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

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Message-ID: <0be9b704-4cc6-7b23-4435-256372e90ffd@gmail.com>
Date:   Thu, 3 Oct 2019 16:47:40 -0700
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Nicolas Saenz Julienne <nsaenzjulienne@...e.de>,
        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 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.
-- 
Florian