[PATCH v2 04/10] PCI/P2PDMA: Clear ACS P2P flags for all devices behind switches

[Logan Gunthorpe]

For peer-to-peer transactions to work the downstream ports in each
switch must not have the ACS flags set. At this time there is no way
to dynamically change the flags and update the corresponding IOMMU
groups so this is done at enumeration time before the the groups are
assigned.

This effectively means that if CONFIG_PCI_P2PDMA is selected then
all devices behind any switch will be in the same IOMMU group.

Signed-off-by: Logan Gunthorpe <logang@xxxxxxxxxxxx>
---
 drivers/pci/Kconfig        |  4 ++++
 drivers/pci/p2pdma.c       | 44 ++++++++++++++++++++++++++++++++++++++++++++
 drivers/pci/pci.c          |  4 ++++
 include/linux/pci-p2pdma.h |  5 +++++
 4 files changed, 57 insertions(+)

diff --git a/drivers/pci/Kconfig b/drivers/pci/Kconfig
index 840831418cbd..a430672f0ad4 100644
--- a/drivers/pci/Kconfig
+++ b/drivers/pci/Kconfig
@@ -138,6 +138,10 @@ config PCI_P2PDMA
 	  it's hard to tell which support it with good performance, so
 	  at this time you will need a PCIe switch.
 
+	  Enabling this option will also disable ACS on all ports behind
+	  any PCIe switch. This effictively puts all devices behind any
+	  switch into the same IOMMU group.
+
 	  If unsure, say N.
 
 config PCI_LABEL
diff --git a/drivers/pci/p2pdma.c b/drivers/pci/p2pdma.c
index 4e1c81f64b29..61af07acd21a 100644
--- a/drivers/pci/p2pdma.c
+++ b/drivers/pci/p2pdma.c
@@ -255,6 +255,50 @@ static struct pci_dev *get_upstream_bridge_port(struct pci_dev *pdev)
 	return up2;
 }
 
+/*
+ * pci_p2pdma_disable_acs - disable ACS flags for ports in PCI
+ *	bridges/switches
+ * @pdev: device to disable ACS flags for
+ *
+ * The ACS flags for P2P Request Redirect and P2P Completion Redirect need
+ * to be disabled on any downstream port in any switch in order for
+ * the TLPs to not be forwarded up to the RC which is not what we want
+ * for P2P.
+ *
+ * This function is called when the devices are first enumerated and
+ * will result in all devices behind any switch to be in the same IOMMU
+ * group. At this time there is no way to "hotplug" IOMMU groups so we rely
+ * on this largish hammer. If you need the devices to be in separate groups
+ * don't enable CONFIG_PCI_P2PDMA.
+ *
+ * Returns 1 if the ACS bits for this device were cleared, otherwise 0.
+ */
+int pci_p2pdma_disable_acs(struct pci_dev *pdev)
+{
+	struct pci_dev *up;
+	int pos;
+	u16 ctrl;
+
+	up = get_upstream_bridge_port(pdev);
+	if (!up)
+		return 0;
+	pci_dev_put(up);
+
+	pos = pci_find_ext_capability(pdev, PCI_EXT_CAP_ID_ACS);
+	if (!pos)
+		return 0;
+
+	dev_info(&pdev->dev, "disabling ACS flags for peer-to-peer DMA\n");
+
+	pci_read_config_word(pdev, pos + PCI_ACS_CTRL, &ctrl);
+
+	ctrl &= ~(PCI_ACS_RR | PCI_ACS_CR);
+
+	pci_write_config_word(pdev, pos + PCI_ACS_CTRL, ctrl);
+
+	return 1;
+}
+
 static bool __upstream_bridges_match(struct pci_dev *upstream,
 				     struct pci_dev *client)
 {
diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
index f6a4dd10d9b0..95ad3cf288c8 100644
--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -16,6 +16,7 @@
 #include <linux/of.h>
 #include <linux/of_pci.h>
 #include <linux/pci.h>
+#include <linux/pci-p2pdma.h>
 #include <linux/pm.h>
 #include <linux/slab.h>
 #include <linux/module.h>
@@ -2826,6 +2827,9 @@ static void pci_std_enable_acs(struct pci_dev *dev)
  */
 void pci_enable_acs(struct pci_dev *dev)
 {
+	if (pci_p2pdma_disable_acs(dev))
+		return;
+
 	if (!pci_acs_enable)
 		return;
 
diff --git a/include/linux/pci-p2pdma.h b/include/linux/pci-p2pdma.h
index 126eca697ab3..f537f521f60c 100644
--- a/include/linux/pci-p2pdma.h
+++ b/include/linux/pci-p2pdma.h
@@ -22,6 +22,7 @@ struct block_device;
 struct scatterlist;
 
 #ifdef CONFIG_PCI_P2PDMA
+int pci_p2pdma_disable_acs(struct pci_dev *pdev);
 int pci_p2pdma_add_resource(struct pci_dev *pdev, int bar, size_t size,
 		u64 offset);
 int pci_p2pdma_add_client(struct list_head *head, struct device *dev);
@@ -41,6 +42,10 @@ int pci_p2pdma_map_sg(struct device *dev, struct scatterlist *sg, int nents,
 void pci_p2pdma_unmap_sg(struct device *dev, struct scatterlist *sg, int nents,
 			 enum dma_data_direction dir);
 #else /* CONFIG_PCI_P2PDMA */
+static inline int pci_p2pdma_disable_acs(struct pci_dev *pdev)
+{
+	return 0;
+}
 static inline int pci_p2pdma_add_resource(struct pci_dev *pdev, int bar,
 		size_t size, u64 offset)
 {
-- 
2.11.0