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

[Stefano Stabellini]

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.