[RFC PATCH 0/3] ARM: Support Cortex-R platform(s)

[Vladimir Murzin]

On 7/12/22 21:44, Stefano Stabellini wrote:
> On Tue, 12 Jul 2022, Arnd Bergmann wrote:
>> On Tue, Jul 12, 2022 at 10:33 AM Vladimir Murzin
>> <vladimir.murzin at arm.com> wrote:
>>> On 6/30/22 09:36, Vladimir Murzin wrote:
>>>>
>>>> I've been running Linux on Cortex-R cores with downstream patches for
>>>> several years already. There are few reasons why we have not got any
>>>> real platform supporting Cortex-R cores so far:
>>>> 1) lack of interest
>>>> 2) lack of easily available platforms
>>>> 3) missing Kconfig bits
>>>>
>>>> During these years I've been receiving questions (mostly in private)
>>>> about running Linux with Cortex-R cores. Use cases vary, but mostly
>>>> fall under "we know Linux and do not want yet another RTOS", also
>>>> people not always care about real-time features of R-class cores and
>>>> see it as an upgrade from M-class cores.
>>>>
>>>> Sometime ago MPS3 platform got support for FPGA image [1] with
>>>> Cortex-R52 cores where Linux can live comfortably.
>>>>
>>>> This patchset addresses #3 and brings support for MPS3 platform
>>>> featuring Cortex-R52
>>>>
>>>> [1] https://developer.arm.com/downloads/-/download-fpga-images
>>>
>>> Anything I can do to make progress with the series or it falls under
>>> "we do not care" category?
>>
>> I think at this point it's an actively bad idea to merge support for Cortex-R.
>>
>> I don't think anyone else cares, but if you can find other people that want
>> this to get merged (and know what they are talking about), you can
>> overrule me on this.
>>
>> The existing MPS2 support is important because it gives developers
>> an easy way to test Cortex-M based NOMMU code, and we still have
>> users on STM32 at least. I do expect the STM32 MCU user base to
>> further shrink, to the point where nobody is updating their kernels any
>> more and we want to remove not just STM32 but all other Cortex-M
>> platforms. All others are already further down the road of decline and
>> MPS2 is not useful by itself.
>>
>> Merging MPS3/Cortex-R52 now feels like a step in the wrong
>> direction, if that leads to a future situation where we remove
>> Cortex-M but keep Cortex-R support around.
> 
> 
> I think this series is really cool if nothing else as a demo, so please
> make it available somewhere on kernel.org or github with some docs on
> how to use it because I am certain someone is going to try it and show
> it as a PoC (R52s are still going to be around for a long time). Your
> efforts are not going to go to waste :-)
> 
> On upstreamability: if the issue was just lack of reviews I can find
> time to step in and review the patches. But if the general idea is that
> we don't want to keep the code upstream in Linux then I am happy to go
> with Arnd's recommendation not to merge.

Thanks everyone for inputs! I've just pushed a branch [1] which contains
the series.

[1] https://gitlab.arm.com/linux-arm/linux-vm/-/tree/staging/rclass-5.19

Cheers
Vladimir