PCI: Fix incorrect value returned from pcie_get_speed_cap()

The macros PCI_EXP_LNKCAP_SLS_*GB are values, not bit masks.  We must mask
the register and compare it against them.

This fixes errors like this:

  amdgpu: [powerplay] failed to send message 261 ret is 0

when a PCIe-v3 card is plugged into a PCIe-v1 slot, because the slot is
being incorrectly reported as PCIe-v3 capable.

6cf57be0f78e, which appeared in v4.17, added pcie_get_speed_cap() with the
incorrect test of PCI_EXP_LNKCAP_SLS as a bitmask.  5d9a63304032, which
appeared in v4.19, changed amdgpu to use pcie_get_speed_cap(), so the
amdgpu bug reports below are regressions in v4.19.

Fixes: 6cf57be0f78e ("PCI: Add pcie_get_speed_cap() to find max supported link speed")
Fixes: 5d9a63304032 ("drm/amdgpu: use pcie functions for link width and speed")
Link: https://bugs.freedesktop.org/show_bug.cgi?id=108704
Link: https://bugs.freedesktop.org/show_bug.cgi?id=108778
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
[bhelgaas: update comment, remove use of PCI_EXP_LNKCAP_SLS_8_0GB and
PCI_EXP_LNKCAP_SLS_16_0GB since those should be covered by PCI_EXP_LNKCAP2,
remove test of PCI_EXP_LNKCAP for zero, since that register is required]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org	# v4.17+
diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
index d068f11..c9d8e3c 100644
--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -5556,9 +5556,13 @@
 	u32 lnkcap2, lnkcap;
 
 	/*
-	 * PCIe r4.0 sec 7.5.3.18 recommends using the Supported Link
-	 * Speeds Vector in Link Capabilities 2 when supported, falling
-	 * back to Max Link Speed in Link Capabilities otherwise.
+	 * Link Capabilities 2 was added in PCIe r3.0, sec 7.8.18.  The
+	 * implementation note there recommends using the Supported Link
+	 * Speeds Vector in Link Capabilities 2 when supported.
+	 *
+	 * Without Link Capabilities 2, i.e., prior to PCIe r3.0, software
+	 * should use the Supported Link Speeds field in Link Capabilities,
+	 * where only 2.5 GT/s and 5.0 GT/s speeds were defined.
 	 */
 	pcie_capability_read_dword(dev, PCI_EXP_LNKCAP2, &lnkcap2);
 	if (lnkcap2) { /* PCIe r3.0-compliant */
@@ -5574,16 +5578,10 @@
 	}
 
 	pcie_capability_read_dword(dev, PCI_EXP_LNKCAP, &lnkcap);
-	if (lnkcap) {
-		if (lnkcap & PCI_EXP_LNKCAP_SLS_16_0GB)
-			return PCIE_SPEED_16_0GT;
-		else if (lnkcap & PCI_EXP_LNKCAP_SLS_8_0GB)
-			return PCIE_SPEED_8_0GT;
-		else if (lnkcap & PCI_EXP_LNKCAP_SLS_5_0GB)
-			return PCIE_SPEED_5_0GT;
-		else if (lnkcap & PCI_EXP_LNKCAP_SLS_2_5GB)
-			return PCIE_SPEED_2_5GT;
-	}
+	if ((lnkcap & PCI_EXP_LNKCAP_SLS) == PCI_EXP_LNKCAP_SLS_5_0GB)
+		return PCIE_SPEED_5_0GT;
+	else if ((lnkcap & PCI_EXP_LNKCAP_SLS) == PCI_EXP_LNKCAP_SLS_2_5GB)
+		return PCIE_SPEED_2_5GT;
 
 	return PCI_SPEED_UNKNOWN;
 }