[patch] enable AHCI device initiated power management

[Johannes Hofmann]

Johannes Hofmann <johannes.hofmann at gmx.de> wrote:
> Hi,
> 
> this patch
> http://leaf.dragonflybsd.org/~hofmann/ahci_dipm.diff
> enables device initiated AHCI link power management.
> 
> Some disks (e.g. the Intel X25-M SSD) don't support host initiated
> power management.
> Therefore also enable device initiated power management when
> ahci power management is set to "aggressive".
> This saves about 0.5W on a Thinkpad with Intel X25-M.
> 
> * Move the currently unused function ahci_pm_set_feature() to ahci.c
>   and make it work for ATA_SATAFT_DEVIPS.
> 
> * Use ahci_set_feature() to enable ATA_SATAFT_DEVIPS when AHCI power
>   management is set to "aggressive".
> 
> * Set ap->link_pwr_mgmt early in ahci_port_link_pwr_mgmt() to ensure
>   it is set when ahci_set_feature() -> ahci_poll() -> ahci_port_intr()
>   is called.
> 
> Inspired-by: Linux

I've updated the patch according to feedback from dillon on irc. The
new version
http://leaf.dragonflybsd.org/~hofmann/ahci_dipm_2.diff
checks whether device initiated link power management is supported and
only enables it for directly attached devices. I don't know whether or
how this could work with port multipliers and it's mostly interesting
for laptops anyway.
This patch also includes the new link_pwr_state sysctl, so it's all in
one patch now.

Cheers,
Johannes