linux-kernel - Re: [PATCH v3 21/22] cpufreq: add support for CPU DVFS based on SCMI message protocol

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Date:   Wed, 4 Oct 2017 16:01:13 +0100
From:   Sudeep Holla <sudeep.holla@....com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Sudeep Holla <sudeep.holla@....com>,
        ALKML <linux-arm-kernel@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        DTML <devicetree@...r.kernel.org>,
        Roy Franz <roy.franz@...ium.com>,
        Harb Abdulhamid <harba@...eaurora.org>,
        Nishanth Menon <nm@...com>, Loc Ho <lho@....com>,
        Alexey Klimov <alexey.klimov@....com>,
        Ryan Harkin <Ryan.Harkin@....com>,
        Jassi Brar <jassisinghbrar@...il.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        linux-pm@...r.kernel.org
Subject: Re: [PATCH v3 21/22] cpufreq: add support for CPU DVFS based on SCMI
 message protocol



On 04/10/17 12:30, Arnd Bergmann wrote:
>> +static int scmi_cpufreq_probe(struct platform_device *pdev)
>> +{
>> +       int ret;
>> +
>> +       handle = devm_scmi_handle_get(&pdev->dev);
>> +
>> +       if (IS_ERR_OR_NULL(handle) || !handle->perf_ops)
>> +               return -EPROBE_DEFER;
> 
> As mentioned before, never create an interface that needs to use
> IS_ERR_OR_NULL(), make it return either NULL on error, or always
> have an error code.
> 

Agreed.

>> +
>> +static struct platform_driver scmi_cpufreq_platdrv = {
>> +       .driver = {
>> +               .name   = "scmi-cpufreq",
>> +       },
>> +       .probe          = scmi_cpufreq_probe,
>> +       .remove         = scmi_cpufreq_remove,
>> +};
> 
> You appear to have split this driver into the 'cpufreq' side and
> the 'protocol' handler in a different file. I already commented that
> the way the main scmi driver knows about all the protocols looks
> bad, here we can see another aspect of the same problem.
> 
> Rather than manually register a platform_device for the purpose
> of connecting it to the cpufreq driver, there should be a way
> to dynamically register the protocol from the cpufreq driver
> and then have both in the same file.
> 

I agree that should be possible. I took this approach for 2 reasons:

1. to avoid all sorts of probe ordering issues
2. we may have system with multiple instances of SCMI. E.g. a system
   may have multiple remote processors, each controlling dvfs/power mgmt
   of a subset of CPUs/devices controller in OS.

I have to admit that I haven't thought too much in details yet. That's
the main idea behind scmi_handle and restricting access to that only to
sub-nodes in it. I am open to suggestions.

-- 
Regards,
Sudeep