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

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!

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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