Re: [BUG] hwmon: (spd5118) resume fails -ENXIO on write protected hubs
From: Guenter Roeck
Date: Mon Aug 17 2026 - 13:44:05 EST
On 8/17/26 09:29, GG wrote:
Hi there,
I apologize in advance if this isn't the right place to send this. I ran into
a quirk on my own machine, went digging, and ended up working out what causes
it, so I figured it was probably worth sharing. Please feel free to ignore all
of this if it's not useful or doesn't apply.
Before you read any further, three things you should probably know:
- I'm not a programmer. I can tell you what my hardware does, and I ran
every test below myself on it, but if you ask me to defend the actual
implementation I'll be out of my depth pretty quickly.
- I worked this out with an AI coding assistant (Claude, claude-opus-5) over
a long back-and-forth debugging session. It suggested things to try and
wrote the instrumented test modules, and I built and ran each one on this
machine and fed the results back. It drafted this write-up too, and I've
edited it.
- So this isn't a formal patch submission and there's deliberately no
Signed-off-by on it. Treat it as a bug report that happens to come with a
suggestion attached, and please feel free to bin the patch entirely and do
something completely different.
The one thing I can offer that might actually be worth something is the
hardware. This machine reproduces the problem every single time, so I'm happy
to test whatever you'd like, in whatever form.
Anyway, here's what I found.
spd5118_resume() fails with -ENXIO on every single suspend/resume cycle here,
and I think it's because the SPD hubs on this board are write protected by the
platform firmware.
Hardware:
Gigabyte Z790 AORUS PRO X, BIOS F11a
Intel i9-14900KS
2x 32GB DDR5, hubs at i2c 12-0051 and 12-0053
Both report: DDR5 temperature sensor: vendor 0x06:0x32 revision 1.6
Kernel 7.1.8 (CachyOS)
This shows up on every resume:
spd5118 12-0051: Failed to write b = 0: -6
spd5118 12-0051: PM: dpm_run_callback(): spd5118_resume returns -6
spd5118 12-0051: PM: spd5118_resume returned -6 after 15085 usecs
spd5118 12-0051: PM: failed to resume async: error -6
The sensor itself works fine either side of a suspend and the configured
limits survive, so nothing is actually broken for me as a user. It's just the
error, plus 5-18ms of pointless failing SMBus traffic per DIMM every time.
The thing that finally made it click is that this has nothing to do with
suspend or resume at all. These hubs refuse writes all the time. Writing
temp1_max back to the exact value it already holds, on a fully booted and idle
system, fails the same way:
$ cat /sys/class/hwmon/hwmon3/temp1_max
55000
$ sudo sh -c 'echo 55000 > /sys/class/hwmon/hwmon3/temp1_max'
sh: line 1: echo: write error: No such device or address
Reads are completely unaffected. During resume, raw i2c_smbus_read_byte_data()
on MR11 and on every other register succeeds on the first try and returns
sensible values. Every single i2c_smbus_write_byte_data() comes back -ENXIO.
Which means this, in spd5118_suspend():
regmap_update_bits(regmap, SPD5118_REG_TEMP_CONFIG, SPD5118_TS_DISABLE,
SPD5118_TS_DISABLE);
is failing silently, because its return value isn't checked. The sensor never
actually gets disabled. But the cache still gets marked dirty, so on resume
regcache_sync() goes off to restore state that was never changed in the first
place.
The error message itself sent me down the wrong path for quite a while. It
doesn't come from the sync, it comes from regcache_sync()'s epilogue, which
force-writes every regmap range selector and then hands that write's error
back as its own return value. For spd5118 in I2C legacy mode that selector is
MR11, which is also a cached writeable register, and with window_start 0 and
window_len 0x100 it sits inside its own paging window. That's why the message
singles out register 0xb, which had me convinced for ages that there was
something specifically wrong with MR11.
