[PATCH] riscv: deprecate CONFIG_MMU=n

[Conor Dooley]

On Mon, Feb 26, 2024 at 11:00:41AM -0800, Charles Lohr wrote:
> WOAH! Please DO NOT deprecate NOMMU. I use the NOMMU build constantly
> and NOMMU Linux on RISC-V is the avenue used by many FPGA soft cores
> for Linux, as well as some limited systems.
> 
> I get new copies of the kernel when there are releases and test them
> frequently to make sure everything is still working as expected.

That is great - it is good to know that people are actively testing.
I was aware that a lot of the soft core folks did run nommu kernels (and
I know some do use XIP also) but everything I ever saw was running on
old kernels (5.x).

> On Mon, Feb 26, 2024 at 8:03 AM Conor Dooley <conor at kernel.org> wrote:
> >
> > On Mon, Feb 26, 2024 at 04:25:24PM +0100, Clément Léger wrote:
> > > I guess I could also mark XIP as deprecated.
> >
> > I'm not so sure, people recently added XIP support to QEMU (and sent
> > kernel fixes in December). XIP is also not nearly as much of a problem
> > to support, there's far less that it does differently, the main barrier
> > was the inability to test it which is no longer the case.
> > That said, XIP is gonna kill itself off I feel as it does not support
> > runtime patching and therefore is extremely limited on extensions, given
> > we use alternatives for all of that (although I suppose if someone has a
> > usecase they could make nasty macros worse and implement a compiletime
> > switch in the alternatives too).

> For us we just don't care about XIP. I mean if someone did push it
> through to fruition, I'd also test and use it, but I urge you please
> do not deprecate this.

XIP does work. What I was talking about here was supporting something
"fancier" than rv{32,64}imafdc.

> While it's sometimes needed a bit of a
> creative build to get everything working, I've never needed to patch
> anything in the kernel beyond patching in a custom console for serial
> output.
> 
> I am happy to discuss the possibility of me and or one of the other
> RISC-V soft (FPGA) core people stepping up to try to be more active,
> but so far we've just been very well serviced by the current NOMMU
> Linux setup.

Most of the issues aren't with nommu actually working, it is the extra
effort in development as it has to be accounted for. I would estimate
that 2/3 of the build issues I report on this list are nommu. The
best thing that you can do to ensure support for things you use is:
a) scream when someone wants to remove it
b) actively let people know you're using it

Seems like you're doing a) but maybe getting someone that provides
Tested-bys whenever you test the releases would be good.

Cheers,
Conor.

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