[Bug 427925] Re: unable to modify scsi host link_power_management_policy

[Andy Whitcroft]

Link power control is complex.  Firstly not all host controllers support
it, and you must determine if that support is there from the host
controller flags.  Secondly power saving has little meaning where there
is not a device present, and indeed if you enable powersaving there
hotplug no longer works.  Where link control is not appropriate any
attempt to set power modes is silently ignored.

The reporter indicates that host0 is the only one which is not taking a
value, that the other 5 do indeed take the setting.  If we look at his
boot dmesg we see the following:

Firstly note the 'pm' in the controller flags, this indicates the there
is power management support:

    [    3.899062] ahci 0000:00:1f.2: flags: 64bit ncq sntf ilck stag pm
led clo pmp pio slum part ems

These are the sata host channels and their capabilities, note that
ata3/4 are fake entries so should be ignored behaviour wise:

    [    3.921463] ata1: SATA max UDMA/133 abar m2048 at 0xd4405000 port 0xd4405100 irq 26
    [    3.921468] ata2: SATA max UDMA/133 abar m2048 at 0xd4405000 port 0xd4405180 irq 26
    [    3.921471] ata3: DUMMY
    [    3.921473] ata4: DUMMY
    [    3.921477] ata5: SATA max UDMA/133 abar m2048 at 0xd4405000 port 0xd4405300 irq 26
    [    3.921482] ata6: SATA max UDMA/133 abar m2048 at 0xd4405000 port 0xd4405380 irq 26

Note that the first link is down and second is up, likely the disk is on
ata2, ie. on host1.  Host 0 we would therefore likely expect not to be
able to control as there is no link:

    [    4.288992] ata1: SATA link down (SStatus 0 SControl 300)
    [    4.984177] ata2: SATA link up 3.0 Gbps (SStatus 123 SControl 300)
    [    5.892161] ata5: SATA link up 1.5 Gbps (SStatus 113 SControl 300)
    [    6.292115] ata6: SATA link down (SStatus 0 SControl 300)

As long as you are able to control host1 and host 4 you are controlling
the links which exist as far as I can tell.

** Changed in: linux (Ubuntu)
     Assignee: Ubuntu Kernel Team (ubuntu-kernel-team) => Andy Whitcroft (apw)

** Changed in: linux (Ubuntu)
       Status: Triaged => Incomplete

-- 
unable to modify scsi host link_power_management_policy
https://bugs.launchpad.net/bugs/427925
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