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SubjectRe: [PATCH] PCI/ASPM: Use 64-bit arithmetic instead of 32-bit
On Tue, Feb 13, 2018 at 01:05:50PM -0600, Gustavo A. R. Silva wrote:
> Hi Andy,
>
> Quoting Andy Shevchenko <andy.shevchenko@gmail.com>:
>
> > On Tue, Feb 13, 2018 at 6:59 PM, Gustavo A. R. Silva
> > <garsilva@embeddedor.com> wrote:
> > > Add suffix ULL to constant 1000 in order to give the compiler complete
> > > information about the proper arithmetic to use. Notice that this
> > > constant is used in a context that expects an expression of type
> > > u64 (64 bits, unsigned).
> > >
> > > The expression threshold_us * 1000 is currently being evaluated
> > > using 32-bit arithmetic.
> >
> > > - u64 threshold_ns = threshold_us * 1000;
> > > + u64 threshold_ns = threshold_us * 1000ULL;
> >
> > Shouldn't be other way around, i.e.
> >
> > (u64)threshold_us ?
> >
>
> Either way works. The thing is that casting threshold_us to u64 may imply
> that there is something wrong with threshold_us, which does not seem to be
> the case. So adding the suffix ULL to the constant 1000 is good enough to
> make the expression be evaluated using 64-bit arithmetic instead of 32-bit.
>
> But, again, either way works.
>
> > But still the question. have you checked all callers? Does it even makes
> > sense?
> >
>
> The proposed patch was due to fact that currently threshold_ns is of type
> u64. But based on the following piece of code (which is the only piece of
> code from where encode_l12_threshold is being called):
>
> * Based on PCIe r3.1, sec 5.5.3.3.1, Figures 5-16 and 5-17, and
> * Table 5-11. T(POWER_OFF) is at most 2us and T(L1.2) is at
> * least 4us.
> */
> l1_2_threshold = 2 + 4 + t_common_mode + t_power_on;
> encode_l12_threshold(l1_2_threshold, &scale, &value);
>
> It seems to me that it makes no sense for threshold_ns to be of type u64,
> because the expression threshold_us * 1000 will never exceed the 32-bit
> limits. So if you agree I can send a patch to change its type to u32
> instead.

Changing it to u32 sounds good to me. I can't remember why I chose
u64 to begin with, but it doesn't look necessary.

Thanks for cleaning this up!

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