[PATCH 1/2] iio: magnetometer: mmc35240: leave cache-only mode before the cache sync

[Shashank Mohan Jain]

mmc35240_resume() marks the register cache dirty and calls
regcache_sync_region() while the regmap is still in cache-only mode.
It calls regcache_cache_only(false) only after the sync.

Since commit afc8e3ee8307 ("regcache: Validate cache_only state in
regcache_sync_region()") a sync in cache-only mode is refused with a
WARN and -EINVAL. Every resume now gives

  WARNING: drivers/base/regmap/regcache.c:536 at regcache_sync_region+0x1a4/0x1c0
  ...
   mmc35240_resume+0x30/0x70
   dpm_run_callback+0x7d/0x1c0
   device_resume+0x175/0x270
  ...
  mmc35240 1-0030: Failed to restore control registers

and the control registers are not written. If the chip lost its state
during suspend, it runs with the reset value of CTRL1 while the cache,
and with it the sampling frequency attribute, still has the configured
value. Writing the same frequency again does not repair this, because
regmap_update_bits() finds no difference to the cache.

Leave cache-only mode first, then mark the cache dirty and sync it.

Reproduced in qemu with the driver bound to an i2c-stub device: set
in_magn_sampling_frequency to 25 (CTRL1 = 0x02), clear CTRL0 and CTRL1
of the stub behind the driver's back to emulate the power loss, and run
a suspend/resume cycle with "devices" in /sys/power/pm_test. Without
this change CTRL1 is still 0x00 after resume and the warning is
printed. With it, and also with the regcache commit reverted, CTRL1 is
0x02 again and there is no warning. Not tested on real hardware.

Fixes: afc8e3ee8307 ("regcache: Validate cache_only state in regcache_sync_region()")
Assisted-by: LLM
Signed-off-by: Shashank Mohan Jain <jain.sm@xxxxxxxxx>
---
Prepared with Claude Code (Anthropic), model Claude Opus 5.5
(claude-opus-5-5).

 drivers/iio/magnetometer/mmc35240.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/drivers/iio/magnetometer/mmc35240.c b/drivers/iio/magnetometer/mmc35240.c
index 6f40cba4b1..7f028d7f1f 100644
--- a/drivers/iio/magnetometer/mmc35240.c
+++ b/drivers/iio/magnetometer/mmc35240.c
@@ -532,14 +532,13 @@ static int mmc35240_resume(struct device *dev)
 	struct mmc35240_data *data = iio_priv(indio_dev);
 	int ret;
 
+	regcache_cache_only(data->regmap, false);
 	regcache_mark_dirty(data->regmap);
 	ret = regcache_sync_region(data->regmap, MMC35240_REG_CTRL0,
 				   MMC35240_REG_CTRL1);
 	if (ret < 0)
 		dev_err(dev, "Failed to restore control registers\n");
 
-	regcache_cache_only(data->regmap, false);
-
 	return 0;
 }
 
-- 
2.43.0