[PATCH/RFC] ARM: dts: marzen: Add FLASH node

[Geert Uytterhoeven]

On Tue, Mar 21, 2023 at 4:01 PM Geert Uytterhoeven <geert at linux-m68k.org> wrote:
> On Mon, Mar 20, 2023 at 7:57 PM Geert Uytterhoeven <geert at linux-m68k.org> wrote:
> > On Mon, Mar 20, 2023 at 6:04 PM Tudor Ambarus <tudor.ambarus at linaro.org> wrote:
> > > Vignesh used to review CFI code, maybe he can intervene. I've never
> > > worked with CFI, but I can try to help. I'll need more debug data though.
> > >
> > > On 3/20/23 16:43, Geert Uytterhoeven wrote:
> > > > Add a device node for the Spansion S29GL512N NOR FLASH on the Marzen
> > > > development board.
> > > >
> > > > Signed-off-by: Geert Uytterhoeven <geert+renesas at glider.be>
> > > > ---
> > > > Although the S29GL512N is a CFI FLASH, using "cfi-flash" instead of
> > > > "mtd-rom" does not work:
> > > >   1. Probing fails with "physmap-flash 0.flash: map_probe failed",
> > >
> > > I would first try to understand why the probe fails.

> I suddenly remembered I have a different board (APE6-EVM), where
> the CFI-FLASH stopped working after adding support for secondary
> CPUs. I always thought that was a hardware quirk...
>
> Turns out the CFI-FLASH on Marzen (quad Cortex-A9) is detected when
> booting with "nosmp":

> My first guess was that the probing process is migrated to a different
> CPU core during probing, but printing smp_processor_id() didn't
> confirm that; it's just running on a different CPU core than CPU0.
> Wrapping the body of cfi_qry_mode_on() inside a get_cpu()/put_cpu()
> pair to prevent migration also didn't fix it.
>
> Is CFI-FLASH known-broken on SMP?

After actively looking for more boards with CFI FLASHes, and finding one
more board where FLASH probing fails on SMP, I dug deeper.
Turns out they all have in common that (a) the CFI FLASH is located at
physical address zero, and (b) the secondary CPU bringup code relies
on mapping (by special hardware) the region at address zero to the
CPU boot code...

Hence fixing this involves making sure that accessing FLASH and bringing
CPU cores online do not happen concurrently...

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert at linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds