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From:   Logan Gunthorpe <logang@deltatee.com>
To:     linux-kernel@vger.kernel.org, linux-pci@vger.kernel.org,
        Bjorn Helgaas <helgaas@kernel.org>
Cc:     Stephen Bates <sbates@raithlin.com>,
        Christoph Hellwig <hch@lst.de>,
        Dan Williams <dan.j.williams@intel.com>,
        Jason Gunthorpe <jgg@ziepe.ca>,
        =?UTF-8?q?Christian=20K=C3=B6nig?= <christian.koenig@amd.com>,
        John Hubbard <jhubbard@nvidia.com>,
        Don Dutile <ddutile@redhat.com>,
        Logan Gunthorpe <logang@deltatee.com>
Date:   Thu, 10 Jun 2021 10:06:04 -0600
Message-Id: <20210610160609.28447-2-logang@deltatee.com>
In-Reply-To: <20210610160609.28447-1-logang@deltatee.com>
References: <20210610160609.28447-1-logang@deltatee.com>
MIME-Version: 1.0
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Subject: [PATCH v1 1/6] PCI/P2PDMA: Rename upstream_bridge_distance() and rework documentation
Precedence: bulk

The function upstream_bridge_distance() has evolved such that it's name
is no longer entirely reflective of what the function does.

The function not only calculates the distance between two peers but also
calculates how the DMA addresses for those two peers should be mapped.

Thus, rename the function to calc_map_type_and_dist() and rework the
documentation to better describe the two pieces of information the
function returns.

Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
---
 drivers/pci/p2pdma.c | 63 ++++++++++++++++++++++----------------------
 1 file changed, 32 insertions(+), 31 deletions(-)

diff --git a/drivers/pci/p2pdma.c b/drivers/pci/p2pdma.c
index 196382630363..6f90e9812f6e 100644
--- a/drivers/pci/p2pdma.c
+++ b/drivers/pci/p2pdma.c
@@ -354,7 +354,7 @@ static bool host_bridge_whitelist(struct pci_dev *a, struct pci_dev *b)
 }
 
 static enum pci_p2pdma_map_type
-__upstream_bridge_distance(struct pci_dev *provider, struct pci_dev *client,
+__calc_map_type_and_dist(struct pci_dev *provider, struct pci_dev *client,
 		int *dist, bool *acs_redirects, struct seq_buf *acs_list)
 {
 	struct pci_dev *a = provider, *b = client, *bb;
@@ -433,17 +433,18 @@ static unsigned long map_types_idx(struct pci_dev *client)
 }
 
