[PATCH 3/3] nvme: Enable autonomous power state transitions
Andy Lutomirski
luto at amacapital.net
Mon Aug 29 16:16:58 PDT 2016
On Aug 29, 2016 8:07 AM, "J Freyensee"
<james_p_freyensee at linux.intel.com> wrote:
>
> 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.
I guess that should be "not using APST" instead.
>
>
> > + 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?
Sure, will do.
>
> > +
> > + 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?
I'll make it a module parameter as Keith suggested.
>
> Nice stuff!
>
>
> >
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