Dead ends, in case it saves anyone else the time:
- Dropping MR11 from the regcache before the sync does silence the "Failed
to write b = 0" message, but resume still fails, because the next register
isn't writeable either.
- MR11 already reads back as 0 on resume, so there's no stale page.
- Reads succeed on the first attempt, so the hub isn't slow to wake up.
- Polling for a write to be accepted for up to 2 seconds never got one
through, and added 3 seconds to resume.
- Pinning i2c-i801 to power/control=on for the whole cycle changes nothing.
The adapter isn't marked suspended at that point either:
__i2c_check_suspended() (drivers/i2c/i2c-core.h) returns -ESHUTDOWN and
emits a "Transfer while suspended" dev_WARN, and neither of those shows up
anywhere in my logs.
The patch below is what ended up working for me here: check whether the device
actually accepted that suspend-time write, and if it didn't, don't bother
marking the cache dirty or syncing on resume. With it applied, resume returns
0 in 0us and the error is gone. I ran 5 suspend/resume cycles on a fresh boot
with no debug kernel options, and /sys/kernel/debug/suspend_stats went from
success=0 failed_resume=0 to success=5 failed_resume=0, with temperatures and
limits unaffected throughout.
One thing I can't check myself: I don't have any hardware with a hub that does
accept writes, so I can't confirm the normal path still behaves properly on a
machine like that.
Candidate change against drivers/hwmon/spd5118.c:
--- a/drivers/hwmon/spd5118.c
+++ b/drivers/hwmon/spd5118.c
@@ -79,6 +79,7 @@ struct spd5118_data {
struct regmap *regmap;
struct mutex nvmem_lock;
bool is_16bit;
+ bool suspend_wp; /* device NAKed the suspend write: nothing to restore */
};
@@ -498,10 +499,27 @@ static int spd5118_suspend(struct device *dev)
return err;
regcache_cache_bypass(regmap, true);
- regmap_update_bits(regmap, SPD5118_REG_TEMP_CONFIG, SPD5118_TS_DISABLE,
- SPD5118_TS_DISABLE);
+ err = regmap_update_bits(regmap, SPD5118_REG_TEMP_CONFIG,
+ SPD5118_TS_DISABLE, SPD5118_TS_DISABLE);
regcache_cache_bypass(regmap, false);
+ /*
+ * Platform firmware may write protect the SPD hub after memory
+ * training, in which case the device serves reads but NAKs every write
+ * with -ENXIO. The temperature sensor was then never disabled, so
+ * there is no device state to restore on resume.
+ */
+ data->suspend_wp = err < 0;
+ if (data->suspend_wp) {
+ dev_dbg(dev, "write protected, skipping state save (%d)\n", err);
+ /* Still avoid bus traffic while suspended. */
+ regcache_cache_only(regmap, true);
+ return 0;
+ }
+
regcache_cache_only(regmap, true);
regcache_mark_dirty(regmap);
@@ -514,6 +532,10 @@ static int spd5118_resume(struct device *dev)
struct regmap *regmap = data->regmap;
regcache_cache_only(regmap, false);
+
+ if (data->suspend_wp)
+ return 0;
+
return regcache_sync(regmap);
}
As far as I can tell this goes back to d1b4c755081a ("hwmon: (spd5118) Add
suspend/resume support"), which first shipped in v6.11.
Thanks for taking a look, and no worries at all if this isn't useful.
This isn't the only problem. NVMEM access does not work if the chip is write
protected, limits can not be set, and it is possible that the temperature
sensor data is not accessible anymore after a suspend/resume sequence.
The driver must be black-listed on affected systems.
There was supposed to be a patch series which would prevent the driver from
probing if the chip is write protected, but that needs to be determined
and reported by the I2C subsystem (or, more specifically, by the I2C controller)
and it was rejected. The driver could try to detect the write protect status
on its own by attempting a write, but that would be too risky.
So your only remedy is to black-list the driver.
Guenter