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 the maintainers and/or almasrymina@google.com for
+   help adding the netmem API.
+
+2. Driver should declare support by setting `netdev->netmem_tx` to the
+   appropriate mode:
+
+   - `NETMEM_TX_DMA`: for physical devices that perform DMA.
+
+   - `NETMEM_TX_NO_DMA`: for virtual or passthrough devices that do
+     not DMA, but still support handling of netmem-backed skbs.