nvme: only consider exit latency when choosing useful non-op power states
When a NVMe is in non-op states, the latency is exlat.
The latency will be enlat + exlat only when the NVMe tries to transit
from operational state right atfer it begins to transit to
non-operational state, which should be a rare case.
Therefore, as Andy Lutomirski suggests, use exlat only when deciding power
states to trainsit to.
Signed-off-by: Kai-Heng Feng <kai.heng.feng@canonical.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
index 0f9cc0c..c07d8d4e 100644
--- a/drivers/nvme/host/core.c
+++ b/drivers/nvme/host/core.c
@@ -1342,7 +1342,7 @@
* transitioning between power states. Therefore, when running
* in any given state, we will enter the next lower-power
* non-operational state after waiting 50 * (enlat + exlat)
- * microseconds, as long as that state's total latency is under
+ * microseconds, as long as that state's exit latency is under
* the requested maximum latency.
*
* We will not autonomously enter any non-operational state for
@@ -1387,7 +1387,7 @@
* lowest-power state, not the number of states.
*/
for (state = (int)ctrl->npss; state >= 0; state--) {
- u64 total_latency_us, transition_ms;
+ u64 total_latency_us, exit_latency_us, transition_ms;
if (target)
table->entries[state] = target;
@@ -1408,12 +1408,15 @@
NVME_PS_FLAGS_NON_OP_STATE))
continue;
- total_latency_us =
- (u64)le32_to_cpu(ctrl->psd[state].entry_lat) +
- + le32_to_cpu(ctrl->psd[state].exit_lat);
- if (total_latency_us > ctrl->ps_max_latency_us)
+ exit_latency_us =
+ (u64)le32_to_cpu(ctrl->psd[state].exit_lat);
+ if (exit_latency_us > ctrl->ps_max_latency_us)
continue;
+ total_latency_us =
+ exit_latency_us +
+ le32_to_cpu(ctrl->psd[state].entry_lat);
+
/*
* This state is good. Use it as the APST idle
* target for higher power states.