Re: [Xen-devel] [PATCH 0/4] ARM: add PSCI host support

[Andre Przywara <andre.przywara@xxxxxxxxxx>]

On 11/25/2013 03:50 PM, Ian Campbell wrote:
> On Mon, 2013-11-25 at 15:21 +0100, Andre Przywara wrote:
> > On 11/25/2013 03:03 PM, Ian Campbell wrote:
> > > On Mon, 2013-11-25 at 13:00 +0000, George Dunlap wrote:
> > > > On Mon, Nov 25, 2013 at 12:02 PM, Andre Przywara
> > > > <andre.przywara@xxxxxxxxxx> wrote:
> > > > > Xen did not make use of the host provided ARM PSCI (Power State
> > > > > Coordination Interface) functionality so far, but relied on platform
> > > > > specific SMP bringup functions.
> > > > > This series adds support for PSCI on the host by reading the required
> > > > > information from the DTB and invoking the appropriate handler when
> > > > > bringing up each single CPU.
> > > > > Since PSCI is defined for both ARM32 and ARM64, I put the code in a
> > > > > file shared by both.
> > > > > The ARM32 code was tested on Midway, but the ARM64 code was compile
> > > > > tested only.
> > > > >
> > > > > This approach seems to be the least intrusive, but one could also use
> > > > > more of the current ARM64 code by copying the PSCI/spin-table
> > > > > distinction code to a shared file and use that from both
> > > > > architectures. However that seems more complicated.
> > > > >
> > > > > Please take a look and complain ;-)
> > > > >
> > > > > Signed-off-by: Andre Przywara <andre.przywara@xxxxxxxxxx>
> > > > Ian, do you agree that this is too late for 4.4?
> > > I'm in two minds. On the one hand none of the existing platforms
> > > currently require this functionality, so it has clearly not been
> > > necessary up to now.
> > >
> > > On the other hand it plays into the strategy of allowing people to
> > > trivially support their platform, and since it is a standard way to do
> > > power control on ARM (albeit quite new and so far uptake is not huge) I
> > > think it is expected that many new platforms will use it.
> > >
> > > Of our current platforms Midway can optionally use PSCI (we have
> > > "native" code at the minute)
> > but which is not upstream yet, right?
> Oh right, I forgot it was still waiting for an Ack from you and thought
> I'd committed it when I had not.
I deliberately held back my ACK: on this one to give PSCI a chance, 
since it turned out to be easier than I thought.
Technically I am OK with Julien's patch, so I can ACK it as well if you 
like.
> > So if you are considering dropping PSCI for 4.4, I'd like to know so
> > that I can ack Julien's "native" SMP patch.
> > I hope at least this patch can make it for 4.4?
> Yes, one or the other should definitely go in for 4.4. It changes the
> argument for the PSCI stuff a bit too, since we can now enable midway
> and make it easier for other platforms at the same time.
That was my thinking. But I see both George's and your point with a 
release manager's hat on, so I am OK with whatever you decide.
Thanks for caring!
Andre

> [...]
> > > An alternative could be requiring for 4.4 that the platform code
> > > explicitly call into/request PSCI for 4.4 and only move to automatically
> > > using it in the absence of the platform code saying otherwise for 4.5.
> > So you are thinking about a change in the priorities?
> I was only suggesting as a way to mitigate risk for 4.4 -- long term we
> should certainly do as Linux does and prefer PSCI. (I confess I wasn't
> sure how this manifests in Linux, if its at odds with what I wrote
> then ...oops)
> >   The Linux kernel
> > prefers PSCI over a native method, which is how I modeled the Xen patch
> > also. This has the advantage of having control in the DTB, so if PSCI
> > fails in Xen, one could do "fdt rm /psci" in u-boot to get the old
> > behavior back.
> >
> > > This has the advantage of being zero risk, but the downside of not being
> > > very well tested (we could enable it for Midway, with the attendant
> > > increase in risk).
> > So are you concerned about one of the existing platforms breaking SMP as
> > soon as it gets PSCI support? One could change the patch to only use
> > PSCI if platform_cpu_up() does _not_ return an explicit "ignore PSCI"
> > value, if that helps.
> I'm addressing George's concerns as release manager about the risk of
> taking any sort of PSCI patches at this stage.
>
> Ian.

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