[GIT PULL] arm: mediatek: DT updates for v5.19

[Matthias Brugger]

On 13/05/2022 14:11, Arnd Bergmann wrote:
> On Fri, May 13, 2022 at 1:21 PM Matthias Brugger <matthias.bgg at gmail.com> wrote:
>>
>> Hi Arnd and Olof,
>>
>> Please have a look on the below 32 bit DT updates for v5.19. Apologies for the
>> late pull request but I got sick and needed my time to recover.
> 
>> Yong Wu (1):
>>         arm: dts: mediatek: Get rid of mediatek, larb for MM nodes
> 
> Can you clarify what this means for backwards compatibility? I see old kernels
> used to parse these properties, does that mean that the updated DT is no
> longer compatible with them?
> 

My understanding is, that backwards compatibility is given when using older DT 
with newer kernel. This should be the common case, normally you update your 
software but nearly never your FW. No compatibility is given when using older 
kernel with newer DT.

I think that's an ongoing debate if we should provide backwards compatibility 
for both kernel and DT.

> If you break compatibility, this should only be done for exceptional reasons,
> and explain the tradeoffs. What is the oldest kernel that is still compatible
> with the updated DT files, and why can't we just keep the properties around?
> 

First kernel version that does not work with the old DT will be v5.18. This 
patch is the outcome of a change in the yaml file [1] which has a Acked-by from 
Rob. I double checked and I wasn't able to find the mail where Rob did give his 
Acked-by... Yong, can you provide a link to that email? I can see you added the 
Acked-by in v2 of the series.

One thing that comes to mind, that we mark the larb phandle as deprecated in the 
yaml file, instead of deleting it. Then we could keep that in the DT files, 
although newer kernels won't work with that. Another option is, to add the code 
deleted by
ba3cd6714aed ("media: mtk-jpeg: Get rid of mtk_smi_larb_get/put")

back as fallback for older DTs. Although I fear there is much more things to fix 
in a lot of drivers. DRM, mdp and memory/mtk-smi.c are some that I found in a 
'quick' look at the problem.

Taking into account that and the fact that we are talking mainly about 
chromebooks and the Bananapi R2 as public available HW, I think we can live with 
the compatibility breakage of newer DTs not working with older kernels.

Anyway as you can see, this patch is just the tip of the ice-berg. So if you 
feel that's something unacceptable we will need to chase people to fix backward 
compatibility.

Regards,
Matthias

[1] 
https://lore.kernel.org/linux-mediatek/20220117070510.17642-2-yong.wu@mediatek.com/