[PATCH 3/4] amba: support masking data abort during identification

[Ahmad Fatoum <a.fatoum@pengutronix.de>]

Registration of an AMBA device requires reading its pid and cid
registers after turning on the device. This is the first I/O done to the
device and may end up crashing if the device tree doesn't describe
the correct clock and power domains or if a previous boot stage
should've enabled them, but didn't.

This is not a likely issue, but when it happens it crashes the system
annoyingly early during device tree populating.

Add a symbol that masks the data abort exception around the
identification code to turn the crash into an error message instead.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
---
 drivers/amba/Kconfig | 19 +++++++++++++++++++
 drivers/amba/bus.c   | 12 +++++++++++-
 2 files changed, 30 insertions(+), 1 deletion(-)

diff --git a/drivers/amba/Kconfig b/drivers/amba/Kconfig
index 444d4ce7435c..be5e053f4b84 100644
--- a/drivers/amba/Kconfig
+++ b/drivers/amba/Kconfig
@@ -1,3 +1,22 @@
 # SPDX-License-Identifier: GPL-2.0-only
 config ARM_AMBA
 	bool
+
+config ARM_AMBA_DABT_MASK
+	bool
+	prompt "Mask data abort while identifying AMBA devices" if COMPILE_TEST
+	depends on ARM_AMBA && ARCH_HAS_DATA_ABORT_MASK
+	help
+	  Registration of an AMBA device requires reading its pid and cid
+	  registers after turning on the device, which can involve enabling
+	  clocks, deasserting resets and powering on power domains.
+	  In some cases, these resources are not available to barebox running
+	  in the normal world and thus need to be setup by a previous
+	  stage bootloader running in the secure world.
+	  If such setup doesn't happen, accessing devices may lead to
+	  a data abort or complete system hang.
+
+	  As a workaround for the former case, this option can be enabled
+	  to gracefully handle data aborts during readout of the PID/CID
+	  registers. The proper solution is fixing your first stage
+	  bootloader or allow barebox access to the missing resources.
diff --git a/drivers/amba/bus.c b/drivers/amba/bus.c
index 6463366fafc5..5ed8336e6124 100644
--- a/drivers/amba/bus.c
+++ b/drivers/amba/bus.c
@@ -10,6 +10,7 @@
 #include <linux/amba/bus.h>
 #include <io.h>
 #include <init.h>
+#include <abort.h>
 
 #define to_amba_driver(d)	container_of(d, struct amba_driver, drv)
 
@@ -132,6 +133,9 @@ int amba_device_add(struct amba_device *dev)
 	if (ret == 0) {
 		u32 pid, cid;
 
+		if (IS_ENABLED(CONFIG_ARM_AMBA_DABT_MASK))
+			data_abort_mask();
+
 		/*
 		 * Read pid and cid based on size of resource
 		 * they are located at end of region
@@ -139,8 +143,14 @@ int amba_device_add(struct amba_device *dev)
 		pid = amba_device_get_pid(tmp, size);
 		cid = amba_device_get_cid(tmp, size);
 
-		if (cid == AMBA_CID)
+		if (IS_ENABLED(CONFIG_ARM_AMBA_DABT_MASK) && data_abort_unmask()) {
+			dev_err(amba_bustype.dev,
+				"data abort during MMIO read of PID/CID for %pOF\n",
+				dev->dev.of_node);
+			ret = -EFAULT;
+		} else if (cid == AMBA_CID) {
 			dev->periphid = pid;
+		}
 
 		if (!dev->periphid)
 			ret = -ENODEV;
-- 
2.39.5