Re: [PATCH v3] serial: 8250_of: Add basic PM runtime support

[Sekhar Nori]

On Wednesday 23 August 2017 12:49 PM, Franklin S Cooper Jr wrote:
> 
> 
> On 08/23/2017 01:34 AM, Sekhar Nori wrote:
>> On Monday 21 August 2017 10:29 PM, Franklin S Cooper Jr wrote:
>>>
>>>
>>> On 08/21/2017 07:20 AM, Sekhar Nori wrote:
>>>> On Thursday 17 August 2017 02:25 AM, Franklin S Cooper Jr wrote:
>>>>> 66AK2G UART instances are not apart of the ALWAYS_ON power domain.
>>>>> Therefore, pm_runtime calls must be made to properly insure the appropriate
>>>>> power domains needed by UART are on. Keep legacy clk api calls since other
>>>>> users of this driver may not support PM runtime.
>>>
>>> There are a significant amount of users across a wide variety of
>>> architectures and boards that I have no means to properly test to insure
>>> I'm avoiding regressions. My current approach which I've seen other
>>> drivers in the past use when in similar situations allows things to work
>>> without regressions.
>>
>> Since the users are all device-tree users, my hope is they can be
>> tracked by looking at the DT files and the maintainers be copied on this
>> patch to make sure nothing breaks for them. I understand its not a
>> trivial list, but its finite :)
>>
>>>>
>>>> Do we really have users like that? And even if there are, cant they use
>>>> PM clock handling support available in drivers/base/power/clock_ops.c ?
>>>
>>> I don't see any current defconfig that enables CONFIG_PM_CLK and only a
>>
>> Thats because its a non-user-selectable def_bool. Most configs should
>> have it enabled after the 'make config' step.
>>
>>> handful of instances where functions from clock_ops.c are actually used.
>>> I don't quite understand what your suggestion is but in general I'm
>>> concerned since any approach to move everyone to different apis is
>>> rather risky especially for a critical driver.
>>>
>>> If I'm missing your point please let me know.
>>
>> I do agree we dont want to break anyone. But at the same time, this
>> patch redundantly enables/disables clock for most known (and possible
>> future) users without no clear indication of who benefits from such
>> redundancy.
>>
>> If we ignore this now, the problem will only get worse as time passes. I
>> suggest biting the bullet now, attempt to reach-out as many affected
>> parties as possible but switch to pm_runtime once and for all.
>>
>>>
>>>>
>>>> The clock enable support itself was added pretty "recently" - about 5
>>>> years back with 0bbeb3c3e84b ("of serial port driver - add
>>>> clk_get_rate() support"). So I doubt any really legacy platforms relied
>>>> on clock support being there. It was added by Murali, I assume for
>>>> Keystone devices. Keystone devices can work with runtime PM using the PM
>>>> clock support pointed to above.
>>>
>>> You might be right but I can't be confident that it is indeed the case.
>>> But its possible that at some point people will start having problems if
>>> they try to use this driver for other UART instances. The currently
>>> could not be aware of an issue because of the bootloader powering things
>>> for them or even that different UART instances could be apart of a non
>>> always on power domain.
>>
>> I dont think you got my point there. I have no disagreement that
>> pm_runtime support is needed. In fact, it should have been that way when
>> clock enable/disable was added. But thats history now.
>>
>>>> Perhaps linux-arm-kernel list should be copied on this submission too,
>>>> since most users of this driver are likely to be there on that list.
>>>>
>>>
>>> Looking at configs that enable CONFIG_SERIAL_OF_PLATFORM I see quite a
>>> bit of users from different arch. ARM, OpenRISC, MicroBlaze, Nios2,
>>> PowerPC, MIPS, Xtensa and Arc.>
>>> Looking at dts that enable some of the compatible it is still a
>>> combination of quite a bit of architectures.
>>
>> Right, I do think there are quite a few users. But they should still be
>> reachable and provide testing inputs for the change.
>>
>>> The current approach I've taken should be safe for all users of this
>>> driver which. I have no issue going another approach as long as its
>>> understood that I'm fairly limited in what I can test when you take into
>>> account the large number of users of this driver.
>>
>> Understood that you are limited by hardware you possess. But we can
>> probably reach out to as many folks affected by this as possible so we
>> get a fair chance at doing the "right thing" without breaking anyone.
> 
> I guess I don't understand the acceptable criteria when we can go
> forward and make this change for something that impacts so many users
> across so many architectures and SoC families.
> 
> I have no issue increasing the audience for a pm_runtime only version of
> this patch to hopefully get more eyes on it. But is the expectation for
> all the various maintainers of all the various dts to come out and say
> they are ok with the change and then we will go forward? Or just atleast

In a perfect world, yes..

> half of them say they are ok with switching and no one else objects?
> What is the criteria so this doesn't stay in indefinite limbo or when
> will we be confident enough that this (some variant of the patch)

.. but I think we are just trying to get as many acks as possible
without someone saying it breaks their system. I don't think we want to
wait indefinitely, just long enough to give everyone a reasonable chance
to test and comment (a couple of weeks).

> doesn't break things for someone? If using pm_runtime only breaks things
> for someone is the answer to revert pm_runtime switch or will the answer
> that they need to fix their SoC to fix the problem.

Its tough to say now what will be the course if switching to pm_runtime
does break someone's system. If it happens, most probably its fixable by
calling pm_clk_add_notifier() for that machine's platform bus.

> Forgive me for all the questions and worrying. This is new grounds for
> me especially when I'm unable to test things to avoid causing problems
> for people.

No problem. You may also wait a while to see if there are opposing ideas
before posting a v4.

Thanks,
Sekhar