Re: [RFC PATCH v2 0/6] ARM: OMAP3+: Introduce ABB driver

[Andrii Tseglytskyi <andrii.tseglytskyi@ti.com>]

On 04/16/2013 10:18 PM, Kevin Hilman wrote:
> Andrii Tseglytskyi <andrii.tseglytskyi@ti.com> writes:
>
>> Hi Kevin,
>>
>> On 04/16/2013 12:53 AM, Kevin Hilman wrote:
>>> Andrii Tseglytskyi <andrii.tseglytskyi@ti.com> writes:
>>>
>>>> From: "Andrii.Tseglytskyi" <andrii.tseglytskyi@ti.com>
>>>>
>>>> Following patch series introduces the Adaptive Body-Bias
>>>> LDO driver, which handles LDOs voltage during OPP change routine.
>>>> Current implementation is based on patch series from
>>>> Mike Turquette:
>>>>
>>>> http://marc.info/?l=linux-omap&m=134931341818379&w=2
>>>>
>>>> ABB transition is a part of OPP changing sequence.
>>>> ABB can operate in the following modes:
>>>> - Bypass mode: Activated when ABB is not required
>>>> - FBB mode: Fast Body Bias mode, used on fast OPPs
>>> Fast?  I thought the 'F' was for Forward?
>> You are right. Should be 'Forward' here.
>>
>>>> - RBB mode: Reverse Body Bias mode, used on slow OPPs
>>>>
>>>> In current implementation ABB is converted to regulator.
>>>> Standalone OPP table is used to store ABB mode, it is defined
>>>> in device tree for each ABB regulator. It has the following format:
>>>>
>>>> operating-points = <
>>>> 	       /* uV   ABB (0 - Bypass, 1 - FBB, 2 - RBB) */
>>>> 	       880000		0
>>>> 	       1060000		1
>>>> 	       1250000		1
>>>> 	       1260000		1
>>>>> ;
>>>> ABB regulator is linked to regulator chain
>>> In addition to Mike's comments (which I completely agree with), it would
>>> be very helfpul to see how this is actually used.  e.g, how the
>>> regulators are chained together, how the proper ordering is managed,
>>> etc. etc.
>> We would like to handle voltage scaling in the following way:
> What I meant is that a detailed description of the use case should be
> included in the changelog.
>
>> cpufreq_cpu0
>> clk_set_rate(cpu0)
>>      |
>>      |-->set_voltage(ABB regulator) /* all ABB related stuff will be
>> handled here */
>>                  |
>>                  |-->set_voltage(smps123 regulator) /* actual voltage
>> scaling */
> -EASCII_ART_WRAP
>
>> This simple model will be extended to handle AVS as a part of the chain.
>> smps123 regulator may be changed to VP/VC regulator.
>>
>> Following example is from integration branch, which already has
>> smps123 regulator.
> I don't know what integration branch you're referring to, and I don't
> know what the smps123 regulator is.
>
>> It demonstrates an example of linkage to chain. ABB regulator is
>> linked with smps123 and cpu0 inside device tree.
>> cpu0 calls set_voltage() function for ABB, and then ABB calls
>> set_voltage() function for smps123 to do actual voltage scaling.
>>
>> diff --git a/arch/arm/boot/dts/omap5.dtsi b/arch/arm/boot/dts/omap5.dtsi
>> index bb5ee70..c8cbbee 100644
>> --- a/arch/arm/boot/dts/omap5.dtsi
>> +++ b/arch/arm/boot/dts/omap5.dtsi
>> @@ -36,7 +36,7 @@
>>          cpus {
>>                  cpu@0 {
>>                          compatible = "arm,cortex-a15";
>> -                       cpu0-supply = <&smps123_reg>;
>> +                       cpu0-supply = <&abb_mpu>;
>>                          operating-points = <
>>                                  /* kHz    uV */
>>                                  /* Only for Nominal Samples */
>> @@ -94,6 +94,7 @@
>>                  reg = <0x4ae07cdc 0x8>,
>>                        <0x4ae06014 0x4>;
>>                  ti,tranxdone_status_mask = <0x80>;
>> +               avs-supply = <&smps123_reg>;
>>                  operating-points = <
>>                          /* uV   ABB */
>>                          880000  0
>>
>> This RFC patch series is verified together with:
>> https://patchwork.kernel.org/patch/2445091/
>>
>> Kevin, what do you think about this model in general? Does it fit to
>> regulator framework?
> I don't know yet, because I don't think the use case has been described
> well enough for me to fully understand it the motiviation behind the
> series.
>
> In addition, there are alternative approaches that seem to have been
> ruled out without describing why.  For example, the regulator framework
> already allows you to override methods with custom hooks (as we do for
> VC/VP controlled regulators already.)  Without thinking about it too
> deeply, it seems this approach could be used to manage the chain of
> events you need as well.  I can imagine there are limitations to this
> approach for what you're trying to do, but I don't feel they have been
> described in the changelog as part of the motivation for this series.
>
> So for now, the guidance I have is this:
>
> First, write changelogs (and cover letters) assuming your audience has
> not been staring at the code as long as you have.  Even if they have
> been staring at the same code, assume they've been staring at mainline,
> and not a random integration branch somewhere.  My general advice is to
> write changelogs in a way that you will understand what you wrote a year
> from now after having forgotten all the details currently in your brains
> cache.  Even better, write them so that I will understand them in a year
> since I forget much better than I remember.
>
> Second, before inventing something new, start with the existing
> framework.  When the existing framework doesn't work, make an argument
> for your new approach or extentions to the framework based on why the
> existing stuff doesn't work.  If you don't do this, the reviewers first
> reaction will almost always be "why don't you use what already exists in
> the framework."  And then you'll have a bunch of back and forth with
> reviewers when you could've explained the reasoning from the beginning.
>
> Hope that helps,
>
> Kevin
>
>
Hi Kevin, Mike

Thanks a lot for your comments.

"regulator chain" approach was added to ABB series to demonstrate how this
approach works in general, and get more comments on this.
It was initially introduced in:

https://lkml.org/lkml/2013/4/5/33 <https://lkml.org/lkml/2013/4/5/33> - 
regulator: query on regulator re-entrance
http://www.spinics.net/lists/linux-omap/msg90022.html - [RFC v1 0/1] 
introduce regulator chain locking scheme

Taking in account your comments - I'll split ABB logic and "regulator 
chain" logic,
ABB has a possibility to work standalone. In this case DVFS framework 
will be responsible
for calls like *regulator_set_voltage(abb_regulator)*.

Could you please exclude "regulator chain" approach from this review and 
take a look to ABB handling logic?

Regards,
Andrii