Re: [PATCH 2/5] pwm: sun4i: Add support for PWM controller on sun6i SoCs

[Icenowy Zheng <icenowy-ymACFijhrKM@public.gmane.org>]

Hi,

14.10.2016, 20:58, "Maxime Ripard" <maxime.ripard-wi1+55ScJUtKEb57/3fJTNBPR1lH4CV8@public.gmane.org>:
> Hi,
>
> On Wed, Oct 12, 2016 at 04:10:03PM +0800, Icenowy Zheng wrote:
>>  > On Wed, Oct 12, 2016 at 12:20 PM, Icenowy Zheng <icenowy-87fMjt6guo8@public.gmane.orgz> wrote:
>>  > > The PWM controller in A31 is different with other Allwinner SoCs, with
>>  > > a control register per channel (in other SoCs the control register is
>>  > > shared), and each channel are allocated 16 bytes of address (but only 8
>>  > > bytes are used.). The register map in one channel is just like a
>>  > > single-channel A10 PWM controller, however, A31 have a different
>>  > > prescaler table than other SoCs.
>>  > >
>>  > > In order to use the driver for all 4 channels, device nodes should be
>>  > > created per channel.
>>  >
>>  > I think Maxime wants you to support the different register offsets
>>  > in this driver, and have all 4 channels in the same device (node).
>>
>>  I think that will make the code much more complex... And in
>>  hardware there may also be 4 controllers... as the register is
>>  aligned at 0x10.
>
> You also have to think about it the other way around. This is exposed
> everywhere as a single device. There may be some undocumented
> registers hidden somewhere in the memory space of that device. How
> would you deal with that without touching the device tree?

Is the reason only they're listed in the one chapter of user manual?

>
> Exposing this as a single device is the best solution both from the
> philosophical point of view, but also from a maintainance aspect.

If we really do so, I will go back to the original patch (pwm-sun6i)
and merge 4 channels.

No other PWM block of Allwinner devices uses 4 control registers, and it's
a significant difference to make it a dedicated driver.

However, I still think we should have 4 nodes, since the 4 channels can work
very dedicatedly, with different control register... This can be a reason to see
them as 4 dedicated controllers.

(And as PWM uses only oscXX, we cannot judge it according to the clock tree,
and Occam's Razor will apply to think it's 4 A10-like PWM controller...)

If we just think it's because it's a whole part, why don't we combine ehci and ohci
to one driver? Just because we can reuse {e,o}hci-platform...

It's the same reason to see it as 4 controllers.

>
> Maxime
>
> --
> Maxime Ripard, Free Electrons
> Embedded Linux and Kernel engineering
> http://free-electrons.com

Regards,
Icenowy

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