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

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:

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On 5/20/21 10:13 PM, Sean Anderson wrote:

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On 5/19/21 3:24 AM, Michal Simek wrote:

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On 5/18/21 12:15 AM, Sean Anderson wrote:

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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.

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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.

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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.

Thanks,
Michal