pci: provide pci_enable_defaults()

One helper function to set/clear specific bit in PCI_COMMAND register is
introduced. Then, provide a function to do most of the common PCI init
work.
Suggested-by: Andrew Jones <drjones@redhat.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

lib/pci.c

@@ -7,6 +7,18 @@
 #include "pci.h"
 #include "asm/pci.h"
 
+void pci_cmd_set_clr(struct pci_dev *dev, uint16_t set, uint16_t clr)
+{
+	uint16_t val = pci_config_readw(dev->bdf, PCI_COMMAND);
+
+	/* No overlap is allowed */
+	assert((set & clr) == 0);
+	val |= set;
+	val &= ~clr;
+
+	pci_config_writew(dev->bdf, PCI_COMMAND, val);
+}
+
 bool pci_dev_exists(pcidevaddr_t dev)
 {
 	return (pci_config_readw(dev, PCI_VENDOR_ID) != 0xffff &&
@@ -246,3 +258,10 @@ void pci_scan_bars(struct pci_dev *dev)
 		}
 	}
 }
+
+void pci_enable_defaults(struct pci_dev *dev)
+{
+	pci_scan_bars(dev);
+	/* Enable device DMA operations */
+	pci_cmd_set_clr(dev, PCI_COMMAND_MASTER, 0);
+}

lib/pci.h

@@ -25,6 +25,8 @@ struct pci_dev {
 
 extern void pci_dev_init(struct pci_dev *dev, pcidevaddr_t bdf);
 extern void pci_scan_bars(struct pci_dev *dev);
+extern void pci_cmd_set_clr(struct pci_dev *dev, uint16_t set, uint16_t clr);
+extern void pci_enable_defaults(struct pci_dev *dev);
 
 extern bool pci_probe(void);
 extern void pci_print(void);