 /*
- * Find the distance through the nearest common upstream bridge between
- * two PCI devices.
+ * Calculate the P2PDMA mapping type and distance between two PCI devices.
  *
- * If the two devices are the same device then 0 will be returned.
+ * If the two devices are the same device then PCI_P2PDMA_MAP_BUS_ADDR
+ * and a distance of 0 will be returned.
  *
  * If there are two virtual functions of the same device behind the same
- * bridge port then 2 will be returned (one step down to the PCIe switch,
- * then one step back to the same device).
+ * bridge port then PCI_P2PDMA_MAP_BUS_ADDR and a distance of 2 will be
+ * returned (one step down to the PCIe switch, then one step back to the
+ * same device).
  *
  * In the case where two devices are connected to the same PCIe switch, the
- * value 4 will be returned. This corresponds to the following PCI tree:
+ * distance of 4 will be returned. This corresponds to the following PCI tree:
  *
  *     -+  Root Port
  *      \+ Switch Upstream Port
@@ -454,31 +455,31 @@ static unsigned long map_types_idx(struct pci_dev *client)
  *
  * The distance is 4 because we traverse from Device A through the downstream
  * port of the switch, to the common upstream port, back up to the second
- * downstream port and then to Device B.
- *
- * Any two devices that cannot communicate using p2pdma will return
- * PCI_P2PDMA_MAP_NOT_SUPPORTED.
+ * downstream port and then to Device B. The mapping type returned will depend
+ * on the ACS redirection setting of the bridges along the path. If ACS
+ * redirect is set on any bridge port in the path then the TLPs will go through
+ * the host bridge. Otherwise PCI_P2PDMA_MAP_BUS_ADDR is returned.
  *
  * Any two devices that have a data path that goes through the host bridge
- * will consult a whitelist. If the host bridges are on the whitelist,
- * this function will return PCI_P2PDMA_MAP_THRU_HOST_BRIDGE.
- *
- * If either bridge is not on the whitelist this function returns
- * PCI_P2PDMA_MAP_NOT_SUPPORTED.
+ * will consult a whitelist. If the host bridge is in the whitelist,
+ * this function will return PCI_P2PDMA_MAP_THRU_HOST_BRIDGE with the
+ * distance set to the number of ports per above. If the device is not
+ * in the whitelist the type will be returned PCI_P2PDMA_MAP_NOT_SUPPORTED.
  *
- * If a bridge which has any ACS redirection bits set is in the path,
- * acs_redirects will be set to true. In this case, a list of all infringing
- * bridge addresses will be populated in acs_list (assuming it's non-null)
- * for printk purposes.
+ * If any ACS redirect bits are set, then the acs_redirects boolean will be
+ * set to true and their pci device name will be appended to the acs_list
+ * seq_buf. This seq_buf is used to print a warning informing the user
+ * how to disable ACS using a command line parameter.
+ * (See calc_map_type_and_dist_warn() below)
  */
 static enum pci_p2pdma_map_type
-upstream_bridge_distance(struct pci_dev *provider, struct pci_dev *client,
+calc_map_type_and_dist(struct pci_dev *provider, struct pci_dev *client,
 		int *dist, bool *acs_redirects, struct seq_buf *acs_list)
 {
 	enum pci_p2pdma_map_type map_type;
 
-	map_type = __upstream_bridge_distance(provider, client, dist,
-					      acs_redirects, acs_list);
+	map_type = __calc_map_type_and_dist(provider, client, dist,
+					    acs_redirects, acs_list);
 
 	if (map_type == PCI_P2PDMA_MAP_THRU_HOST_BRIDGE) {
 		if (!cpu_supports_p2pdma() &&
@@ -494,8 +495,8 @@ upstream_bridge_distance(struct pci_dev *provider, struct pci_dev *client,
 }
 
 static enum pci_p2pdma_map_type
-upstream_bridge_distance_warn(struct pci_dev *provider, struct pci_dev *client,
-			      int *dist)
+calc_map_type_and_dist_warn(struct pci_dev *provider, struct pci_dev *client,
+			    int *dist)
 {
 	struct seq_buf acs_list;
 	bool acs_redirects;
@@ -505,8 +506,8 @@ upstream_bridge_distance_warn(struct pci_dev *provider, struct pci_dev *client,
 	if (!acs_list.buffer)
 		return -ENOMEM;
 
-	ret = upstream_bridge_distance(provider, client, dist, &acs_redirects,
-				       &acs_list);
+	ret = calc_map_type_and_dist(provider, client, dist, &acs_redirects,
+				     &acs_list);
 	if (acs_redirects) {
 		pci_warn(client, "ACS redirect is set between the client and provider (%s)\n",
 			 pci_name(provider));
@@ -565,11 +566,11 @@ int pci_p2pdma_distance_many(struct pci_dev *provider, struct device **clients,
 		}
 
 		if (verbose)
-			ret = upstream_bridge_distance_warn(provider,
-					pci_client, &distance);
+			ret = calc_map_type_and_dist_warn(provider, pci_client,
+							  &distance);
 		else
-			ret = upstream_bridge_distance(provider, pci_client,
-						       &distance, NULL, NULL);
+			ret = calc_map_type_and_dist(provider, pci_client,
+						     &distance, NULL, NULL);
 
 		pci_dev_put(pci_client);
 
-- 
2.20.1