PCI: Don't restrict MPS for slots below Root Ports

[karo-tx-linux.git] / drivers / pci / probe.c

diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
index 0591b087a6c31b8db7429d27b6c02602bc7f1eac..94b1d227ddcd3ee4556c12672c70fba27d4d9759 100644 (file)
--- a/drivers/pci/probe.c
+++ b/drivers/pci/probe.c
@@ -1491,23 +1491,23 @@ static int pcie_find_smpss(struct pci_dev *dev, void *data)
        if (!pci_is_pcie(dev))
                return 0;
 
-       /* For PCIE hotplug enabled slots not connected directly to a
-        * PCI-E root port, there can be problems when hotplugging
-        * devices.  This is due to the possibility of hotplugging a
-        * device into the fabric with a smaller MPS that the devices
-        * currently running have configured.  Modifying the MPS on the
-        * running devices could cause a fatal bus error due to an
-        * incoming frame being larger than the newly configured MPS.
-        * To work around this, the MPS for the entire fabric must be
-        * set to the minimum size.  Any devices hotplugged into this
-        * fabric will have the minimum MPS set.  If the PCI hotplug
-        * slot is directly connected to the root port and there are not
-        * other devices on the fabric (which seems to be the most
-        * common case), then this is not an issue and MPS discovery
-        * will occur as normal.
+       /*
+        * We don't have a way to change MPS settings on devices that have
+        * drivers attached.  A hot-added device might support only the minimum
+        * MPS setting (MPS=128).  Therefore, if the fabric contains a bridge
+        * where devices may be hot-added, we limit the fabric MPS to 128 so
+        * hot-added devices will work correctly.
+        *
+        * However, if we hot-add a device to a slot directly below a Root
+        * Port, it's impossible for there to be other existing devices below
+        * the port.  We don't limit the MPS in this case because we can
+        * reconfigure MPS on both the Root Port and the hot-added device,
+        * and there are no other devices involved.
+        *
+        * Note that this PCIE_BUS_SAFE path assumes no peer-to-peer DMA.
         */
-       if (dev->is_hotplug_bridge && (!list_is_singular(&dev->bus->devices) ||
-           pci_pcie_type(dev) != PCI_EXP_TYPE_ROOT_PORT))
+       if (dev->is_hotplug_bridge &&
+           pci_pcie_type(dev) != PCI_EXP_TYPE_ROOT_PORT)
                *smpss = 0;
 
        if (*smpss > dev->pcie_mpss)