linux-kernel - Re: [PATCH 2/2] dt-bindings: cpufreq: Document operating-points-v2-sunxi-cpu

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Message-ID: <5721bddc-4995-c38f-3f45-7a4589a1ccb5@sholland.org>
Date:   Fri, 5 Apr 2019 22:26:20 -0500
From:   Samuel Holland <samuel@...lland.org>
To:     Maxime Ripard <maxime.ripard@...tlin.com>,
        Yangtao Li <tiny.windzz@...il.com>
Cc:     vireshk@...nel.org, nm@...com, sboyd@...nel.org,
        robh+dt@...nel.org, mark.rutland@....com, wens@...e.org,
        rjw@...ysocki.net, davem@...emloft.net, mchehab+samsung@...nel.org,
        gregkh@...uxfoundation.org, nicolas.ferre@...rochip.com,
        linux-pm@...r.kernel.org, devicetree@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] dt-bindings: cpufreq: Document
 operating-points-v2-sunxi-cpu

On 4/5/19 9:55 AM, Maxime Ripard wrote:
> Hi,
> 
> On Fri, Apr 05, 2019 at 06:24:55AM -0400, Yangtao Li wrote:
>> Allwinner Process Voltage Scaling Tables defines the voltage and
>> frequency value  based on the speedbin blown in the efuse combination.
>> The sunxi-cpufreq-nvmem driver reads the efuse value from the SoC to
>> provide the OPP framework with required information.
>> This is used to determine the voltage and frequency value for each
>> OPP of operating-points-v2 table when it is parsed by the OPP framework.
>>
>> This change adds documentation for the DT bindings.
>> The "operating-points-v2-sunxi-cpu" DT extends the "operating-points-v2"
>> with following parameters:
>> - nvmem-cells (NVMEM area containig the speedbin information)
>> - opp-supported-hw: A single 32 bit bitmap value,
>>   representing compatible HW:
>> 			0:      speedbin 0
>> 			1:      speedbin 1
>> 			2:      speedbin 2
>> 			3-31:   unused
>>
>> Signed-off-by: Yangtao Li <tiny.windzz@...il.com>
>> ---
>>  .../bindings/opp/sunxi-nvmem-cpufreq.txt      | 235 ++++++++++++++++++
>>  1 file changed, 235 insertions(+)
>>  create mode 100644 Documentation/devicetree/bindings/opp/sunxi-nvmem-cpufreq.txt
>>
>> diff --git a/Documentation/devicetree/bindings/opp/sunxi-nvmem-cpufreq.txt b/Documentation/devicetree/bindings/opp/sunxi-nvmem-cpufreq.txt
>> new file mode 100644
>> index 000000000000..80201d4e5147
>> --- /dev/null
>> +++ b/Documentation/devicetree/bindings/opp/sunxi-nvmem-cpufreq.txt
>> @@ -0,0 +1,235 @@
>> +Allwinner Technologies, Inc. NVMEM CPUFreq and OPP bindings
>> +===================================
>> +
>> +For some SoCs, the CPU frequency subset and voltage value of each OPP
>> +varies based on the silicon variant in use. Allwinner Process Voltage
>> +Scaling Tables defines the voltage and frequency value  based on the
>> +speedbin blown in the efuse combination. The sunxi-cpufreq-nvmem driver
>> +reads the efuse value from the SoC to provide the OPP framework with
>> +required information.
>> +
>> +Required properties:
>> +--------------------
>> +In 'cpus' nodes:
>> +- operating-points-v2: Phandle to the operating-points-v2 table to use.
>> +
>> +In 'operating-points-v2' table:
>> +- compatible: Should be
>> +	- 'operating-points-v2-sunxi-cpu'.
>> +- nvmem-cells: A phandle pointing to a nvmem-cells node representing the
>> +		efuse registers that has information about the
>> +		speedbin that is used to select the right frequency/voltage
>> +		value pair.
>> +		Please refer the for nvmem-cells
>> +		bindings Documentation/devicetree/bindings/nvmem/nvmem.txt
>> +		and also examples below.
>> +
>> +In every OPP node:
>> +- opp-supported-hw: A single 32 bit bitmap value, representing compatible HW.
>> +		    Bitmap:
>> +			0:	speedbin 0
>> +			1:	speedbin 1
>> +			2:	speedbin 2
>> +			3-31:	unused
> 
> I'm wondering if that's the right approach.
> 
> I guess we could also have three different OPP tables, and pass them
> all three through a phandle array, and have the kernel code select
> which one is relevant based on the SID content
> 
> Another option would be to use the OF_DYNAMIC code to fill
> operating-points-v2 at kernel boot, before (or when) cpufreq kicks in.
> 
> ATF could also do that work.

While ATF has code to read and parse the DTB (which it uses to set up the PMIC),
it's not really in a position to modify the DTB. For one thing, it only has
access to the DTB appended to u-boot, not any DTB loaded from disk. And for two,
that would add a significant amount of code, when ATF is already reaching its
size limits.

Cheers,
Samuel