[PATCH RFC V2 RESEND] nvme: make providing NGUID as UUID usage less scary

[AlanCui4080 <me@alancui.cc>]

Since we decide to show the prp passthrough usage warning at device
initial discovery stage and degrade it to a informational message,
here is a warning that I think are essentially the same.

grep -C3 -rn "dev_warn_once" ./drivers/nvme/*
```
./drivers/nvme/host/sysfs.c-147-         * we have no UUID set
./drivers/nvme/host/sysfs.c-148-         */
./drivers/nvme/host/sysfs.c-149-        if (uuid_is_null(&ids->uuid)) {
./drivers/nvme/host/sysfs.c:150:                dev_warn_once(dev,
./drivers/nvme/host/sysfs.c-151-                        "No UUID available providing old NGUID\n");
./drivers/nvme/host/sysfs.c-152-                return sysfs_emit(buf, "%pU\n", ids->nguid);
./drivers/nvme/host/sysfs.c-153-        }
```

This warning will only complain for the first partition of the first device also.
And for NVMe devices, according to the NVM-Express-1_4 specification p.175 Figure 251:
"Bit 9 (UUID List): ...", UUID is not mandatory. So let's degrade it into a
informational message on initial discovery for every device.

Signed-off-by: Alan Cui <me@alancui.cc>
---
v2: move the hint to nvme_init_ns_head instead of nvme_validate_ns
v2: clean the commit message lines

In my understanding, the nvme_init_ns_head will be called each time
a namespace created, and it does some checking on ids so we also place
the hint there, and here is the result of dmesg applied current patch.

resend v2: add Ccs to all maintainers
resend v2: see https://lists.infradead.org/pipermail/linux-nvme/2026-May/062831.html

After discussion, I begin to think we should treat this as a warning,
since the way kernel generate UUID by NGUID is not a valid RFC4122 UUID,
and NVMe specification requires "The Universally Unique Identifier is defined in RFC4122"

However the "No UUID available providing old NGUID" is should be reported
per device anyway.

[    4.502052] nvme nvme1: pci function 0000:05:00.0
[    4.502060] nvme nvme0: pci function 0000:01:00.0
[    4.508544] nvme nvme1: missing or invalid SUBNQN field.
[    4.508664] nvme nvme1: D3 entry latency set to 8 seconds
[    4.512422] nvme nvme0: missing or invalid SUBNQN field.
[    4.517581] nvme nvme0: allocated 32 MiB host memory buffer (1 segment).
[    4.523519] nvme nvme0: 8/0/0 default/read/poll queues
[    4.524761] nvme nvme1: 16/0/0 default/read/poll queues
[    4.525438] nvme nvme0: No UUID available, uuid_show providing old NGUID
[    4.526683] nvme nvme1: No UUID available, uuid_show providing old NGUID
[    4.527243]  nvme0n1: p1 p2 p3 p4 p5 p6
[    4.528351]  nvme1n1: p1

diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
index 766e9cc4ffca..70856856a99c 100644
--- a/drivers/nvme/host/core.c
+++ b/drivers/nvme/host/core.c
@@ -4007,6 +4007,10 @@ static int nvme_init_ns_head(struct nvme_ns *ns, struct nvme_ns_info *info)
 		ctrl->quirks |= NVME_QUIRK_BOGUS_NID;
 	}
 
+	if (uuid_is_null(&info->ids.uuid)) {
+		dev_info(ctrl->device, "No UUID available, uuid_show providing old NGUID\n");
+	}
+
 	mutex_lock(&ctrl->subsys->lock);
 	head = nvme_find_ns_head(ctrl, info->nsid);
 	if (!head) {
diff --git a/drivers/nvme/host/sysfs.c b/drivers/nvme/host/sysfs.c
index 16c6fea4b2db..ea8bce9887f9 100644
--- a/drivers/nvme/host/sysfs.c
+++ b/drivers/nvme/host/sysfs.c
@@ -147,8 +147,6 @@ static ssize_t uuid_show(struct device *dev, struct device_attribute *attr,
 	 * we have no UUID set
 	 */
 	if (uuid_is_null(&ids->uuid)) {
-		dev_warn_once(dev,
-			"No UUID available providing old NGUID\n");
 		return sysfs_emit(buf, "%pU\n", ids->nguid);
 	}
 	return sysfs_emit(buf, "%pU\n", &ids->uuid);
--
2.54.0