On Wed, Sep 07, 2016 at 03:20:14PM +0100, Peter Maydell wrote:
> On 7 September 2016 at 13:58, Christoffer Dall
> <christoffer.dall@xxxxxxxxxx> wrote:
> > On Wed, Sep 07, 2016 at 11:48:52AM +0100, Vladimir Murzin wrote:
> >> On 06/09/16 17:55, Christoffer Dall wrote:
> >> > On Tue, Sep 06, 2016 at 02:23:16PM +0100, Vladimir Murzin wrote:
> >> >>
> >> >> Sorry, missed this one
> >> >>
> >> >> On 05/09/16 12:29, Christoffer Dall wrote:
> >> >>>>
> >> >>>>> +static bool __hyp_text __has_useable_gicv3_cpuif(void)
> >> >>>>> +{
> >> >>>>> + if (IS_ENABLED(CONFIG_ARM_GIC_V3) && (read_sysreg(ID_PFR1) >> 28))
> >> >>> Do we have a define for bit 28 we could use?
> >> >>
> >> >> I'll check it.
> >> >>
> >> >>>
> >> >>> Does this actually work on all v7 boards? The v7 ARM ARM seems to state
> >> >>> that this bitfield is Reserved, UNK. Does that somehow mean 'is going
> >> >>> to be zero'?
> >> >>
> >> >> It is how v7ARM ARM I have defines UNK
> >> >>
> >> >> An abbreviation indicating that software must treat a field as
> >> >> containing an UNKNOWN value. Hardware must implement the bit as read as
> >> >> 0, or all 0s for a bit field. Software must not rely on the field
> >> >> reading as zero.
> >> >>
> >> >> It seems goes under 'is going to be zero' case, no?
> >> >>
> >> > The last sentence is disturbing to me, and feels slightly contradicting
> >> > itself. Reading the UNKNOWN description doesn't help much either.
> >> >
> >> > Perhaps you can ask around internally and figure out what the precise
> >> > answer to this is?
> >>
> >> Since it is kind of implementation dependant thing the precise answer
> >> from here hardly help, IMO. We still have non-zero chance to see
> >> something scary.
> >
> > Well, if the precise answer is: This will actually always return 0
> > because of X and Y, then your code is fine.
>
> I think the "must not rely on the field reading as zero" wording
> in the case of the ID registers is intended to mean "in a
> future rev of the architecture we may assign these bits,
> and your code mustn't do something that will break if the
> bits then read as something other than zero". (And indeed
> in v8 bits 28..31 have an assigned meaning.) It doesn't
> mean there'll be v7 hardware out there with non-zero
> values, because that would be breaking the hardware's part
> of the UNKNOWN contract ("must implement the bit as read as 0").
>
ok, that was the kind of answer I was hoping for. In that case, this is
a non-issue.
Thanks for the clarification.
-Christoffer
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