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

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?
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?

and sunxi is going to need it whenever SMP
is enabled (patches to u-boot are circulating now).

That is a good point. We would get sunxi SMP support basically for free, also the timing for this is independent of any Xen release cycle.

I'm inclined towards punting on this for 4.4.0 but be open to the idea
of adding it in 4.4.1 if it turns out to be something that people are
needing in practice.

4.4.1 sounds OK for me. But it would be nice to have the native SMP support then in 4.4.0.

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? 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.


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