[PATCH 3/3] nvme: Enable autonomous power state transitions

[J Freyensee]

On Mon, 2016-08-29 at 02:25 -0700, Andy Lutomirski wrote:
> NVME devices can advertise multiple power states.  These states can
> be either "operational" (the device is fully functional but possibly
> slow) or "non-operational" (the device is asleep until woken up).
> Some devices can automatically enter a non-operational state when
> idle for a specified amount of time and then automatically wake back
> up when needed.
> 
> The hardware configuration is a table.  For each state, an entry in
> the table indicates the next deeper non-operational state, if any,
> to autonomously transition to and the idle time required before
> transitioning.
> 
> This patch teaches the driver to program APST so that each
> successive non-operational state will be entered after an idle time
> equal to 100% of the total latency (entry plus exit) associated with
> that state.  A sysfs attribute 'apst_max_latency_ns' gives the
> maximum acceptable latency in ns; non-operational states with total
> latency greater than this value will not be used.  As a special
> case, apst_max_latency_ns=0 will disable APST entirely.
> 
> On hardware without APST support, apst_max_latency_ns will not be
> exposed in sysfs.
> 
> In theory, the device can expose "default" APST table, but this
> doesn't seem to function correctly on my device (Samsung 950), nor
> does it seem particularly useful.  There is also an optional
> mechanism by which a configuration can be "saved" so it will be
> automatically loaded on reset.  This can be configured from
> userspace, but it doesn't seem useful to support in the driver.
> 
> On my laptop, enabling APST seems to save nearly 1W.
> 
> The hardware tables can be decoded in userspace with nvme-cli.
> 'nvme id-ctrl /dev/nvmeN' will show the power state table and
> 'nvme get-feature -f 0x0c -H /dev/nvme0' will show the current APST
> configuration.
> 
> Signed-off-by: Andy Lutomirski <luto at kernel.org>
> ---
>  drivers/nvme/host/core.c | 167
> +++++++++++++++++++++++++++++++++++++++++++++++
>  drivers/nvme/host/nvme.h |   6 ++
>  include/linux/nvme.h     |   6 ++
>  3 files changed, 179 insertions(+)
> 
> diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
> index 3f7561ab54dc..042137ad2437 100644
> --- a/drivers/nvme/host/core.c
> +++ b/drivers/nvme/host/core.c
> @@ -1223,6 +1223,98 @@ static void nvme_set_queue_limits(struct
> nvme_ctrl *ctrl,
>  	blk_queue_write_cache(q, vwc, vwc);
>  }
>  
> +static void nvme_configure_apst(struct nvme_ctrl *ctrl)
> +{
> +	/*
> +	 * APST (Autonomous Power State Transition) lets us program
> a
> +	 * table of power state transitions that the controller will
> +	 * perform automatically.  We configure it with a simple
> +	 * heuristic: we are willing to spend at most 2% of the time
> +	 * transitioning between power states.  Therefore, when
> running
> +	 * in any given state, we will enter the next lower-power
> +	 * non-operational state after waiting 100 * (enlat + exlat)
> +	 * microseconds, as long as that state's total latency is
> under
> +	 * the requested maximum latency.
> +	 *
> +	 * We will not autonomously enter any non-operational state
> for
> +	 * which the total latency exceeds
> apst_max_latency_ns.  Users
> +	 * can set apst_max_latency_ns to zero to turn off APST.
> +	 */
> +
> +	unsigned apste;
> +	struct nvme_feat_auto_pst *table;
> +	int ret;
> +
> +	if (!ctrl->apsta)
> +		return;	/* APST isn't supported. */
> +
> +	if (ctrl->npss > 31) {
> +		dev_warn(ctrl->device, "NPSS is invalid; disabling
> APST\n");

Quick question.  A little bit below in a later if() block, apste is set
to 0 to turn off APST, which is to be used later in a
nvme_set_features() call to actually turn it off.  You wouldn't want to
also set apste to zero too and call a nvme_set_features() to "disable
APST"?

I guess I'm a little confused on the error statement, "disabling APST",
when it doesn't seem like anything is being done to actually disable
APST, it's just more of an invalid state retrieved from the HW.
 

> +		return;
> +	}
> +
> +	table = kzalloc(sizeof(*table), GFP_KERNEL);
> +	if (!table)
> +		return;
> +
> +	if (ctrl->apst_max_latency_ns == 0) {
> +		/* Turn off APST. */
> +		apste = 0;
> +	} else {
> +		__le64 target = cpu_to_le64(0);
> +		int state;
> +
> +		/*
> +		 * Walk through all states from lowest- to highest-
> power.
> +		 * According to the spec, lower-numbered states use
> more
> +		 * power.  NPSS, despite the name, is the index of
> the
> +		 * lowest-power state, not the number of states.
> +		 */
> +		for (state = (int)ctrl->npss; state >= 0; state--) {
> +			u64 total_latency_us, transition_ms;
> +
> +			if (target)
> +				table->entries[state] = target;
> +
> +			/*
> +			 * Is this state a useful non-operational
> state for
> +			 * higher-power states to autonomously
> transition to?
> +			 */
> +			if (!(ctrl->psd[state].flags & 2))
> +				continue;  /* It's an operational
> state. */
> +
> +			total_latency_us =
> +				(u64)cpu_to_le32(ctrl-
> >psd[state].entry_lat) +
> +				+ cpu_to_le32(ctrl-
> >psd[state].exit_lat);
> +			if (total_latency_us * 1000 > ctrl-
> >apst_max_latency_ns)
> +				continue;
> +
> +			/*
> +			 * This state is good.  Use it as the APST
> idle
> +			 * target for higher power states.
> +			 */
> +			transition_ms = total_latency_us + 19;
> +			do_div(transition_ms, 20);
> +			if (transition_ms >= (1 << 24))
> +				transition_ms = (1 << 24);

Is it possible to use a macro for this bit shift as its used more than
once?

> +
> +			target = cpu_to_le64((state << 3) |
> +					     (transition_ms << 8));
> +		}
> 

snip...
.
.
.

> +	/*
> +	 * By default, allow up to 25ms of APST-induced
> latency.  This will
> +	 * have no effect on non-APST supporting controllers (i.e.
> any
> +	 * controller with APSTA == 0).
> +	 */
> +	ctrl->apst_max_latency_ns = 25000000;

Is it possible to make that a #define please?

Nice stuff!


>