[PATCH 1/2] lib: utils: Implement "64bit-mmio" property parsing

[Anup Patel]

> -----Original Message-----
> From: Jessica Clarke <jrtc27 at jrtc27.com>
> Sent: 17 April 2021 18:58
> To: Anup Patel <Anup.Patel at wdc.com>
> Cc: guoren at kernel.org; Alistair Francis <Alistair.Francis at wdc.com>; Anup
> Patel <anup at brainfault.org>; OpenSBI <opensbi at lists.infradead.org>; Guo
> Ren <guoren at linux.alibaba.com>; Xiang W <wxjstz at 126.com>
> Subject: Re: [PATCH 1/2] lib: utils: Implement "64bit-mmio" property parsing
> 
> On 17 Apr 2021, at 14:26, Anup Patel <Anup.Patel at wdc.com> wrote:
> >
> >
> >
> >> -----Original Message-----
> >> From: Jessica Clarke <jrtc27 at jrtc27.com>
> >> Sent: 17 April 2021 18:47
> >> To: Anup Patel <Anup.Patel at wdc.com>
> >> Cc: guoren at kernel.org; Alistair Francis <Alistair.Francis at wdc.com>;
> >> Anup Patel <anup at brainfault.org>; OpenSBI
> >> <opensbi at lists.infradead.org>; Guo Ren <guoren at linux.alibaba.com>;
> >> Xiang W <wxjstz at 126.com>
> >> Subject: Re: [PATCH 1/2] lib: utils: Implement "64bit-mmio" property
> >> parsing
> >>
> >> On 17 Apr 2021, at 14:16, Anup Patel <Anup.Patel at wdc.com> wrote:
> >>>
> >>>
> >>>
> >>>> -----Original Message-----
> >>>> From: Jessica Clarke <jrtc27 at jrtc27.com>
> >>>> Sent: 17 April 2021 17:16
> >>>> To: guoren at kernel.org
> >>>> Cc: Alistair Francis <Alistair.Francis at wdc.com>; Anup Patel
> >>>> <anup at brainfault.org>; OpenSBI <opensbi at lists.infradead.org>; Guo
> >> Ren
> >>>> <guoren at linux.alibaba.com>; Anup Patel <Anup.Patel at wdc.com>;
> Xiang
> >> W
> >>>> <wxjstz at 126.com>
> >>>> Subject: Re: [PATCH 1/2] lib: utils: Implement "64bit-mmio"
> >>>> property parsing
> >>>>
> >>>> On 17 Apr 2021, at 12:41, Jessica Clarke <jrtc27 at jrtc27.com> wrote:
> >>>>>
> >>>>> On 13 Apr 2021, at 04:44, guoren at kernel.org wrote:
> >>>>>>
> >>>>>> From: Guo Ren <guoren at linux.alibaba.com>
> >>>>>>
> >>>>>> Figure out CLINT has_64bit_mmio from DT node and using antonym
> >>>>>> for compatibility.
> >>>>>>
> >>>>>> Signed-off-by: Guo Ren <guoren at linux.alibaba.com>
> >>>>>> Cc: Anup Patel <anup.patel at wdc.com>
> >>>>>> Cc: Xiang W <wxjstz at 126.com>
> >>>>>> ---
> >>>>>> lib/utils/fdt/fdt_helper.c | 3 ++-
> >>>>>> 1 file changed, 2 insertions(+), 1 deletion(-)
> >>>>>>
> >>>>>> diff --git a/lib/utils/fdt/fdt_helper.c
> >>>>>> b/lib/utils/fdt/fdt_helper.c index bf19ff9..909de01 100644
> >>>>>> --- a/lib/utils/fdt/fdt_helper.c
> >>>>>> +++ b/lib/utils/fdt/fdt_helper.c
> >>>>>> @@ -442,8 +442,9 @@ int fdt_parse_clint_node(void *fdt, int
> >>>> nodeoffset, bool for_timer,
> >>>>>> 	if (clint->hart_count < count)
> >>>>>> 		clint->hart_count = count;
> >>>>>>
> >>>>>> -	/* TODO: We should figure-out CLINT has_64bit_mmio from
> DT node
> >>>> */
> >>>>>> 	clint->has_64bit_mmio = TRUE;
> >>>>>> +	if (fdt_getprop(fdt, nodeoffset, "clint,has-no-64bit-mmio",
> >>>>>> +&count))
> >>>>>
> >>>>> I do not see this specified in clint.yaml.
> >>>>
> >>>> Also, the RISC-V privileged is very clear:
> >>>>
> >>>>> For RV64, naturally aligned 64-bit memory accesses to the mtime
> >>>>> and mtimecmp registers are atomic.
> >>>>
> >>>> So I would say your CLINT violates the spec and thus should not
> >>>> claim to be a RISC-V or SiFive-compatible CLINT.
> >>>
> >>> There is no CLINT spec maintained by RISC-V international.
> >>>
> >>> All implementation have SiFive compatible CLINT or some variant of it.
> >>
> >> That was a quote from the ratified privileged spec. The memory
> >> mapping may not be specified by RISC-V Intl, but the fact that
> >> mtime(cmp) are 64-bit registers is very much specified.
> >
> > Yes, mtime(cmp) has to be a 64bit wide as-per ratified spec but the
> > spec does not require 64bit MMIO to be supported for these MMIO
> registers.
> >
> > In fact, for RV32 64bit MMIO is not possible at all because machine
> > word is 32bits wide.
> >
> > The T-Head implementation does not violate the mtime(cmp)
> requirements
> > of ratified spec but it is certainly bit strange because they have a
> > RV64 system but their custom CLINT only supports 32bit MMIO accesses.
> 
> The quoted part of the spec says that, on RV64, aligned 64-bit accesses are
> atomic. There is the clear implication there that 64-bit accesses are allowed.
> This is entirely a hardware bug that violates the spec.

It does not say that aligned 64bit accesses have to be supported. There is
no mandate in the statement.

It only says if 64bit accesses are supported then they have to be atomic.

Regards,
Anup