Add configurable ramstage support for minimal PCI scanning

This CL has changes that allow us to enable a configurable
ramstage, and one change that allows us to minimize PCI
scanning. Minimal scanning is a frequently requested feature.

To enable it, we add two new variables to src/Kconfig
CONFIGURABLE_RAMSTAGE
is the overall variable controlling other options for minimizing the
ramstage.

MINIMAL_PCI_SCANNING is how we indicate we wish to enable minimal
PCI scanning.

Some devices must be scanned in all cases, such as 0:0.0.

To indicate which devices we must scan, we add a new mandatory
keyword to sconfig

It is used in place of on, off, or hidden, and indicates
a device is enabled and mandatory. Mandatory
devices are always scanned. When MINIMAL_PCI_SCANNING is enabled,
ONLY mandatory devices are scanned.

We further add support in src/device/pci_device.c to manage
both MINIMAL_PCI_SCANNING and mandatory devices.

Finally, to show how this works in practice, we add mandatory
keywords to 3 devices on the qemu-q35.

TEST=
1. This is tested and working on the qemu-q35 target.
2. On CML-Hatch

Before CL:
Total Boot time: ~685ms

After CL:
Total Boot time: ~615ms

Change-Id: I2073d9f8e9297c2b02530821ebb634ea2a5c758e
Signed-off-by: Ronald G. Minnich <rminnich@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/36221
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Jeremy Soller <jeremy@system76.com>

src/Kconfig

@@ -354,6 +354,21 @@ config RAMPAYLOAD
 	  Skip PCI enumeration logic and only allocate BAR for fixed devices
 	  (bootable devices, TPM over GSPI).
 
+config CONFIGURABLE_RAMSTAGE
+	bool "Enable a configurable ramstage."
+	default y if ARCH_X86
+	help
+	  A configurable ramstage allows you to select which parts of the ramstage
+	  to run. Currently, we can only select a minimal PCI scanning step.
+	  The minimal PCI scanning will only check those parts that are enabled
+	  in the devicetree.cb. By convention none of those devices should be bridges.
+
+config MINIMAL_PCI_SCANNING
+	bool "Enable minimal PCI scanning"
+	depends on CONFIGURABLE_RAMSTAGE
+	help
+	  If this option is enabled, coreboot will scan only devices
+	  marked as mandatory in devicetree.cb
 endmenu
 
 menu "Mainboard"

src/device/pci_device.c

@@ -1195,6 +1195,12 @@ void pci_scan_bus(struct bus *bus, unsigned int min_devfn,
 	 * non-existence and single function devices.
 	 */
 	for (devfn = min_devfn; devfn <= max_devfn; devfn++) {
+		if (CONFIG(MINIMAL_PCI_SCANNING)) {
+			dev = pcidev_path_behind(bus, devfn);
+			if (!dev || !dev->mandatory)
+				continue;
+		}
+
 		/* First thing setup the device structure. */
 		dev = pci_scan_get_dev(bus, devfn);
 

src/include/device/device.h

@@ -119,7 +119,10 @@ struct device {
 	unsigned int  initialized : 1; /* 1 if we have initialized the device */
 	unsigned int    on_mainboard : 1;
 	unsigned int    disable_pcie_aspm : 1;
-	unsigned int    hidden : 1;	/* set if we should hide from UI */
+	/* set if we should hide from UI */
+	unsigned int    hidden : 1;
+	/* set if this device is used even in minimum PCI cases */
+	unsigned int    mandatory : 1;
 	u8 command;
 	uint16_t hotplug_buses; /* Number of hotplug buses to allocate */
 

src/mainboard/emulation/qemu-q35/devicetree.cb

@@ -5,10 +5,10 @@ chip mainboard/emulation/qemu-q35
 		end
 	end
 	device domain 0 on
-		device pci 0.0 on end		# northbridge (q35)
+		device pci 0.0 mandatory end		# northbridge (q35)
 		chip southbridge/intel/i82801ix
 			# present unconditionally
-			device pci 1f.0 on end	# LPC
+			device pci 1f.0 mandatory end	# LPC
 			device pci 1f.2 on end	# SATA
 			device pci 1f.3 on end	# SMBus