Subject: [PATCH] PCI: Fix endianness issues in pci_bus_read_config()

From: Gerd Bayer <[email-masked]>
Date: Thu, 31 Jul 2025 19:38:58 +0200

Simple pointer-casts to map byte and word reads from PCI config space
into dwords (i.e. u32) produce unintended results on big-endian systems.
Add the necessary adjustments under compile-time switch
CONFIG_CPU_BIG_ENDIAN.

pci_bus_read_config() was just introduced with
https://lore.kernel.org/all/[email-masked]/

Signed-off-by: Gerd Bayer <[email-masked]>
---

Hi Hans, hi Bjorn,

Sorry to spill this endianness aware code into drivers/pci, feel free to
suggest a cleaner approach. This has fixed the issues seen on s390 systems
Otherwise it is just compile-tested for x86 and arm64.

Since this is still sitting in the a pull-request for upstream, I'm not sure if this
warrants a Fixes: tag.

Thanks,
Gerd
---
 drivers/pci/access.c | 25 +++++++++++++++++--------
 1 file changed, 17 insertions(+), 8 deletions(-)

diff --git a/drivers/pci/access.c b/drivers/pci/access.c
index ba66f55d2524..77a73b772a28 100644
--- a/drivers/pci/access.c
+++ b/drivers/pci/access.c
@@ -89,15 +89,24 @@ int pci_bus_read_config(void *priv, unsigned int devfn, int where, u32 size,
 			u32 *val)
 {
 	struct pci_bus *bus = priv;
+	int rc;
 
-	if (size == 1)
-		return pci_bus_read_config_byte(bus, devfn, where, (u8 *)val);
-	else if (size == 2)
-		return pci_bus_read_config_word(bus, devfn, where, (u16 *)val);
-	else if (size == 4)
-		return pci_bus_read_config_dword(bus, devfn, where, val);
-	else
-		return PCIBIOS_BAD_REGISTER_NUMBER;
+	if (size == 1) {
+		rc = pci_bus_read_config_byte(bus, devfn, where, (u8 *)val);
+#if (IS_ENABLED(CONFIG_CPU_BIG_ENDIAN))
+		*val = ((*val >> 24) & 0xff);
+#endif
+	} else if (size == 2) {
+		rc = pci_bus_read_config_word(bus, devfn, where, (u16 *)val);
+#if (IS_ENABLED(CONFIG_CPU_BIG_ENDIAN))
+		*val = ((*val >> 16) & 0xffff);
+#endif
+	} else if (size == 4) {
+		rc = pci_bus_read_config_dword(bus, devfn, where, val);
+	} else {
+		rc =  PCIBIOS_BAD_REGISTER_NUMBER;
+	}
+	return rc;
 }
 
 int pci_generic_config_read(struct pci_bus *bus, unsigned int devfn,
-- 
2.48.1