Disable decoding of BARs by devices before we trash the value in the BAR

by writing all 1's to it to determine its length.  This fixes issues with
MCFG on at least some machines where a trashed BAR claimed subsequent
attempts at PCI config transactions because the addresses in the MCFG
window fell in the decoding range of the BAR.

In general it is a bad idea to leave the BARs enabled while we are
frobbing with them in this manner.

Sleuthing by:  tegge
MFC after:     1 week

1 files changed, 18 insertions, 0 deletions

diff --git a/sys/dev/pci/pci.c b/sys/dev/pci/pci.c
index 295e68d..a027e99 100644
--- a/sys/dev/pci/pci.c
+++ b/sys/dev/pci/pci.c
@@ -2291,9 +2291,27 @@ pci_add_map(device_t pcib, device_t bus, device_t dev,
 	struct resource *res;
 
 	map = PCIB_READ_CONFIG(pcib, b, s, f, reg, 4);
+
+	/*
+	 * Disable decoding via the command register before
+	 * determining the BARs length since we will be placing them
+	 * in a weird state.
+	 */
+	cmd = PCIB_READ_CONFIG(pcib, b, s, f, PCIR_COMMAND, 2);
+	PCIB_WRITE_CONFIG(pcib, b, s, f, PCIR_COMMAND,
+	    cmd & ~(PCI_BAR_MEM(map) ? PCIM_CMD_MEMEN : PCIM_CMD_PORTEN), 2);
+
+	/*
+	 * Determine the BAR's length by writing all 1's.  The bottom
+	 * log_2(size) bits of the BAR will stick as 0 when we read
+	 * the value back.
+	 */
 	PCIB_WRITE_CONFIG(pcib, b, s, f, reg, 0xffffffff, 4);
 	testval = PCIB_READ_CONFIG(pcib, b, s, f, reg, 4);
+
+	/* Restore the BAR and command register. */
 	PCIB_WRITE_CONFIG(pcib, b, s, f, reg, map, 4);
+	PCIB_WRITE_CONFIG(pcib, b, s, f, PCIR_COMMAND, cmd, 2);
 
 	if (PCI_BAR_MEM(map))
 		type = SYS_RES_MEMORY;