[aarch64-port-dev ] RFR: 8079203: aarch64: need to cater for different partner implementations

[Mark Rutland]

On Tue, May 05, 2015 at 03:23:41PM +0100, Edward Nevill wrote:
> On Fri, 2015-05-01 at 15:23 +0100, Mark Rutland wrote:
> 
> > From a look at the proposed patch, I guess this assumes a uniform SMP
> > system (i.e. no big.LITTLE)?
> 
> Not quite: The assumption is that any CPU implementer == ARM could be
> big little and therefore could contain A53 and therefor the A53
> feature must be enabled.

Ah, I see.

> > A while back [1] the /proc/cpuinfo format was fixed to show information per-cpu
> > (example form a Juno system below), and it looks like the CPU information
> > parsed will only refer to ther final CPU listed, and may not be representative
> > of the system as a whole.
> > 
> > That format has been packported to the various stable kernels too.
> > 
> 
> Nonetheless, we must still cater for olde style /proc/cpuinfo. I have
> modified the webrev below so that if it is a new style /proc/cpuinfo,
> it will only enable the A53 feature if it can positively identify an
> A53 core.

Great!

> Otherwise, if it is an old style /proc/cpuinfo it will assume the A53
> feature needs enabling if it finds an A57 (or, of course if it is
> identified as an A53).

Sure. The old format is something that will hopefully die off
eventually, but unfortunately we're stuck with it for the timebeing.

> http://cr.openjdk.java.net/~enevill/8079203/webrev.01
> 
> > Thanks,
> > Mark.
> 
> You're welcome. It would be much easier if the kernel exported this
> information in a machine readable form. I hate having to grub around
> in /proc/cpuinfo to find this information, and I know that others in
> the OpenJDK community hate this also. Its fragile and non portable.
> 
> Why can the kernel not make MIDR readable at EL0 and provide a
> HWCAP_BIGLITTLE in auxv.

We're looking into exposing such information to userspace in a more
structured manner at the moment, but we don't have an RFC just yet. I
take it that you would be interested when that appears?

Is there anything in particular other than MIDR that you'd like to see
exposed?

Thanks,
Mark.