Add constant flags to indicate two devices are not supported or whether
the data path goes through the host bridge instead of using the negative
values -1 and -2.

This helps annotate the code better, but the main reason is so we
can cache the result in an xarray which does not allow us to store
negative values.

Signed-off-by: Logan Gunthorpe <log...@deltatee.com>
---
 drivers/pci/p2pdma.c | 52 ++++++++++++++++++++++++++------------------
 1 file changed, 31 insertions(+), 21 deletions(-)

diff --git a/drivers/pci/p2pdma.c b/drivers/pci/p2pdma.c
index 234476226529..e8ec86e1dd00 100644
--- a/drivers/pci/p2pdma.c
+++ b/drivers/pci/p2pdma.c
@@ -273,6 +273,20 @@ static bool root_complex_whitelist(struct pci_dev *dev)
        return false;
 }
 
+enum {
+       /*
+        * Thes arbitrary offset are or'd onto the upstream distance
+        * calculation for the following conditions:
+        */
+
+       /* The data path includes the host-bridge */
+       P2PDMA_THRU_HOST_BRIDGE         = 0x02000000,
+       /* The data path is forced through the host-bridge due to ACS */
+       P2PDMA_ACS_FORCES_UPSTREAM      = 0x04000000,
+       /* The data path is not supported by P2PDMA */
+       P2PDMA_NOT_SUPPORTED            = 0x08000000,
+};
+
 /*
  * Find the distance through the nearest common upstream bridge between
  * two PCI devices.
@@ -297,22 +311,17 @@ static bool root_complex_whitelist(struct pci_dev *dev)
  * 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 don't have a common upstream bridge will return -1.
- * In this way devices on separate PCIe root ports will be rejected, which
- * is what we want for peer-to-peer seeing each PCIe root port defines a
- * separate hierarchy domain and there's no way to determine whether the root
- * complex supports forwarding between them.
+ * Any two devices that cannot communicate using p2pdma will return the 
distance
+ * with the flag P2PDMA_NOT_SUPPORTED.
  *
- * In the case where two devices are connected to different PCIe switches,
- * this function will still return a positive distance as long as both
- * switches eventually have a common upstream bridge. Note this covers
- * the case of using multiple PCIe switches to achieve a desired level of
- * fan-out from a root port. The exact distance will be a function of the
- * number of switches between Device A and Device B.
+ * 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,
+ * then the distance will be returned with the flag P2PDMA_THRU_HOST_BRIDGE 
set.
+ * If either bridge is not on the whitelist, the flag P2PDMA_NOT_SUPPORTED will
+ * be set.
  *
  * If a bridge which has any ACS redirection bits set is in the path
- * then this functions will return -2. This is so we reject any
- * cases where the TLPs are forwarded up into the root complex.
+ * then this functions will flag the result with P2PDMA_ACS_FORCES_UPSTREAM.
  * In this case, a list of all infringing bridge addresses will be
  * populated in acs_list (assuming it's non-null) for printk purposes.
  */
@@ -359,9 +368,9 @@ static int upstream_bridge_distance(struct pci_dev 
*provider,
         */
        if (root_complex_whitelist(provider) &&
            root_complex_whitelist(client))
-               return 0x1000 + dist_a + dist_b;
+               return (dist_a + dist_b) | P2PDMA_THRU_HOST_BRIDGE;
 
-       return -1;
+       return (dist_a + dist_b) | P2PDMA_NOT_SUPPORTED;
 
 check_b_path_acs:
        bb = b;
@@ -379,7 +388,7 @@ static int upstream_bridge_distance(struct pci_dev 
*provider,
        }
 
        if (acs_cnt)
-               return -2;
+               return P2PDMA_NOT_SUPPORTED | P2PDMA_ACS_FORCES_UPSTREAM;
 
        return dist_a + dist_b;
 }
@@ -395,16 +404,17 @@ static int upstream_bridge_distance_warn(struct pci_dev 
*provider,
                return -ENOMEM;
 
        ret = upstream_bridge_distance(provider, client, &acs_list);
-       if (ret == -2) {
-               pci_warn(client, "cannot be used for peer-to-peer DMA as ACS 
redirect is set between the client and provider (%s)\n",
+       if (ret & P2PDMA_ACS_FORCES_UPSTREAM) {
+               pci_warn(client, "ACS redirect is set between the client and 
provider (%s)\n",
                         pci_name(provider));
                /* Drop final semicolon */
                acs_list.buffer[acs_list.len-1] = 0;
                pci_warn(client, "to disable ACS redirect for this path, add 
the kernel parameter: pci=disable_acs_redir=%s\n",
                         acs_list.buffer);
+       }
 
-       } else if (ret < 0) {
-               pci_warn(client, "cannot be used for peer-to-peer DMA as the 
client and provider (%s) do not share an upstream bridge\n",
+       if (ret & P2PDMA_NOT_SUPPORTED) {
+               pci_warn(client, "cannot be used for peer-to-peer DMA as the 
client and provider (%s) do not share an upstream bridge or whitelisted host 
bridge\n",
                         pci_name(provider));
        }
 
@@ -468,7 +478,7 @@ int pci_p2pdma_distance_many(struct pci_dev *provider, 
struct device **clients,
 
                pci_dev_put(pci_client);
 
-               if (ret < 0)
+               if (ret & P2PDMA_NOT_SUPPORTED)
                        not_supported = true;
 
                if (not_supported && !verbose)
-- 
2.20.1

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