Re: [PATCH v3 2/2] clocksource: Add support for Xilinx AXI Timer

From: Sean Anderson
Date: Fri Jun 18 2021 - 17:24:20 EST

On 6/16/21 8:12 AM, Michal Simek wrote:
> Hi Uwe,
> On 5/25/21 8:11 AM, Uwe Kleine-König wrote:
>> Hello Sean, hello Michal,
>> On Mon, May 24, 2021 at 09:00:51AM +0200, Michal Simek wrote:
>>> On 5/20/21 10:13 PM, Sean Anderson wrote:
>>>> On 5/19/21 3:24 AM, Michal Simek wrote:
>>>>> On 5/18/21 12:15 AM, Sean Anderson wrote:
>>>>>> This could be deprecated, but cannot be removed since existing device
>>>>>> trees (e.g. qemu) have neither clocks nor clock-frequency properties.
>>>>> Rob: Do we have any obligation to keep properties for other projects?
>> If a binding is in the wild and used to be documented, it has to stay.
>>>>>>> 4. Make driver as module
>>>>>>> 5. Do whatever changes you want before adding pwm support
>>>>>>> 6. Extend DT binding doc for PWM support
>>>>>>> 7. Add PWM support
>>>>>> Frankly, I am inclined to just leave the microblaze timer as-is. The PWM
>>>>>> driver is completely independent. I have already put too much effort into
>>>>>> this driver, and I don't have the energy to continue working on the
>>>>>> microblaze timer.
>>>>> I understand. I am actually using axi timer as pwm driver in one of my
>>>>> project but never had time to upstream it because of couple of steps above.
>>>>> We need to do it right based on steps listed above. If this is too much
>>>>> work it will have to wait. I will NACK all attempts to add separate
>>>>> driver for IP which we already support in the tree.
>>>> 1. Many timers have separate clocksource and PWM drivers. E.g. samsung,
>>>> renesas TPU, etc. It is completely reasonable to keep separate
>>>> drivers for these purposes. There is no Linux requirement that each
>>>> device have only one driver, especially if it has multiple functions
>>>> or ways to be configured.
>>> It doesn't mean that it was done properly and correctly. Code
>>> duplication is bad all the time.
>> IMHO it's not so much about code duplication. Yes, code duplication is
>> bad and should be prevented if possible. But it's more important to not
>> introduce surprises. So I think it should be obvious from reading the
>> device tree source which timer is used to provide the PWM. I don't care
>> much if this is from an extra property (like xilinx,provide-pwm),
>> overriding the compatible or some other explicit mechanism. IIUC in this
>> suggested patch the selection is implicit and so this isn't so nice.
>>>> 2. If you want to do work on a driver, I'm all for it. However, if you
>>>> have not yet submitted that work to the list, you should not gate
>>>> other work behind it. Saying that X feature must be gated behind Y
>>>> *even if X works completely independently of Y* is just stifling
>>>> development.
>>> I gave you guidance how I think this should be done. I am not gating you
>>> from this work. Your patch is not working on Microblaze arch which is
>>> what I maintain. And I don't want to go the route that we will have two
>>> drivers for the same IP without integration. We were there in past and
>>> it is just pain.
>>> I am expecting that PWM guys will guide how this should be done
>>> properly. I haven't heard any guidance on this yet.
>>> Thierry/Uwe: Any comment?
>> Not sure I can and want to provide guidance here. This is not Perl, but
>> still TIMTOWTDI. If it was me who cared here, I'd look into the
>> auxiliary bus (Documentation/driver-api/auxiliary_bus.rst) to check if
>> it can help to solve this problem.
> I recently got patches for cadence TTC driver
> (drivers/clocksource/timer-cadence-ttc.c) for PWM support too. It is the
> second and very similar case. This driver is used on Zynq as clock
> source and can be also use as PWM. I can't believe that there are no
> other examples how to deal with these timers which are used for PWM
> generation.

The approach I took in v4 is that probe functions and driver callbacks
live in drivers/timer and drivers/pwm, and common functions live in
drivers/mfd (although I may move them to drivers/timer since Lee Jones
doesn't like them there).

I would greatly appreciate if you could review v4. It has been on the
list for three weeks now with no comments from either you or Uwe.