Re: [RFC PATCH] ata: ahci: Enable DEVSLP by default on x86 modern standby platform

[Hans de Goede]

Hi,

On 03-07-18 10:57, Hans de Goede wrote:
> Hi,
>
> On 03-07-18 00:08, Srinivas Pandruvada wrote:
> > Hi Hans,
> >
> > On Mon, 2018-07-02 at 23:27 +0200, Hans de Goede wrote:
>
> <snip>
>
> > > > > SATA IP block doesn't get turned off till SATA is in DEVSLP mode.
> > > > > Here
> > > > > user has to either use scsi-host sysfs or tools like powertop to
> > > > > set
> > > > > the sata-host link_power_management_policy to min_power.
> > > > >
> > > > > This change sets by default link power management policy to
> > > > > min_power
> > > > > for some platforms. To avoid regressions, the following
> > > > > conditions
> > > > > are used:
> > > > > - The kernel config is already set to use med_power_with_dipm or
> > > > > deeper
> > > > > - System is a modern standby system using ACPI low power idle
> > > > > flag
> > > > > - The platform is not blacklisted for Suspend to Idle and suspend
> > > > > to idle is used instead of S3
> > > > > This combination will make sure that systems are fairly recent
> > > > > and
> > > > > since getting shipped with modern standby standby, the DEVSLP
> > > > > function
> > > > > is already validated.
> > > > >
> > > > > Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@xxxxxxxxx
> > > > > el.com>
> > > >
> > > > Seems sane to me. Hans, what do you think?
> > >
> > > I think this is going in the right direction, but min_power enables
> > > both DEVSLP and HIPM, AFAIK Windows at least in the past did not
> > > enable HIPM be default.
> >
> > Windows and Linux will need to meet the same criteria to reach SLP_S0
> > state. OS is not deciding whether we meet criteria or not, it is
> > firmware deciding, and it will look for SATA IP to be in certain
> > predefined state, before generating this SLP_S0 signal.
> >
> >
> > > Srinivas can you figure out what Windows does wrt HIPM?
> >
> > We can see that under Windows the SATA rail voltage is dropped. I don't
> > know if there is any other method other than DEVSLP configuration. But
> > I will surely check around.
>
> I understand that we need DEVSLP, but AFAIK DEVSLP is entered automatically
> after being in slumber for a defined time. The question is how we get into
> slumber. Currently with the new med_power_with_dipm policy we always use
> DIPM to enter slumber. Your suggested switch to min_power means also
> enabling HIPM. What I'm suggesting is that it might better to have
> DIPM + DEVSLP rather then DIPM + HIPM + DEVSLP.
>
> So basically the question is does windows use:
>
> a) DIPM + DEVSLP; or
> b) DIPM + HIPM + DEVSLP
>
> If b. is the case, then I'm fine with your patch as is, but if Windows
> does a. then we should mimic that, which requires a new power-level as
> our min_power level is b.

So digging a bit deeper I just realized that another important difference
between med_power_with_dipm and min_power is that min_power by default
set the ASP bits making the link go to the slumber state instead of to
the partial (power-saving) state.

According to:
https://www.intel.com/content/dam/doc/reference-guide/sata-devices-implementation-recommendations.pdf

ASP defaults to off in the iRST drivers. But that is a document from
before DEVSLP got introduced.

So we need to know if Windows and/or the iRST drivers use HIPM and
ASP by default on these systems. If they do then using min_power
is fine. If they don't use one or the other we are going to need
a new policy reflecting those settings and use that.

Regards,

Hans