linux-kernel - Re: [PATCH v2 0/8] ARM: sun9i: SMP support with Multi-Cluster Power Management

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Message-ID: <20180104180409.GD12239@red-moon>
Date:   Thu, 4 Jan 2018 18:04:09 +0000
From:   Lorenzo Pieralisi <lorenzo.pieralisi@....com>
To:     Maxime Ripard <maxime.ripard@...e-electrons.com>
Cc:     Chen-Yu Tsai <wens@...e.org>, Russell King <linux@...linux.org.uk>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Mylene JOSSERAND <mylene.josserand@...e-electrons.com>,
        devicetree@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, linux-sunxi@...glegroups.com,
        Nicolas Pitre <nicolas.pitre@...aro.org>,
        Dave Martin <Dave.Martin@....com>
Subject: Re: [PATCH v2 0/8] ARM: sun9i: SMP support with Multi-Cluster Power
 Management

On Thu, Jan 04, 2018 at 03:58:38PM +0100, Maxime Ripard wrote:
> On Thu, Jan 04, 2018 at 10:37:46PM +0800, Chen-Yu Tsai wrote:
> > This is v2 of my sun9i SMP support with MCPM series which was started
> > over two years ago [1]. We've tried to implement PSCI for both the A80
> > and A83T. Results were not promising. The issue is that these two chips
> > have a broken security extensions implementation. If a specific bit is
> > not burned in its e-fuse, most if not all security protections don't
> > work [2]. Even worse, non-secure access to the GIC become secure. This
> > requires a crazy workaround in the GIC driver which probably doesn't work
> > in all cases [3].
> > 
> > Nicolas mentioned that the MCPM framework is likely overkill in our
> > case [4]. However the framework does provide cluster/core state tracking
> > and proper sequencing of cache related operations. We could rework
> > the code to use standard smp_ops, but I would like to actually get
> > a working version in first.
> > 
> > Much of the sunxi-specific MCPM code is derived from Allwinner code and
> > documentation, with some references to the other MCPM implementations,
> > as well as the Cortex's Technical Reference Manuals for the power
> > sequencing info.
> > 
> > One major difference compared to other platforms is we currently do not
> > have a standalone PMU or other embedded firmware to do the actually power
> > sequencing. All power/reset control is done by the kernel. Nicolas
> > mentioned that a new optional callback should be added in cases where the
> > kernel has to do the actual power down [5]. For now however I'm using a
> > dedicated single thread workqueue. CPU and cluster power off work is
> > queued from the .{cpu,cluster}_powerdown_prepare callbacks. This solution
> > is somewhat heavy, as I have a total of 10 static work structs. It might
> > also be a bit racy, as nothing prevents the system from bringing a core
> > back before the asynchronous work shuts it down. This would likely
> > happen under a heavily loaded system with a scheduler that brings cores
> > in and out of the system frequently. In simple use-cases it performs OK.
> 
> It all looks sane to me
> Acked-by: Maxime Ripard <maxime.ripard@...e-electrons.com>

It does not to me, sorry. You do not need MCPM (and workqueues) to
do SMP bring-up.

Nico explained why, just do it:

commit 905cdf9dda5d ("ARM: hisi/hip04: remove the MCPM overhead")

Lorenzo