PCI/bwctrl: Enable only if more than one speed is supported

If a PCIe port only supports a single speed, enabling bandwidth control
is pointless:  There's no need to monitor autonomous speed changes, nor
can the speed be changed.

Not enabling it saves a small amount of memory and compute resources,
but also fixes a boot hang reported by Niklas:  It occurs when enabling
bandwidth control on Downstream Ports of Intel JHL7540 "Titan Ridge 2018"
Thunderbolt controllers.  The ports only support 2.5 GT/s in accordance
with USB4 v2 sec 11.2.1, so the present commit works around the issue.

PCIe r6.2 sec 8.2.1 prescribes that:

   "A device must support 2.5 GT/s and is not permitted to skip support
    for any data rates between 2.5 GT/s and the highest supported rate."

Consequently, bandwidth control is currently only disabled if a port
doesn't support higher speeds than 2.5 GT/s.  However the Implementation
Note in PCIe r6.2 sec 7.5.3.18 cautions:

   "It is strongly encouraged that software primarily utilize the
    Supported Link Speeds Vector instead of the Max Link Speed field,
    so that software can determine the exact set of supported speeds on
    current and future hardware.  This can avoid software being confused
    if a future specification defines Links that do not require support
    for all slower speeds."

In other words, future revisions of the PCIe Base Spec may allow gaps
in the Supported Link Speeds Vector.  To be future-proof, don't just
check whether speeds above 2.5 GT/s are supported, but rather check
whether *more than one* speed is supported.

Fixes: 665745f ("PCI/bwctrl: Re-add BW notification portdrv as PCIe BW controller")
Closes: https://lore.kernel.org/r/db8e457fcd155436449b035e8791a8241b0df400.camel@kernel.org
Link: https://lore.kernel.org/r/3564908a9c99fc0d2a292473af7a94ebfc8f5820.1734428762.git.lukas@wunner.de
Reported-by: Niklas Schnelle <niks@kernel.org>
Tested-by: Niklas Schnelle <niks@kernel.org>
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Reviewed-by: Jonathan Cameron <Jonthan.Cameron@huawei.com>
Reviewed-by: Mario Limonciello <mario.limonciello@amd.com>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>

Author: l1k

drivers/pci/pcie/portdrv.c

@@ -265,12 +265,14 @@ static int get_port_device_capability(struct pci_dev *dev)
 	    (pcie_ports_dpc_native || (services & PCIE_PORT_SERVICE_AER)))
 		services |= PCIE_PORT_SERVICE_DPC;
 
+	/* Enable bandwidth control if more than one speed is supported. */
 	if (pci_pcie_type(dev) == PCI_EXP_TYPE_DOWNSTREAM ||
 	    pci_pcie_type(dev) == PCI_EXP_TYPE_ROOT_PORT) {
 		u32 linkcap;
 
 		pcie_capability_read_dword(dev, PCI_EXP_LNKCAP, &linkcap);
-		if (linkcap & PCI_EXP_LNKCAP_LBNC)
+		if (linkcap & PCI_EXP_LNKCAP_LBNC &&
+		    hweight8(dev->supported_speeds) > 1)
 			services |= PCIE_PORT_SERVICE_BWCTRL;
 	}