[PATCH 4/5] lib: utils/timer: Allow ACLINT MTIMER supporting only 32-bit MMIO

[Anup Patel]

On Sat, Aug 7, 2021 at 7:55 PM Jessica Clarke <jrtc27 at jrtc27.com> wrote:
>
> On 24 Jul 2021, at 13:25, Anup Patel <anup.patel at wdc.com> wrote:
> >
> > We can have ACLINT MTIMER devices which only support 32-bit MMIO
> > accesses on RV64 system so this patch adds a boolean DT property
> > "mtimer,no-64bit-mmio" to detect this from MTIMER DT node.
>
> Can the ACLINT not just mandate 64-bit accesses must work? FreeBSD and
> Linux aren’t going to want to support such broken implementations, and
> I would hope at least the upcoming platform spec will mandate 64-bit
> access, if not the ACLINT itself.

This has been discussed on the AIA mailing lists.

The ACLINT spec can't mandate 64-bit MMIO access because there is
a cost associated in using wider bus between harts and ACLINT device
to support 64-bit MMIO accesses.

Andrew confirmed that the RISC-V privilege spec does not explicitly
state that 64-MMIO accesses are mandatory for MTIME and MTIMECMP
on RV64 systems.

Refer:
https://lists.riscv.org/g/tech-aia/message/114
https://lists.riscv.org/g/tech-aia/message/115
https://lists.riscv.org/g/tech-aia/message/117
https://lists.riscv.org/g/tech-aia/message/118
https://lists.riscv.org/g/tech-aia/message/119
https://lists.riscv.org/g/tech-aia/message/120


Regards,
Anup



>
> Jess
>
> > Signed-off-by: Anup Patel <anup.patel at wdc.com>
> > ---
> > lib/utils/timer/fdt_timer_mtimer.c | 3 +++
> > 1 file changed, 3 insertions(+)
> >
> > diff --git a/lib/utils/timer/fdt_timer_mtimer.c b/lib/utils/timer/fdt_timer_mtimer.c
> > index b08ed38..15a36ed 100644
> > --- a/lib/utils/timer/fdt_timer_mtimer.c
> > +++ b/lib/utils/timer/fdt_timer_mtimer.c
> > @@ -58,6 +58,9 @@ static int timer_mtimer_cold_init(void *fdt, int nodeoff,
> >               mt->mtime_size = size[0];
> >               mt->mtimecmp_addr = addr[1];
> >               mt->mtimecmp_size = size[1];
> > +             /* Parse additional ACLINT MTIMER properties */
> > +             if (fdt_getprop(fdt, nodeoff, "mtimer,no-64bit-mmio", &rc))
> > +                     mt->has_64bit_mmio = false;
> >       }
> >
> >       rc = aclint_mtimer_cold_init(mt, mtmaster);
> > --
> > 2.25.1
> >
> >
> > --
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>