hw/pci-host/gt64120: Fix endianness handling

The GT-64120 PCI controller requires special handling where:
1. Host bridge(bus 0 ,device 0) must never be byte-swapped
2. Other devices follow MByteSwap bit in GT_PCI0_CMD

The previous implementation incorrectly  swapped all accesses, breaking
host bridge detection (lspci -d 11ab:4620).

Changes made:
1. Removed gt64120_update_pci_cfgdata_mapping() and moved data_mem initialization
  to gt64120_realize() for cleaner setup
2. Implemented custom read/write handlers that:
   - Preserve host bridge accesses (extract32(config_reg,11,13)==0)
   - apply swapping only for non-bridge devices in big-endian mode

Fixes: 145e2198 ("hw/mips/gt64xxx_pci: Endian-swap using PCI_HOST_BRIDGE MemoryRegionOps")
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2826

Signed-off-by: Rakesh Jeyasingh <rakeshjb010@gmail.com>
Tested-by: Thomas Huth <thuth@redhat.com>
Link: https://lore.kernel.org/r/20250429170354.150581-2-rakeshjb010@gmail.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
(cherry picked from commit e5894fd6f411c113e2b5f62811e96eeb5b084381)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

hw/pci-host/gt64120.c

@@ -320,38 +320,6 @@ static void gt64120_isd_mapping(GT64120State *s)
     memory_region_transaction_commit();
 }
 
-static void gt64120_update_pci_cfgdata_mapping(GT64120State *s)
-{
-    /* Indexed on MByteSwap bit, see Table 158: PCI_0 Command, Offset: 0xc00 */
-    static const MemoryRegionOps *pci_host_data_ops[] = {
-        &pci_host_data_be_ops, &pci_host_data_le_ops
-    };
-    PCIHostState *phb = PCI_HOST_BRIDGE(s);
-
-    memory_region_transaction_begin();
-
-    /*
-     * The setting of the MByteSwap bit and MWordSwap bit in the PCI Internal
-     * Command Register determines how data transactions from the CPU to/from
-     * PCI are handled along with the setting of the Endianness bit in the CPU
-     * Configuration Register. See:
-     * - Table 16: 32-bit PCI Transaction Endianness
-     * - Table 158: PCI_0 Command, Offset: 0xc00
-     */
-
-    if (memory_region_is_mapped(&phb->data_mem)) {
-        memory_region_del_subregion(&s->ISD_mem, &phb->data_mem);
-        object_unparent(OBJECT(&phb->data_mem));
-    }
-    memory_region_init_io(&phb->data_mem, OBJECT(phb),
-                          pci_host_data_ops[s->regs[GT_PCI0_CMD] & 1],
-                          s, "pci-conf-data", 4);
-    memory_region_add_subregion_overlap(&s->ISD_mem, GT_PCI0_CFGDATA << 2,
-                                        &phb->data_mem, 1);
-
-    memory_region_transaction_commit();
-}
-
 static void gt64120_pci_mapping(GT64120State *s)
 {
     memory_region_transaction_begin();
@@ -645,7 +613,6 @@ static void gt64120_writel(void *opaque, hwaddr addr,
     case GT_PCI0_CMD:
     case GT_PCI1_CMD:
         s->regs[saddr] = val & 0x0401fc0f;
-        gt64120_update_pci_cfgdata_mapping(s);
         break;
     case GT_PCI0_TOR:
     case GT_PCI0_BS_SCS10:
@@ -1024,6 +991,48 @@ static const MemoryRegionOps isd_mem_ops = {
     },
 };
 
+static bool bswap(const GT64120State *s) 
+{
+    PCIHostState *phb = PCI_HOST_BRIDGE(s);
+    /*check for bus == 0 && device == 0, Bits 11:15 = Device , Bits 16:23 = Bus*/
+    bool is_phb_dev0 = extract32(phb->config_reg, 11, 13) == 0;
+    bool le_mode = FIELD_EX32(s->regs[GT_PCI0_CMD], GT_PCI0_CMD, MByteSwap);
+    /* Only swap for non-bridge devices in big-endian mode */
+    return !le_mode && !is_phb_dev0;
+}
+
+static uint64_t gt64120_pci_data_read(void *opaque, hwaddr addr, unsigned size)
+{
+    GT64120State *s = opaque;
+    uint32_t val = pci_host_data_le_ops.read(opaque, addr, size);
+
+    if (bswap(s)) {
+        val = bswap32(val);
+    }
+    return val;
+}
+
+static void gt64120_pci_data_write(void *opaque, hwaddr addr, 
+    uint64_t val, unsigned size)
+{
+    GT64120State *s = opaque;
+
+    if (bswap(s)) {
+        val = bswap32(val); 
+    }
+    pci_host_data_le_ops.write(opaque, addr, val, size);  
+}
+
+static const MemoryRegionOps gt64120_pci_data_ops = {
+    .read = gt64120_pci_data_read,
+    .write = gt64120_pci_data_write,
+    .endianness = DEVICE_LITTLE_ENDIAN,
+    .valid = {
+        .min_access_size = 4,
+        .max_access_size = 4,
+    },
+};
+
 static void gt64120_reset(DeviceState *dev)
 {
     GT64120State *s = GT64120_PCI_HOST_BRIDGE(dev);
@@ -1178,7 +1187,6 @@ static void gt64120_reset(DeviceState *dev)
 
     gt64120_isd_mapping(s);
     gt64120_pci_mapping(s);
-    gt64120_update_pci_cfgdata_mapping(s);
 }
 
 static void gt64120_realize(DeviceState *dev, Error **errp)
@@ -1202,6 +1210,12 @@ static void gt64120_realize(DeviceState *dev, Error **errp)
     memory_region_add_subregion_overlap(&s->ISD_mem, GT_PCI0_CFGADDR << 2,
                                         &phb->conf_mem, 1);
 
+    memory_region_init_io(&phb->data_mem, OBJECT(phb),
+                          &gt64120_pci_data_ops,
+                          s, "pci-conf-data", 4);
+    memory_region_add_subregion_overlap(&s->ISD_mem, GT_PCI0_CFGDATA << 2,
+                                        &phb->data_mem, 1);
+
 
     /*
      * The whole address space decoded by the GT-64120A doesn't generate