linux-kernel - Re: [PATCH] ARM: dt: check MPIDR on MP devices built without SMP

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Date:   Fri, 04 Oct 2019 10:36:08 +0200
From:   Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
To:     Florian Fainelli <f.fainelli@...il.com>,
        linux-kernel@...r.kernel.org
Cc:     Russell King <linux@...linux.org.uk>,
        linux-arm-kernel@...ts.infradead.org,
        "kernelci.org bot" <bot@...nelci.org>, wahrenst@....net
Subject: Re: [PATCH] ARM: dt: check MPIDR on MP devices built without SMP

On Thu, 2019-10-03 at 16:47 -0700, Florian Fainelli wrote:
> On 10/3/19 12:39 PM, Nicolas Saenz Julienne wrote:
> > On Thu, 2019-10-03 at 11:08 -0700, Florian Fainelli wrote:
> > > On 10/2/19 4:45 AM, Nicolas Saenz Julienne wrote:
> > > > Currently, in arm_dt_init_cpu_maps(), the hwid of the boot CPU is read
> > > > from MPIDR on SMP devices and set to 0 for non SMP. This value is then
> > > > matched with the DT cpu nodes' reg property in order to find the boot
> > > > CPU in DT.
> > > 
> > > The code you change is about the "mpidr" variable, yet in your commit
> > > message you refer to "hwid", that is a tad confusing for the reader.
> > 
> > Sorry, it's indeed pretty confusing. I'll send a new version with a fixed
> > description if there is no major push back.
> > 
> > > > On MP devices build without SMP the cpu DT node contains the expected
> > > > MPIDR yet the hwid is set to 0. With this the function fails to match
> > > > the cpus and uses the default CPU logical map. Making it impossible to
> > > > get the CPU's DT node further down the line. This causes issues with
> > > > cpufreq-dt, as it triggers warnings when not finding a suitable DT node
> > > > on CPU0.
> > > > 
> > > > Change the way we choose whether to get MPIDR or not. Instead of
> > > > depending on SMP check the number of CPUs defined in DT. Anything > 1
> > > > means MPIDR will be available.
> > > 
> > > Except if someone accidentally wrote their Device Tree such as to have >
> > > 1 CPU nodes, yet the CPU is not MP capable and reading the MPIDR
> > > register does return the expected value, but that is wrong anyway.
> > 
> > An UP device will most likely not have a MPIDR. That said I'm not sure this
> > is
> > always true. As per ARM1176JZ's TRM[1], the RPi1 CPU, if one was to get the
> > MPIDR it would raise an undefined exception.
> > 
> > The way I see it's an acceptable outcome as the DT is clearly wrong.
> 
> It is, although you probably want to use of_get_available_child_count()
> instead of of_get_child_count() since we could imagine that a boot
> loader or some other boot program mangling the DT could intentionally
> put a 'status = "disabled"' property on the non-boot CPU node for
> whatever reason.

Good point, I'll fix it on v2.


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