Re: [PATCH 0/2] ata: libahci: devslp fixes

[Rajat Jain]

On Fri, Mar 8, 2019 at 12:57 AM Hans de Goede <hdegoede@xxxxxxxxxx> wrote:
>
> Hi,
>
> On 08-03-19 01:04, Srinivas Pandruvada wrote:
> > On Thu, 2019-03-07 at 15:07 -0800, Rajat Jain wrote:
> >> Hello,
> >>
> >> On Thu, Mar 7, 2019 at 12:37 PM Hans de Goede <hdegoede@xxxxxxxxxx>
> >> wrote:
> >>>
> >>> Hi,
> >>>
> >>> On 07-03-19 21:27, Gwendal Grignou wrote:
> >>>> Srinivas,
> >>>>
> >>>> I am looking at problem on a laptop machine that suspends to
> >>>> S01x, but
> >>>> link_management is set to max_performance, because the machine is
> >>>> connected to a charger.
> >>>
> >>> What is setting it to max_performance when charging?  I assume
> >>> chrome-os is
> >>> running something in userspace to do this (like TLP, but I guess
> >>> you are not
> >>> using TLP) ?
> >>
> >> Yes, we have a udev script that does this.
> >>
> >>>
> >>> Have you run benchmarks with max_performance vs the default?
> >>> I seriously doubt there will be a significant difference, esp.
> >>> with a chrome-os style workload.
> >>>
> >>>> Given DVLSP must be set before the laptop suspends ["""One of the
> >>>> requirement for modern x86 system to enter lowest power
> >>>> mode  (SLP_S0)
> >>>> is SATA IP block to be off."""], the machine never reaches S01x.
> >>>> Does it make sense to change the target_lpm_policy at suspend
> >>>> (ata_port_suspend()) to min_power and bring it back to the
> >>>> original
> >>>> value on resume?
> >>>
> >>> If userspace messes with the setting, then userspace should also
> >>> put it back before suspending...
> >>>
> >>> The upstream kernel's default behavior is to have the target level
> >>> set
> >>> to a fixed level independent of the charging state. Could it be
> >>> this
> >>> fixed level is actually max-performance ? If that is the default
> >>> the
> >>> kernel comes up with, that would indicate a kernel bug.
> >>
> >> Side note: max-performance indeed can be the default forced by the
> >> kernel for some (broken) SATA devices:
> >>
> >>          if (dev->horkage & ATA_HORKAGE_NOLPM) {
> >>                  ata_dev_warn(dev, "LPM support broken, forcing
> >> max_power\n");
> >>                  dev->link->ap->target_lpm_policy = ATA_LPM_MAX_POWER;
> >>          }
> >>
> >> So definitely these systems won't be able to go into S0ix today.
> >>
> >> But I think the main idea that we are asking is:
> >>
> >> 1) Yes, we acknowledge that the userspace has set it max-performance.
> >>
> >> 2) However, given that the kernel already knows that:
> >>         - while in suspend, there is no real value in retaining the
> >> max-performance.
> >>         - On the contrary, we know system will fail to go into lower
> >> power mode because of max-suspend.
> >>
> >> 3) Does it not make sense to use this knowledge and switch to
> >> min_power when we are actually going to suspend (even if user
> >> specified max-performance), and restore max-performance on resume?
> >
> > It is all about regressions. Hence we added multiple conditions for
> > setting default to min power.
> > It may cause issues for some SATAs, which may not recover once enters
> > slumber or DEVSLP. There is also case where user having issues with
> > default LPM policy hence he changed policy to max performance. We can't
> > detect that.
> > So it will be much safer if user space change policy to default before
> > calling suspend.

Now I understand, and agree.

>
> Right, or simply do not mess with the setting in the first place!
>
> I noticed you did not answer this part of my original reply:
>
> "Have you run benchmarks with max_performance vs the default?
> I seriously doubt there will be a significant difference, esp.
> with a chrome-os style workload."
>
> I seriously doubt the max-performance setting has a user
> noticeable impact on performance. So I repeat has someone
> actually measured this with real-world chrome-os workloads ?

The reason we're setting it to max-performance is not really
performance - but as a work around to a broken SATA device (Transcend
SSD) that can't handle DEVSLP in active state (similar to what
Srinivas said). Putting it to min_power while suspended was OK because
it'd be in reset anyway at that time. Thanks for your explanations and
help.

Rajat


>
> Regards,
>
> Hans
>