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

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.

> 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.

> > 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.


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