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

[Hans de Goede]

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.

> > We may need a new med_power_with_dipm_and_devslp level if windows
> > does
> > not enable HIPM by default on these systems. In hind sight we really
> > should have separate bools for each, rather then coding this in
> > a single level, ah well.
> Agree. We can certainly do this for even safer implementation.

Ack.

Regards,

Hans