device: Add method to configure bus mastering based on Kconfig

The bus master bit is set at many places in coreboot's code, but the
reason for that is not quite clear. We examined not setting the
bus master bit whereever possible and tried booting without it,
which worked fine for internal PCI devices but not for PCIe. As a PCIe
device we used a Samsung M.2 NVMe SSD.

For security reasons, we would like to disable bus mastering where
possible. Depending on the device, bus mastering might get enabled
by the operating system (e.g. for iGPU) and it might be required for
some devices to work properly. However, the idea is to leave it disabled
and configure the IOMMU first before enabling it.

To have some sort of "backwards compatibility", add a method which
configures bus mastering based on an additional config option. Since
CB:42460 makes usage of this treewide, enable it by default to keep the
current behaviour for now.

Tested with Siemens/Chili, a Coffee Lake based platform.

Change-Id: I876c48ea3fb4f9cf7b6a5c2dcaeda07ea36cbed3
Signed-off-by: Felix Singer <felix.singer@secunet.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/42459
Reviewed-by: Nico Huber <nico.h@gmx.de>
Reviewed-by: Subrata Banik <subrata.banik@intel.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/device/Kconfig

@@ -519,6 +519,19 @@ config PCIEXP_PLUGIN_SUPPORT
 	bool
 	default y
 
+config PCI_ALLOW_BUS_MASTER
+	bool "Allow PCI bus master bit to be enabled by coreboot"
+	default y
+	help
+	  For security reasons, bus mastering should be enabled as late as
+	  possible. In coreboot, it's usually not necessary and payloads
+	  should only enable it for devices they use. Since not all payloads
+	  enable bus mastering properly yet, this option gives some sort of
+	  "backwards compatibility" and is enabled by default to keep the
+	  traditional behaviour for now. This is currently necessary, for
+	  instance, for libpayload based payloads as the drivers don't enable
+	  bus mastering for PCI bridges.
+
 endif # PCI
 
 if PCIEXP_PLUGIN_SUPPORT

src/include/device/pci.h

@@ -128,6 +128,18 @@ static inline int pci_base_address_is_memory_space(unsigned int attr)
 }
 
 void pci_dev_disable_bus_master(const struct device *dev);
+
+static __always_inline
+#if ENV_PCI_SIMPLE_DEVICE
+void pci_dev_request_bus_master(pci_devfn_t dev)
+#else
+void pci_dev_request_bus_master(const struct device *dev)
+#endif /* ENV_PCI_SIMPLE_DEVICE */
+{
+	if (CONFIG(PCI_ALLOW_BUS_MASTER))
+		pci_or_config16(dev, PCI_COMMAND, PCI_COMMAND_MASTER);
+}
+
 #endif /* CONFIG_PCI */
 
 void pci_early_bridge_init(void);