Kristen Carlson Accardi wrote:
On Sat, 12 Apr 2008 00:28:13 -0400
Jeff Garzik <jeff@xxxxxxxxxx> wrote:
..
Thinking about the bigger pictures, powering off the phy is something we
want to do in a lot more cases than this, but there is a stumbling
block: we wander into the realm of policy.
For most users most of the time, empty SATA ports are needlessly
powered. The problem is that, at any given moment, a device may be
hot-plugged, so we must be ready for that. We need some way for the
user to let the driver know that they will not be hotplugging anything
anytime soon, permitting power savings to be enabled.
A compromise solution that avoids adding a userspace "knob" has also
been proposed (by Tejun, I think?): power up the phy every N seconds,
check for device, power down phy if nothing. That should provide some
power savings, though not as much as with a "knob" switched to "hotplug:
off"
..
How long does a poll actually take, in real time?
Probably a hundred milliseconds or two, to see if the PHY syncs up?
So with a knob set to, say 2 seconds (sysfs), then this would
save 90% of the power of a permanent "off". That's pretty good,
and setting that same knob to "infinity" (-1 ?) would achieve 100%.
Definitely a good way to go, IMHO.
I think that for the common case - we should use HPCP to determine if
the suggested use of the port is for hot plug. I can see your point
about wanting to give the user the option to just disregard the intended
use of the port and do what they want, but I say we don't make that
the default behavior. And, I don't like the idea of adding another
wakeup in the driver to do polling - seems like 99% of users are
going to be just fine with a knob - and that they should only
have to use the knob to override the default (or how bout a
module param?). I don't think we should compromise power for a
feature that most people are unlikely to use (if HPCP is not set).
..
I worry that this is far too x86 / vendor specific.
Most of the SATA ports I have here, for example, probably lack this HPCP bit
even on x86, and on other arch's it likely doesn't exist at all, right?
For arch/system/device that does have HPCP implemented, then sure,
it could be used to automatically tweak the sysfs knob.
But from userspace perhaps, rather than kernel ?
Thanks
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