[PATCH] nvme: log when APST skips a state due to latency cap

[Mike Bommarito <michael.bommarito@gmail.com>]

nvme_configure_apst() silently skips any non-operational power state
whose exit_lat exceeds ctrl->ps_max_latency_us (set from the
default_ps_max_latency_us module parameter, default 100,000 us).

When the kernel skips a drive's deepest non-op state for this reason,
the user gets no signal that anything happened.  This is a frequent
gotcha on Intel platforms that depend on the drive reaching its
deepest power state to allow the PCIe root port to enter L1.x and the
PCH Southport to power-gate, which in turn is a prerequisite for the
platform reaching S0ix during s2idle suspend.

Add a dev_info_once() at the skip site that names the state, prints
the actual exit_lat versus the current cap, and points the user at
the module parameter to raise it.  Using _once keeps the line to a
single instance per controller; users that want to see every probe
event can still enable dev_dbg.

Assisted-by: Claude:claude-opus-4-7
Signed-off-by: Mike Bommarito <michael.bommarito@gmail.com>
---
Build- and boot-tested on Framework Laptop 13 (Intel Core Ultra 5 125H)
running Linux 7.1-rc4 with this patch applied, across multiple boots
over three days.  No regressions observed in NVMe probe or APST
configuration with the installed WD Black SN850X (15b7:5030).

Design context: the conservative default cap and the per-device-quirk
mitigation were discussed in 2017 at

    https://lore.kernel.org/all/20170606095454.GA13569@infradead.org/

and retained.  This patch only adds observability for the silent-skip
case; behavior is unchanged.

 drivers/nvme/host/core.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
index c3032d6ad6b1..220e844e852c 100644
--- a/drivers/nvme/host/core.c
+++ b/drivers/nvme/host/core.c
@@ -2943,8 +2943,13 @@ static int nvme_configure_apst(struct nvme_ctrl *ctrl)
 			continue;
 
 		exit_latency_us = (u64)le32_to_cpu(ctrl->psd[state].exit_lat);
-		if (exit_latency_us > ctrl->ps_max_latency_us)
+		if (exit_latency_us > ctrl->ps_max_latency_us) {
+			dev_info_once(ctrl->device,
+				      "APST: skipping non-op PS%d (exit_lat %llu us > cap %llu us); raise nvme_core.default_ps_max_latency_us to enable\n",
+				      state, exit_latency_us,
+				      ctrl->ps_max_latency_us);
 			continue;
+		}
 
 		total_latency_us = exit_latency_us +
 			le32_to_cpu(ctrl->psd[state].entry_lat);
-- 
2.53.0