[BUG/PATCH] Platform/RaspberryPi: Only enable IORT when 3G limit is disabled.

["Jeremy Linton" <jeremy.linton@arm.com>]

The 3G limit, and the 2G IORT are intended to solve
the same linux problem, and limit PCI DMA operations
to the first 3G of RAM. Older linux kernels, as
used with RHEL/Centos, trigger an assertion*
when a DMA operation starts that doesn't fit within
the 2G range specified by the IORT.

The simple solution is to only enable the IORT
when the 3G flag is disabled.

* https://github.com/pftf/RPi4/issues/123

Fixes: dac891da5cf3 ("Platform/RaspberryPi/AcpiTables: add a IORT ACPI table to limit XHCI DMA")
Signed-off-by: Jeremy Linton <jeremy.linton@arm.com>
---
 Platform/RaspberryPi/Drivers/ConfigDxe/ConfigDxe.c | 26 +++++++++++++++++-----
 1 file changed, 21 insertions(+), 5 deletions(-)

diff --git a/Platform/RaspberryPi/Drivers/ConfigDxe/ConfigDxe.c b/Platform/RaspberryPi/Drivers/ConfigDxe/ConfigDxe.c
index 19ef950f10..9581bc41e1 100644
--- a/Platform/RaspberryPi/Drivers/ConfigDxe/ConfigDxe.c
+++ b/Platform/RaspberryPi/Drivers/ConfigDxe/ConfigDxe.c
@@ -217,13 +217,23 @@ SetupVariables (
     ASSERT_EFI_ERROR (Status);
   }
 
-  if (mModelFamily >= 4 && mModelInstalledMB > 3 * 1024) {
+  if (mModelFamily >= 4) {
+    if (mModelInstalledMB > 3 * 1024) {
+      /*
+       * This allows changing PcdRamLimitTo3GB in forms.
+       */
+      Status = PcdSet32S (PcdRamMoreThan3GB, 1);
+      ASSERT_EFI_ERROR (Status);
+    } else {
+      Status = PcdSet32S (PcdRamMoreThan3GB, 0);
+      ASSERT_EFI_ERROR (Status);
+    }
+
     /*
-     * This allows changing PcdRamLimitTo3GB in forms.
+     * Enable the "3G ram limit" on 2GB device because we
+     * are going to utlize that limit as a new/old kernel
+     * flag and also disable the IORT if the 3G limit is set.
      */
-    Status = PcdSet32S (PcdRamMoreThan3GB, 1);
-    ASSERT_EFI_ERROR (Status);
-
     Size = sizeof (UINT32);
     Status = gRT->GetVariable (L"RamLimitTo3GB",
                                &gConfigDxeFormSetGuid,
@@ -755,6 +765,12 @@ HandleDynamicNamespace (
     DEBUG ((DEBUG_ERROR, "Found namespace table not in table list.\n"));
 
     return FALSE;
+  case SIGNATURE_32 ('I', 'O', 'R', 'T'):
+    // only enable the IORT on machines with >3G and no limit
+    // to avoid problems with rhel/centos
+    if (PcdGet32 (PcdRamLimitTo3GB)) {
+      return FALSE;
+    }
   }
 
   return TRUE;
-- 
2.13.7