[PATCH] IntelSiliconPkg IntelVTdDxe: Optimize when func 0 is not implemented

[Star Zeng <star.zeng@intel.com>]

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=1169

PCI spec:
They are also required to always implement function 0 in the device.
Implementing other functions is optional and may be assigned in any
order (i.e., a two-function device must respond to function 0 but
can choose any of the other possible function numbers (1-7) for the
second function).

This patch updates ScanPciBus() to not scan other functions if
function 0 is not implemented.

Test done:
Added debug code below in the second loop of ScanPciBus(),
compared the debug logs with and without this patch, many
non-0 unimplemented functions are skipped correctly.

  DEBUG ((
    DEBUG_INFO,
    "%a() B%02xD%02xF%02x VendorId: %04x DeviceId: %04x\n",
    __FUNCTION__,
    Bus,
    Device,
    Function,
    VendorID,
    DeviceID
    ));

Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Rangasai V Chaganty <rangasai.v.chaganty@intel.com>
Cc: Tomson Chang <tomson.chang@intel.com>
Cc: Jenny Huang <jenny.huang@intel.com>
Cc: Amy Chan <amy.chan@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Star Zeng <star.zeng@intel.com>
---
 IntelSiliconPkg/Feature/VTd/IntelVTdDxe/PciInfo.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/IntelSiliconPkg/Feature/VTd/IntelVTdDxe/PciInfo.c b/IntelSiliconPkg/Feature/VTd/IntelVTdDxe/PciInfo.c
index 36750b3f1d9c..305995de032c 100644
--- a/IntelSiliconPkg/Feature/VTd/IntelVTdDxe/PciInfo.c
+++ b/IntelSiliconPkg/Feature/VTd/IntelVTdDxe/PciInfo.c
@@ -1,6 +1,6 @@
 /** @file
 
-  Copyright (c) 2017, Intel Corporation. All rights reserved.<BR>
+  Copyright (c) 2017 - 2018, Intel Corporation. All rights reserved.<BR>
   This program and the accompanying materials
   are licensed and made available under the terms and conditions of the BSD License
   which accompanies this distribution.  The full text of the license may be found at
@@ -247,6 +247,12 @@ ScanPciBus (
       VendorID  = PciSegmentRead16 (PCI_SEGMENT_LIB_ADDRESS(Segment, Bus, Device, Function, PCI_VENDOR_ID_OFFSET));
       DeviceID  = PciSegmentRead16 (PCI_SEGMENT_LIB_ADDRESS(Segment, Bus, Device, Function, PCI_DEVICE_ID_OFFSET));
       if (VendorID == 0xFFFF && DeviceID == 0xFFFF) {
+        if (Function == 0) {
+          //
+          // If function 0 is not implemented, do not scan other functions.
+          //
+          break;
+        }
         continue;
       }
 
-- 
2.7.0.windows.1