[PATCH v5 2/3] iio: imu: inv_icm42600: log whoami mismatch instead of failing probe

A WHOAMI value differing from the one the compatible implies aborts
probe with -ENODEV, which prevents a register-compatible part described
with a fallback compatible from probing at all.

The devicetree compatible is authoritative for which part is fitted:
log the mismatch at info level and continue. The message keeps a
breadcrumb for anyone seeing an unexpected part change on their device
versus the devicetree; tmp117 takes the same trade at the same level
for its fallback matches.

A whoami of 0x00 or 0xff still fails probe: SPI has no transfer-level
ack, so those values mean nothing answered. Otherwise the probe outcome
on mismatch changes for all parts the driver supports. The check sits
before the mismatch comparison; it does not depend on hw->whoami, since
no supported part uses either value (the WHOAMI values the driver
defines span 0x40-0x5C). inv_icm45600 carries the same guard nested in
its mismatch branch.

Tested via a backport to a Fairphone 6 running a 7.1-based kernel: its
ICM-42630 (WHOAMI 0x0C), described with an icm42631 fallback compatible
and matched as icm42631, probes with the one informational line, and
accelerometer, gyroscope and temperature reads work.

Suggested-by: Conor Dooley <[email protected]>
Suggested-by: Jonathan Cameron <[email protected]>
Assisted-by: Claude:claude-fable-5
Signed-off-by: Jorijn van der Graaf <[email protected]>
---
v5: the no-response guard moves before the whoami-mismatch comparison
as its own check (Andy); the four Link trailers are dropped (Andy,
Jonathan); retested on the device.
v4 kept the whoami read, logging the mismatch at info level
(Jonathan, Conor), and added inv_icm45600's no-response guard; both
prints use %#04x so the two ids render at equal width.
v3 dropped the check and the whoami definitions entirely.
New in v2, as a demotion of the error to a warning.

 .../iio/imu/inv_icm42600/inv_icm42600_core.c    | 17 ++++++++++++++---
 1 file changed, 14 insertions(+), 3 deletions(-)

diff --git a/drivers/iio/imu/inv_icm42600/inv_icm42600_core.c b/drivers/iio/imu/inv_icm42600/inv_icm42600_core.c
index dc97d8a274e3..be8cbecdd92e 100644
--- a/drivers/iio/imu/inv_icm42600/inv_icm42600_core.c
+++ b/drivers/iio/imu/inv_icm42600/inv_icm42600_core.c
@@ -8,6 +8,7 @@
 #include <linux/interrupt.h>
 #include <linux/irq.h>
 #include <linux/kernel.h>
+#include <linux/limits.h>
 #include <linux/module.h>
 #include <linux/mutex.h>
 #include <linux/pm_runtime.h>
@@ -509,10 +510,20 @@ static int inv_icm42600_setup(struct inv_icm42600_state *st,
 	ret = regmap_read(st->map, INV_ICM42600_REG_WHOAMI, &val);
 	if (ret)
 		return ret;
+
+	/*
+	 * SPI interface has no ack mechanism.
+	 * 0xFF or 0x00 whoami means no response from the device.
+	 */
+	if (val == U8_MAX || val == 0)
+		return dev_err_probe(dev, -ENODEV,
+				     "invalid whoami %#04x expected %#04x (%s)\n",
+				     val, hw->whoami, hw->name);
+
 	if (val != hw->whoami) {
-		dev_err(dev, "invalid whoami %#02x expected %#02x (%s)\n",
-			val, hw->whoami, hw->name);
-		return -ENODEV;
+		dev_info(dev,
+			 "device id %#04x is not the %#04x associated with the FW-specified device (%s), probably using a valid fallback compatible\n",
+			 val, hw->whoami, hw->name);
 	}
 	st->name = hw->name;
 
-- 
2.55.0