[PATCH v2 2/2] pci: imx6: support kernels built in Thumb-2 mode

[Stefan Agner <stefan@agner.ch>]

Add a fault handler which handles immediate reads in Thumb-2
mode. Install the appropriate handler depending on which mode
the kernel has been built. This avoids an "Unhandled fault:
external abort on non-linefetch (0x1008) at 0xf0a80000"
during boot on a device with a PCIe switch connected.

Link: https://lore.kernel.org/linux-pci/20181126161645.8177-1-stefan@agner.ch/
Signed-off-by: Stefan Agner <stefan@agner.ch>
---
Changes since v1:
- Added Thumb-2 32-bit instruction support (tested by inserting .w
  instructions in arch/arm/include/asm/io.h)
- Avoid dereferencing if fault happened in user mode

 drivers/pci/controller/dwc/pci-imx6.c | 59 ++++++++++++++++++++++++++-
 1 file changed, 58 insertions(+), 1 deletion(-)

diff --git a/drivers/pci/controller/dwc/pci-imx6.c b/drivers/pci/controller/dwc/pci-imx6.c
index 54a29e441303..a9bbbf176c4a 100644
--- a/drivers/pci/controller/dwc/pci-imx6.c
+++ b/drivers/pci/controller/dwc/pci-imx6.c
@@ -29,6 +29,7 @@
 #include <linux/reset.h>
 #include <linux/pm_domain.h>
 #include <linux/pm_runtime.h>
+#include <asm/opcodes.h>
 
 #include "pcie-designware.h"
 
@@ -305,6 +306,59 @@ static int imx6q_pcie_abort_handler(unsigned long addr,
 	return 1;
 }
 
+static int imx6q_pcie_abort_handler_thumb2(unsigned long addr,
+		unsigned int fsr, struct pt_regs *regs)
+{
+	unsigned long pc = instruction_pointer(regs);
+	unsigned long instr;
+
+	if (user_mode(regs))
+		return 1;
+
+	instr = __mem_to_opcode_thumb32(*(unsigned long *)pc);
+
+	if (__opcode_is_thumb32(instr)) {
+		/* Load word/byte and halfword immediate offset */
+		if ((instr & 0xff100000UL) == 0xf8100000UL) {
+			int reg = (instr >> 12) & 0xf;
+			unsigned long val;
+
+			if ((instr & 0x00700000UL) == 0x00100000UL)
+				val = 0xff;
+			else if ((instr & 0x00700000UL) == 0x00300000UL)
+				val = 0xffff;
+			else
+				val = 0xffffffffUL;
+
+			regs->uregs[reg] = val;
+			regs->ARM_pc += 4;
+			return 0;
+		}
+	} else {
+		instr = __mem_to_opcode_thumb16(*(unsigned long *)pc);
+
+		/* Load word/byte and halfword immediate offset */
+		if (((instr & 0xe800) == 0x6800) ||
+		    ((instr & 0xf800) == 0x8800)) {
+			int reg = instr & 0x7;
+			unsigned long val;
+
+			if (instr & 0x1000)
+				val = 0xff;
+			else if (instr & 0x8000)
+				val = 0xffff;
+			else
+				val = 0xffffffffUL;
+
+			regs->uregs[reg] = val;
+			regs->ARM_pc += 2;
+			return 0;
+		}
+	}
+
+	return 1;
+}
+
 static int imx6_pcie_attach_pd(struct device *dev)
 {
 	struct imx6_pcie *imx6_pcie = dev_get_drvdata(dev);
@@ -1075,6 +1129,8 @@ static struct platform_driver imx6_pcie_driver = {
 
 static int __init imx6_pcie_init(void)
 {
+	bool thumb2 = IS_ENABLED(CONFIG_THUMB2_KERNEL);
+
 	/*
 	 * Since probe() can be deferred we need to make sure that
 	 * hook_fault_code is not called after __init memory is freed
@@ -1082,7 +1138,8 @@ static int __init imx6_pcie_init(void)
 	 * we can install the handler here without risking it
 	 * accessing some uninitialized driver state.
 	 */
-	hook_fault_code(8, imx6q_pcie_abort_handler, SIGBUS, 0,
+	hook_fault_code(8, thumb2 ? imx6q_pcie_abort_handler_thumb2 :
+			imx6q_pcie_abort_handler, SIGBUS, 0,
 			"external abort on non-linefetch");
 
 	return platform_driver_register(&imx6_pcie_driver);
-- 
2.19.1