uart(4): Add a concept of "unique" serial devices

FreeBSD detects serial ports twice: First, very early in the boot
process, in order to obtain a usable console; and second, during
the device probe/attach process.  When a UART is discovered during
device probing, FreeBSD attempts to determine whether it is a
device which was already being used as a console; without this,
the console doesn't work in userland.

Unfortunately it's possible for a UART to be mapped to a different
location in memory when it is discovered on a bus than it has when
it is announced via the ACPI SPCR table; this breaks the matching
process, which relies on comparing bus addresses.

To address this, we introduce a concept of "unique" serial devices,
i.e. devices which are guaranteed to be present *only once* on any
system.  If we discover one of these during device probing, we can
match it to a same-PCI-vendor-and-device-numbers console which was
announced via the ACPI SPCR table, regardless of the differing bus
addresses.

At present, the only unique serial device is the "Amazon PCI serial
device" (vendor 0x1d0f, device 0x8250) found in some EC2 instances.
This unbreaks the serial console on those systems.

Reviewed by:	imp
MFC after:	3 days
Sponsored by:	https://www.patreon.com/cperciva
Differential Revision:	https://reviews.freebsd.org/D34703

sys/dev/uart/uart_bus_pci.c

@@ -43,6 +43,7 @@ __FBSDID("$FreeBSD$");
 
 #include <dev/uart/uart.h>
 #include <dev/uart/uart_bus.h>
+#include <dev/uart/uart_cpu.h>
 
 #define	DEFAULT_RCLK	1843200
 
@@ -76,6 +77,11 @@ struct pci_id {
 	int		regshft;
 };
 
+struct pci_unique_id {
+	uint16_t	vendor;
+	uint16_t	device;
+};
+
 #define PCI_NO_MSI	0x40000000
 #define PCI_RID_MASK	0x0000ffff
 
@@ -214,6 +220,44 @@ uart_pci_match(device_t dev, const struct pci_id *id)
 	return ((id->vendor == vendor && id->device == device) ? id : NULL);
 }
 
+extern SLIST_HEAD(uart_devinfo_list, uart_devinfo) uart_sysdevs;
+
+/* PCI vendor/device pairs of devices guaranteed to be unique on a system. */
+static const struct pci_unique_id pci_unique_devices[] = {
+{ 0x1d0f, 0x8250 }	/* Amazon PCI serial device */
+};
+
+/* Match a UART to a console if it's a PCI device known to be unique. */
+static void
+uart_pci_unique_console_match(device_t dev)
+{
+	struct uart_softc *sc;
+	struct uart_devinfo * sysdev;
+	const struct pci_unique_id * id;
+	uint16_t vendor, device;
+
+	sc = device_get_softc(dev);
+	vendor = pci_get_vendor(dev);
+	device = pci_get_device(dev);
+
+	/* Is this a device known to exist only once in a system? */
+	for (id = pci_unique_devices; ; id++) {
+		if (id == &pci_unique_devices[nitems(pci_unique_devices)])
+			return;
+		if (id->vendor == vendor && id->device == device)
+			break;
+	}
+
+	/* If it matches a console, it must be the same device. */
+	SLIST_FOREACH(sysdev, &uart_sysdevs, next) {
+		if (sysdev->pci_info.vendor == vendor &&
+		    sysdev->pci_info.device == device) {
+			sc->sc_sysdev = sysdev;
+			sysdev->bas.rclk = sc->sc_bas.rclk;
+		}
+	}
+}
+
 static int
 uart_pci_probe(device_t dev)
 {
@@ -251,6 +295,13 @@ uart_pci_probe(device_t dev)
 	/* Bail out on error. */
 	if (result > 0)
 		return (result);
+	/*
+	 * If we haven't already matched this to a console, check if it's a
+	 * PCI device which is known to only exist once in any given system
+	 * and we can match it that way.
+	 */
+	if (sc->sc_sysdev == NULL)
+		uart_pci_unique_console_match(dev);
 	/* Set/override the device description. */
 	if (id->desc)
 		device_set_desc(dev, id->desc);

sys/dev/uart/uart_cpu.h

@@ -52,6 +52,14 @@ struct uart_ops {
 extern bus_space_tag_t uart_bus_space_io;
 extern bus_space_tag_t uart_bus_space_mem;
 
+/*
+ * PCI ID used for matching "unique" devices to a console.
+ */
+struct uart_pci_info {
+	uint16_t vendor;
+	uint16_t device;
+};
+
 /*
  * Console and debug port device info.
  */
@@ -72,6 +80,7 @@ struct uart_devinfo {
 	void	*cookie;		/* Type dependent use. */
 	struct mtx *hwmtx;
 	struct uart_softc *sc;		/* valid only from start of attach */
+	struct uart_pci_info pci_info;
 };
 
 int uart_cpu_eqres(struct uart_bas *, struct uart_bas *);

sys/dev/uart/uart_cpu_acpi.c

@@ -36,6 +36,8 @@ __FBSDID("$FreeBSD$");
 
 #include <machine/bus.h>
 
+#include <dev/pci/pcireg.h>
+
 #include <dev/uart/uart.h>
 #include <dev/uart/uart_bus.h>
 #include <dev/uart/uart_cpu.h>
@@ -182,6 +184,11 @@ uart_cpu_acpi_spcr(int devtype, struct uart_devinfo *di)
 		    (int)spcr->BaudRate);
 		goto out;
 	}
+	if (spcr->PciVendorId != PCIV_INVALID &&
+	    spcr->PciDeviceId != PCIV_INVALID) {
+		di->pci_info.vendor = spcr->PciVendorId;
+		di->pci_info.device = spcr->PciDeviceId;
+	}
 
 	/* Apply device tweaks. */
 	if ((cd->cd_quirks & UART_F_IGNORE_SPCR_REGSHFT) ==