[linux-sunxi] [PATCH 2/5] arm: allwinner: a64: drop the dummy vcc3v3 regulator in Pine64 DT

[Andre Przywara]

Hi,

On 21/07/17 15:38, Maxime Ripard wrote:
> On Fri, Jul 21, 2017 at 02:02:18PM +0100, Andre Przywara wrote:
>> Hi,
>>
>> On 21/07/17 13:49, Icenowy Zheng wrote:
>>>
>>>
>>> 于 2017年7月21日 GMT+08:00 下午8:45:39, Andre Przywara <andre.przywara at arm.com> 写到:
>>>> Hi,
>>>>
>>>> On 19/07/17 17:10, Icenowy Zheng wrote:
>>>>> The Pine64 DT used to contain a dummy vcc3v3 regulator, in order to
>>>>> satisfy some device nodes when proper AXP803 regulator support is
>>>>> available. It's in fact the DCDC1 regulator of AXP803.
>>>>>
>>>>> Drop the dummy regulator, and fix the reference of this regulator to
>>>>> DCDC1.
>>>>
>>>> Do we really need to have this?
>>>> While I see that this is technically correct, it breaks older kernels,
>>>> which miss the AXP driver. So we can't use this DT for syncing it into
>>>> U-Boot anymore, while still expecting various kernels (for instance
>>>> from
>>>> distribution installers) to work via UEFI (for which U-Boot provides
>>>> the
>>>> DT). That would be a shame, because we start to see generic arm64
>>>> distribution installers to work out of the box.
>>>>
>>>> I see these solutions:
>>>> 1) We drop this patch, instead add a comment that technically it's
>>>> DCDC1. I believe we can't really turn off DCDC1 anyway.
>>>> 2) We keep theses patches, but don't sync them to U-Boot to have a
>>>> universal DT in there which works with every kernel.
>>>> 3) We keep these patches *and* sync them to U-Boot, but add the fixed
>>>> regulator back in via a U-Boot specific .dtsi "overlay" snippet. This
>>>> would take care of the parts that break compatibility. The end result
>>>> would be similar to 2), then.
>>>>
>>>> The easiest and most maintainable would be 1), but I am OK with 3) as
>>>> well, though I am not sure this won't get messy in the future and will
>>>> work for every change that we make.
>>>>
>>>> What do you think?
>>>
>>> 4) Do nothing.
>>>
>>> We only promise old DTs will run with newer kernel, but
>>> we don't promise newer DTs to run with old kernel.And
>>> U-Boot is intended to update less frequently than Linux.
>>>
>>> When updateing U-Boot, please update kernel as well.
>>
>> Which means you tie your firmware to a kernel. I know this is the old
>> embedded approach, but we should really get rid of this, as I don't see
>> how this will work nicely with the Pinebook, for instance (which is not
>> really "embedded" anymore).
>>
>> U-Boot sits on the SPI flash there, and you are expected to just run any
>> (not only Linux) distribution from a USB pen drive, for instance, with
>> that one firmware version, using UEFI. This already works today, but is
>> only sustainable if we have forward DT compatibility as well.
> 
> We've been discussing this over and over and over again.

Don't tell me ;-)
But apart from "We don't care" I haven't got a real solution out of this
discussion.

> You're using the pinebook as an example, fine.

I believe the current approach for supporting Allwinner boards is rooted
in some embedded world, where shipping firmware together with some
kernel is standard, especially if there is no on-board storage anyway.

But the Pinebook is clearly not embedded and comes with SPI flash to
boot from, so people might expect to install some Linux distribution on it.
And with the UEFI support in U-Boot we have a good solution for this
(check the debian-testing arm64 installer), and so far this works: every
extension we did to the DT was still fine with older kernels - this
particular feature might not work (say Ethernet in kernels < 2.13-rc1),
but at least it doesn't hurt or introduces regressions.

> Please give me the full documented,
> reviewed and acked-by binding for all the features the pinebook has.
> 
> If you can't, this discussion is pointless, since you will expect
> changes in the DT.

It's not about *changes* per se, it's about breaking compatibility,
which can be avoided.
As long as we just *add* features (DE2/HDMI, for instance) and don't
introduce regressions, touching the DT is fine.

And yes: I expect some hiccups with this, but also would hope for
finding some solutions (like the ones sketched in my original email).

Cheers,
Andre.