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Date:   Wed, 23 Jan 2019 17:21:15 +0000
From:   Sudeep Holla <sudeep.holla@arm.com>
To:     Scott Branden <scott.branden@broadcom.com>
Cc:     Mark Rutland <mark.rutland@arm.com>,
        Pramod Kumar <pramod.kumar@broadcom.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        Suzuki K Poulose <Suzuki.Poulose@arm.com>,
        Dave Martin <dave.martin@arm.com>,
        Rob Herring <robh@kernel.org>,
        Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>,
        Steve Capper <steve.capper@arm.com>,
        Sudeep Holla <sudeep.holla@arm.com>,
        BCM Kernel Feedback <bcm-kernel-feedback-list@broadcom.com>,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH RFC 1/1] arm64: Use PSCI calls for CPU stop when hotplug
 is supported
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On Wed, Jan 23, 2019 at 09:05:26AM -0800, Scott Branden wrote:
> Hi Mark,
>
> Hopefully I can shed some light on the use case inline.
>
> On 2019-01-23 8:48 a.m., Mark Rutland wrote:
> > On Mon, Jan 21, 2019 at 11:30:02AM +0530, Pramod Kumar wrote:
> > > On Mon, Jan 21, 2019 at 11:28 AM Pramod Kumar <pramod.kumar@broadcom.com>
> > > wrote:
> > >
> > >      Need comes from a specific use case where one Accelerator card(SoC) is
> > >      plugged in a sever over a PCIe interface.  This Card gets supply from a
> > >      battery, which could provide very less power for a very small time, in case
> > >      of any power loss. Once Card switches to battery, this has to reduce its
> > >      power consumption to its lowest point and back-up the DDR contents asap
> > >      before battery gets fully drained off.
> > In this example is Linux running on the server, or on the accelerator?
> Accelerator
> >
> > What precisely are you trying to back up from DDR, and why?
> Data in DDR is being written to disk at this time (disk is connected to
> accelerator)
> >
> > What is responsible for backing up that contents?
>
> A low power M-class processor and DMA engine which continues necessary
> operations to transfer DDR memory to disk.
>
> The high power processors on the accelerator running linux needed to be
> halted ASAP on this power loss event and M0 take over. Graceful shutdown of
> linux and other peripherals is unnecessary (and we don't have the power
> necessary to do so).
>

It may be unnecessary for your use-case, but not recommended.

> >
> > >      Since battery can provide limited power for a very short time hence need to
> > >      transition to lowest power. As per the transition process , CPUs power
> > >      domain has to be off but before that it needs to flush out its content to
> > >      system memory(L3) so that content could be backed-up by a MCU, a controller
> > >      consuming very less power. Since we can not afford plugging-out every
> > >      individual CPUs in sequence hence uses  ipi_cpu_stop for all other CPUs
> > >      which ultimately switch to ATF to flush out all the CPUs caches and comes
> > >      out of coherency domain so that its power rails could be switched-off.
> > If you're stopping CPUs from completely arbitrary states, what is the
> > benefit of saving the RAM contents?
>
> Some of the RAM contains data that was in the process of being written to
> disk by the accelerator.
>
> This data must be saved to disk and the high power CPUs consume too much
> power to continue performing this operation.
>

Why will suspend to ram or idle not work ? It will power off the secondaries
which this patch is trying to achieve, but in more sane way so that no
data/state is lost/corrupted as I stated earlier.

> >
> > CPUs might be running with IRQs disabled for an arbitrarily long time,
>
> In an embedded linux system we control everything running.
>

By which I assume you have patches to do all sorts of things to make this
work and this patch standalone is of no use :)

I don't like this as it's not scalable to big systems as this is in the
same code path as system off/reset.

--
Regards,
Sudeep