On 7/12/22 21:44, Stefano Stabellini wrote:

On Tue, 12 Jul 2022, Arnd Bergmann wrote:

quoted

On Tue, Jul 12, 2022 at 10:33 AM Vladimir Murzin
[off-list ref] wrote:

quoted

On 6/30/22 09:36, Vladimir Murzin wrote:

quoted

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

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